Hadean zircon formed due to hydrated ultramafic protocrust melting

Anastassia Y. Borisova, Anne Nédélec, Nail R. Zagrtdenov, Michael J. Toplis, Wendy A. Bohrson, Oleg G. Safonov, Ilya N. Bindeman, Oleg E. Melnik, Gleb S. Pokrovski, Georges Ceuleneer, Klaus Peter Jochum, Brigitte Stoll, Ulrike Weis, Andrew Y. Bychkov, Andrey A. Gurenko

Geology (2021)


“Hadean zircons, from the Jack Hills (Western Australia) and other localities, are currently the only window into the earliest terrestrial felsic crust, the formation of which remains enigmatic. Based upon new experimental results, generation of such early crust has been hypothesized to involve the partial melting of hydrated peridotite interacting with basaltic melt at low pressure (<10 km), but it has yet to be demonstrated that such liquids can indeed crystallize zircons comparable to Jack Hills zircon. We used thermodynamic and geochemical modeling to test this hypothesis. The predicted zircon saturation temperatures of <750 °C, together with the model zircon Th, U, Nb, Hf, Y, and rare earth element (REE) contents at 700 °C, δ18OVSMOW (Vienna standard mean ocean water) signatures, and co-crystallizing mineral assemblage were compared to those of the Jack Hills zircon. This comparison was favorable with respect to crystallization temperature, most trace-element contents, and mineral inclusions in zircon. The discrepancy in δ18OVSMOW signatures may be explained by hotter conditions of Hadean protocrust hydration. Our work supports the idea that felsic magma generation at shallow depths involving a primordial weathered ultramafic protocrust and local basaltic intrusions is indeed a viable mechanism for the formation of felsic crust on early Earth.”

Cellular remains in a ~3.42-billion-year-old subseafloor hydrothermal environment

Barbara Cavalazzi, Laurence Lemelle, Alexandre Simionovici, Sherry L. Cady, Michael J. Russell, Elena Bailo, Roberto Canteri, Emanuele Enrico, Alain Manceau, Assimo Maris, Murielle Salomé, Emilie Thomassot, Nordine Bouden, Rémi Tucoulou, Axel Hofmann

Science Advances 14 Jul 2021:
Vol. 7, no. 29, eabf3963

“Subsurface habitats on Earth host an extensive extant biosphere and likely provided one of Earth’s earliest microbial habitats. Although the site of life’s emergence continues to be debated, evidence of early life provides insights into its early evolution and metabolic affinity. Here, we present the discovery of exceptionally well-preserved, ~3.42-billion-year-old putative filamentous microfossils that inhabited a paleo-subseafloor hydrothermal vein system of the Barberton greenstone belt in South Africa. The filaments colonized the walls of conduits created by low-temperature hydrothermal fluid. Combined with their morphological and chemical characteristics as investigated over a range of scales, they can be considered the oldest methanogens and/or methanotrophs that thrived in an ultramafic volcanic substrate.”

A CO2 greenhouse efficiently warmed the early Earth and decreased seawater 18O/16O before the onset of plate tectonics

Daniel Herwartz, Andreas Pack, and Thorsten J. Nagel

PNAS June 8, 2021 118 (23) e2023617118

“The low 18O/16O stable isotope ratios (δ18O) of ancient chemical sediments imply ∼70 °C Archean oceans if the oxygen isotopic composition of seawater (sw) was similar to modern values. Models suggesting lower δ18Osw of Archean seawater due to intense continental weathering and/or low degrees of hydrothermal alteration are inconsistent with the triple oxygen isotope composition (Δ’17O) of Precambrian cherts. We show that high CO2 sequestration fluxes into the oceanic crust, associated with extensive silicification, lowered the δ18Osw of seawater on the early Earth without affecting the Δ’17O. Hence, the controversial long-term trend of increasing δ18O in chemical sediments over Earth’s history partly reflects increasing δ18Osw due to decreasing atmospheric pCO2. We suggest that δ18Osw increased from about −5‰ at 3.2 Ga to a new steady-state value close to −2‰ at 2.6 Ga, coinciding with a profound drop in pCO2 that has been suggested for this time interval. Using the moderately low δ18Osw values, a warm but not hot climate can be inferred from the δ18O of the most pristine chemical sediments. Our results are most consistent with a model in which the “faint young Sun” was efficiently counterbalanced by a high-pCO2 greenhouse atmosphere before 3 Ga.”

Ingredients for microbial life preserved in 3.5 billion-year-old fluid inclusions

Helge Mißbach, Jan-Peter Duda, Alfons M. van den Kerkhof, Volker Lüders, Andreas Pack, Joachim Reitner & Volker Thiel

Nature Communications volume 12, Article number: 1101 (2021)

“It is widely hypothesised that primeval life utilised small organic molecules as sources of carbon and energy. However, the presence of such primordial ingredients in early Earth habitats has not yet been demonstrated. Here we report the existence of indigenous organic molecules and gases in primary fluid inclusions in c. 3.5-billion-year-old barites (Dresser Formation, Pilbara Craton, Western Australia). The compounds identified (e.g., H2S, COS, CS2, CH4, acetic acid, organic (poly-)sulfanes, thiols) may have formed important substrates for purported ancestral sulfur and methanogenic metabolisms. They also include stable building blocks of methyl thioacetate (methanethiol, acetic acid) – a putative key agent in primordial energy metabolism and thus the emergence of life. Delivered by hydrothermal fluids, some of these compounds may have fuelled microbial communities associated with the barite deposits. Our findings demonstrate that early Archaean hydrothermal fluids contained essential primordial ingredients that provided fertile substrates for earliest life on our planet.”

Elements for the Origin of Life on Land: A Deep-Time Perspective from the Pilbara Craton of Western Australia

Martin J. Van Kranendonk, Raphael Baumgartner, Tara Djokic, Tsutomu Ota, Luke Steller, Ulf Garbe, and Eizo Nakamura

Astrobiology, Vol. 21, No. 1
Published Online: 5 Jan 2021


“For decades, deep sea hydrothermal vents have been a preferred setting for the Origin of Life, but “The Water Problem” as relates to polymerization of organic molecules, together with a propensity to dilute critical prebiotic elements as well as a number of other crucial factors, suggests that a terrestrial hot spring field with the capacity for wet–dry cycling and element concentration may represent a more likely candidate. Here, we investigate a 3.5 billion-year-old, anoxic hot spring setting from the Pilbara Craton (Australia) and show that its hydrothermal veins and compositionally varied pools and springs concentrated all of the essential elements required for prebiotic chemistry (including B, Zn, Mn, and K, in addition to C, H, N, O, P, and S). Temporal variability (seasonal to decadal), together with the known propensity of hot springs for wet–dry cycling and information exchange, would lead to innovation pools with peaks of fitness for developing molecules. An inference from the chemical complexity of the Pilbara analogue is that life could perhaps get started quickly on planets with volcanoes, silicate rocks, an exposed land surface, and water, ingredients that should form the backbone in the search for life in the Universe.”

A Reconstructed Subaerial Hot Spring Field in the ∼3.5 Billion-Year-Old Dresser Formation, North Pole Dome, Pilbara Craton, Western Australia

Tara Djokic, Martin J. Van Kranendonk, Kathleen A. Campbell, Jeff R. Havig, Malcolm R. Walter, and Diego M. Guido

Astrobiology, Vol. 21, No. 1
Published Online: 5 Jan 2021


“Recent discoveries of geyserite and siliceous sinter with textural biosignatures in the ∼3.5 Ga Dresser Formation of the Pilbara Craton, Western Australia, extended the record of inhabited subaerial hot springs on Earth by ∼3 billion years, back to the time when siliceous sinter deposits are known to have formed on Mars (e.g., at Columbia Hills, Gusev Crater). Here, we present more detailed lithostratigraphic, petrographic and geochemical data collected from 100 measured sections across a ∼14 km strike length in the Dresser Formation. The data indicate deposition of a wide range of hot spring and associated deposits in a restricted interval that directly overlies a hydrothermally influenced volcanic caldera lake facies, with shoreline stromatolites. Hot spring deposits show abrupt lateral facies changes and include associated channelized clastic deposits that support fluvial, subaerial hot spring deposition. All Dresser hot spring and associated lithofacies have direct analogs with proximal, middle, and distal apron hot spring facies that are characteristic of those from New Zealand, Yellowstone National Park, USA, and Argentina. Rare earth element and yttrium geochemistry shows that the Dresser geyserite shares identical patterns with Phanerozoic hot spring sinters. This geochemical data further supports textural and contextual evidence that indicate the Dresser geyserite formed as a subaerial hot spring sinter. Further, the Dresser hot spring deposits are temporally associated with a diverse suite of textural biosignatures that indicate a thriving microbial community existed within in a Paleoarchean hot spring field. The results presented here underscore the importance of continued study of the early geological record for astrobiological research. In particular these findings reinforce the long-standing hypothesis that hydrothermal systems are optimal places to search for past life on Mars.”

Inclusions in impact-formed zircon a 1 s a tracer of target rock lithology: Implications for Hadean continental crust composition and abundance

Jordan P. Faltys, Matthew M. Wielicki

Available online 4 September 2020, 105761


• Inclusions in terrestrial impact-formed zircon are reflective of their source.
• Impact-formed zircons thus can be used to trace surface compositions.
• Implications of the inclusion population within Hadean zircon suggest an abundant early felsic crustal component.”

“Impacts play an essential role in the formation and possible habitability on early Earth and are the dominant mechanism controlling surface geomorphology on many solar system bodies. Without a significant >4 Ga rock record, geochemical variations and micron-scale inclusions within detrital Hadean zircon (Earth’s oldest material – primarily from the Jack Hills, Western Australia) have provided valuable insights into the near-surface conditions of this crucial period in Earth’s evolution. We report primary inclusion mineralogy of impact-formed zircon and the host impact melt from the Sudbury and Morokweng impact structures with estimates of target rock lithology to probe the composition of Earth’s earliest crust given the possibility that Hadean grains were created in ancient impacts. Applying the theory of uniformitarianism between preserved and ancient impactites and utilizing geochemical proxies, we highlight the compositional evolution of an impact melt from a given target rock and the associated primary inclusion assemblages. Whole-rock X-ray Fluorescence and thick-section analyses coupled with primary inclusion populations from known terrestrial impact-formed zircon yield proportional mineralogy of the impact melt sheet from which they crystallized. Additionally, target rock lithology consistently plots within an area on a Total Alkali Silica (TAS) diagram centered amongst the granophyre and noritic units of an impact melt. Deducing similar trends of the impact melt mineralogy combined with TAS and Quartz-Alkali-Plagioclase ternary diagram melt pathways, we infer that if Hadean zircons from Jack Hills were impact-formed, the target rock lithology (i.e. Earth’s earliest crust) would be between the composition of quartz-rich granitoid and tonalite-trondhjemite-granodiorite.”

Boron partitioning between zircon and melt: Insights into Hadean, modern arc, and pegmatitic settings

Wriju Chowdhury, Dustin Trail, Elizabeth Bell

Chemical Geology
Available online 25 June 2020, 119763

“There is a lot of debate surrounding the geodynamic environment of the Hadean Earth, and moreover, the only definitively known fragments from the first 500 Ma are detrital zircons. This makes the combination of natural and experimentally obtained data a powerful tool to unravel the chemistry of the early Earth. We have used an incompatible marker element, B, to partially derive the chemistry of the parent melts of the Hadean and modern zircons. We report an experimental calibration for temperature dependent B partitioning between zircon and a hydrous weakly peraluminous granitic melt.
where DBzrc/melt is the zircon-melt partition coefficient for B and T is temperature in K.

This calibration has been applied to natural samples, viz., Hadean zircons from the Jack Hills (JH), Australia, Phanerozoic zircons from the Lachlan Fold Belt (LFB), Australia and three pegmatitic zircons from Seiland Igneous province, Norway, Paicoma Canyon, California and Freeman Mine, North Carolina. Our results present direct evidence of B being present in the Hadean crust. The zircons from JH and LFB are rather poor in B (8–80 ppb), but comparable to each other, while the pegmatites have as much as ten times the [B] (~0.35–0.45 ppm). Application of our experimental calibration yields calculated B melt concentrations of 10–90 ppm for the JH and LFB zircons. Such values for melt [B] (concentrations) are similar to the modern upper continental crust (17 ppm) and volcanic arcs, but are high when to compared to OIBs (0.6–1.8 ppm) and MORBs (1.3 ppm). The Lachlan zircons have [B] values that are broadly similar to the detrital Hadean and Archean zircons, and these have been presented as a point of comparison between Hadean and modern zircons. The pegmatite zircons return calculated melt values that are much higher than the Australian zircons (244–701 ppm). One of the zircons show a variation in calculated Ti-in-zircon crystallization T from the core to the rim (716 °C – 916 °C). Two of the other zircons crystallized at 664 ± 14 °C and 598 ± 13 °C (2 s.e.; not taking into consideration a ~50o T uncertainty due to imperfectly constrained silica/titania activities) and show no variation in [B] or T. Finally, B has been proposed to have been a possible stabilizing agent for ribose aqueous solutions and could have played a part in the formation of carbohydrates and proteins which were building blocks for RNA. Our documented presence of B in the Hadean crust at calculated concentrations similar to a modern volcanic arc setting, makes the role of B in ribose stabilization at least possible on the primordial Earth.”

Reappraisal of purported ca. 3.7 Ga stromatolites from the Isua Supracrustal Belt (West Greenland) from detailed chemical and structural analysis

Mike J. Zawaski, Nigel M. Kelly, Omero Felipe Orlandini, Claire I.O. Nichols, Abigail C. Allwood, Stephen J. Mojzsis

Earth and Planetary Science Letters
Volume 545, 1 September 2020, 116409


• Claimed ca. 3.7 Ga Isua stromatolites are instead granoblastic quartz + dolomite boudins.
• Absence of internal convex upward laminae precludes classification as stromatolites.
• 3-D structural analysis reveals the boudins form ridges parallel to regional deformation.
• Parallel micro- and macro-structural and chemical analyses are essential to assess biogenicity of any proposed bio-structures in ancient, deformed terranes.”

“The biogenicity of proposed stromatolite structures from Eoarchean (ca. 3.71 Ga) rocks of the Isua Supracrustal Belt (ISB) in West Greenland is under debate. The structures in question are found within a suite of multiply deformed greenschist- to amphibolite-facies metamorphic rocks. To assess their premise as primary sedimentary features – as opposed to products of strain localization in layered, variably ductile rocks – we report new field mapping at the appropriate scale and resolution from the original discovery Sites reported by Nutman et al. (2016). Our new map was used to guide micro- and macro-structural investigations and comprehensive geochemical sampling. Here, we report detailed field characterization and structural analysis to show that the structures are linear inverted ridges aligned with azimuths of local and regional fold axes and parallel to linear structures; they are not deformed conical stromatolites. Combined major element (e.g., Ca, Mg, Si) scanning μXRF maps, and electron backscatter diffraction (EBSD) patterns collected on fresh surfaces cut perpendicular and parallel to the ridges attest to the lack of any residual sedimentary laminae (e.g., compositional layering) within these structures’ cores. Internal layering previously inferred for these features instead arises from variable weathering of outcrop surfaces that otherwise conceals granoblastic quartz ± dolomite cored boudins that sit between semi-continuous competent layers of enveloping quartzite in a calc-silicate schist. The morphology of boudins reflects viscosity contrasts of the different ductile layers during deformation. Therefore, these features are not of sedimentary origin. Furthermore, discontinuous field relationships and absence of primary sedimentary structures that could serve as way-up indicators preclude confident assignment of these outcrops as being structurally overturned, as originally argued. Collectively, our results show that the Isua structures are the expected result of a tectonic fabric that preserves no fine-scale primary sedimentary structures and were probably never stromatolites.!”

In support of rare relict ∼3700 Ma stromatolites from Isua (Greenland)
Allen P. Nutman, Vickie C. Bennett, Clark R.L. Friend, Martin J. Van Kranendonk
Earth and Planetary Science Letters
In Press, Corrected Proof, Available online 3 March 2021
Reply: The Isua (Greenland) “relict stromatolites” cannot be confidently interpreted as original sedimentary structures
Mike J. Zawaski, Nigel M. Kelly, Omero Felipe Orlandini, Claire I.O. Nichols, Abigail C.Allwood, Stephen J. Mojzsis
Earth and Planetary Science Letters, In Press, Corrected Proof, Available online 3 March 2021

Formation of early Archean Granite-Greenstone Terranes from a globally chondritic mantle: Insights from igneous rocks of the Pilbara Craton, Western Australia

Andreas Petersson, Anthony I.S. Kemp, Chris M.Gray, Martin J.Whitehouse

Chemical Geology
Available online 22 June 2020, 119757

“The continental crust grows via juvenile additions from the mantle. However, the timing of initial continent stabilisation and the rate of subsequent continental growth during the first billion years of Earth history is widely debated, in part due to uncertainty over the composition of the mantle source of new crust. Well-preserved Archean granite-greenstone terranes, as present within the Pilbara Craton (Western Australia), provide insights into the sources of felsic magmas and the processes of continental growth and evolution in the distant geological past at the regional scale. Here, we present zircon UPb, O and Hf isotope data from ancient gneissic and granitic rocks of the Pilbara Craton, to decipher magma sources and the timing and processes of craton growth. There is no evidence for depleted mantle compositions, and the simplest interpretation is that the crust of the Pilbara Craton was generated from mantle with a chondritic Hf isotope composition. Our results indicate crustal addition at ~3.59 Ga, represented by emplacement of gabbroic to anorthositic rocks. We suggest that the formation of these igneous rocks, and the foundering of the complementary residues, triggered extensive melting of hot, upwelling mantle, leading to the subsequent accumulation of the >12 km thick greenstone belt eruptive sequences from 3.53 Ga, with emplacement of coeval felsic magmas at depth. This process shaped the initial crustal configuration of the proto-craton, which subsequently underwent gravitationally driven overturn and reworking to generate stable, cratonic continental crust with the distinctive dome and keel architecture. The zircon Hf and O isotope signatures of the Pilbara igneous rocks from ~3.59–3.4 Ga do not support remelting of an ancient (> 3.8 Ga) basement, and reinforce the overwhelmingly chondritic to near-chondritic zircon Hf isotope composition of Eoarchean meta-igneous rocks from a number of different Archean cratons. A corollary of this remarkable global consistency is that a significant volume of the mantle maintained a chondritic composition for the LuHf system from the formation of the Earth into the Paleoarchean (up to 3.6–3.5 Ga), as would be the case if stabilised volumes of felsic continental crust prior to 3.5 Ga were relatively small. One implication is that the common assumption of a linear evolution of depleted mantle from 4.5 Ga to the present day is inappropriate for determining the timing and volume of continental crust extraction in the Archean. The nearly identical early evolution of the Pilbara and Kaapvaal cratons suggests a common process to generate Archean granite-greenstone terranes that does not require extensive reworking of ancient crust, but rather involves juvenile crustal addition above persistent zones of upwelling, chondritic mantle.”

Compositional heterogeneity of Archean mantle estimated from Sr and Nd isotopic systematics of basaltic rocks from North Pole, Australia, and the Isua supracrustal belt, Greenland

Hitomi Nakamura, Ayane Sano, Saya Kagami, Tetsuya Yokoyama, Akira Ishikawa, Tsuyoshi Komiya, Hikaru Iwamori

Precambrian Research
Available online 6 June 2020, 105803


• MORBs and OIBs record the mantle differentiation-homogenization history in the Archean.
• Archean basalts from Western Australia and West Greenland were chemically examined.
• Results indicate efficient plate recycling and differentiation before ∼3800 Ma.
• Mantle-crust was then homogenized (to ∼3460 Ma) with subsequent differentiation.”

“Compositional variability found in modern mid-ocean ridge basalt (MORB) and ocean island basalt (OIB) reflects differentiation processes associated with material recycling in the mantle-crust system. To investigate the timing at which this recycling system was established and how it transformed into the present-day system, we present geochemical analyses of the Archean basalts from North Pole (NP) in the East Pilbara Craton, Western Australia, and the Isua supracrustal belt (ISB), southern West Greenland. These rocks represent Archean accretionary complexes with ages of ∼3.5 Ga and 3.7–3.8 Ga, respectively. We analyzed the trace element contents including rare earth elements (REEs), and Sr and Nd isotopic compositions of the basalts, which may represent MORBs and OIBs, from NP and ISB.

Their trace-element compositions are broadly similar, but show distinct geochemical characteristics particularly with respect to REEs that probably reflect differences in both the source mantle and degree of melting. Such differences are also evident in their initial Nd isotopic compositions, which were estimated based on equilibrium partitioning of REEs and well-defined isochron ages. In contrast, the Sr isotopic compositions of the NP and ISB basalts are highly variable and their isochron ages are inconsistent with previous studies. Furthermore, the partitioning of Rb and Sr in the NP basalts indicates disequilibrium, suggesting that the Rb-Sr system has been disturbed by post-igneous alteration and metamorphism.

Based on these observations, we propose the following model to explain the temporal variations in the geochemical composition of the Archean mantle: (i) ∼3800 Ma: recycling of plate material and melting occurred quite readily and, therefore, MORBs and OIBs were produced from differentiated mantle sources; (ii) 3460 Ma to ∼3800 Ma: mantle-crust mixing occurred as the result of an extreme event, such as mantle overturning, reducing the compositional variation of the mantle; and (iii) after ∼3460 Ma: mantle heterogeneity gradually developed in the material-recycling system, re-establishing the compositional differences between MORBs and OIBs. This model requires an extreme event to drive the homogenization during stage (ii), which may provide new insights into the evolution of the crust-mantle system.”

Unraveling the complexity of zircons from the 4.0-2.9 Ga Acasta Gneiss Complex

Ann M. Bauer, Jeffrey D. Vervoort, Christopher M. Fisher

Geochimica et Cosmochimica Acta
In Press, Journal Pre-proof, Available online 1 June 2020

“The Acasta Gneiss Complex (AGC) of the Northwest Territories, Canada, contains some of the oldest evolved terrestrial crust and has a sustained record of more than a billion years of magmatism (>4.0 to 2.9 Ga). These rocks provide an opportunity to investigate the nature of continental crust formation on the early Earth. Because complexities in zircons and bulk rocks are characteristic of the early Earth record—including the AGC—strategies are needed to extract accurate and meaningful age and isotopic information. In order to evaluate the accuracy of the early Earth Hf isotope record, we examined AGC zircons from a range of lithologies using paired chemical abrasion isotope dilution U-Pb age and solution Lu-Hf isotope analysis and compared these with previous results obtained using laser ablation split-stream (LASS) analysis. We describe an approach whereby LASS is used to identify rocks with the least complex zircon, and, when appropriate, solution methods are then used to refine the age and Hf isotopic composition to the highest precision. This two-pronged analytical approach results in a more robust determination of the age and Hf isotopic record of complex rocks and zircons and allows identification of complexity in the Hf isotopic record that would not be apparent by solution analysis alone, thereby refining the record of magmatic evolution on the early Earth. Despite the better precision, solution techniques are unsuitable for rocks with complexly zoned zircons. In particular, zircon from the two oldest AGC rocks in this study have positive initial εHf values when determined by solution analysis (+2.8 and +4.9 at 4.0 and 3.9 Ga, respectively) but have negative εHf values when determined by LASS (-3.4 and -4.7, respectively). This is attributable to the presence of later radiogenic overgrowths on the zircon grains that are incorporated in solution analyses but that can be avoided using LASS. This provides important clarity to the AGC Hf isotope record. In total, all but one of the AGC rocks we analyzed, including the oldest samples, have negative εHf values and indicate derivation from an enriched reservoir; none of these samples—in contrast to whole rock Nd isotope compositions—have sufficiently positive εHf values to indicate their derivation from a depleted mantle reservoir. Despite the presence of ancient AGC crust, there is no record of corresponding mantle depletion. This implies that extraction of Hadean crust in this region did not happen in sufficient volume to result in widespread mantle depletion in the AGC source by the Eoarchean. Our results underscore the importance of identifying different components in ancient zircon—and the rocks that contain them—and accurately determining the age and isotopic composition of those components. This is critically important for clarifying the record of the formation of enriched crust and development of the depleted mantle in Earth’s early history.”

An andesitic source for Jack Hills zircon supports onset of plate tectonics in the Hadean

Simon Turner, Simon Wilde, Gerhard Wörner, Bruce Schaefer & Yi-Jen Lai

Nature Communications, Volume 11, Article number: 1241 (2020)

“The composition and origin of Earth’s early crust remains hotly debated. Here we use partition coefficients to invert the trace element composition of 4.3–3.3 Gyr Jack Hills zircons to calculate the composition of the melts from which they crystallised. Using this approach, the average SiO2 content of these melts was 59 ± 6 wt. % with Th/Nb, Dy/Yb and Sr/Y ratios of 2.7 ± 1.9, 0.9 ± 0.2 and 1.6 ± 0.7, respectively. Such features strongly indicate that the protolith for the Jack Hills zircons was not an intra-plate mafic rock, nor a TTG (tondjhemite-tonalite-granodiorite) or a Sudbury-like impact melt. Instead, the inferred equilibrium melts are much more similar to andesites formed in modern subduction settings. We find no evidence for any secular variation between 4.3 and 3.3 Gyr implying little change in the composition or tectonic affinity of the Earth’s early crust from the Hadean to Mesoarchaean.”

Sulfidization of 3.48 billion-year-old stromatolites of the Dresser Formation, Pilbara Craton: Constraints from in-situ sulfur isotope analysis of pyrite

Raphael.J. Baumgartner, Stefano Caruso, Marco L. Fiorentini, Martin J.Van Kranendonk, Laure Martin, Heejin Jeon, Anais Pagès, David Wacey

Chemical Geology
Volume 538, 5 April 2020, 119488

“This study reports in–situ sulfur isotope analyses (32S, 33S, 34S and 36S) of pyrite in strongly sulfidized stromatolites from the ~3.48 billion–year–old Dresser Formation, Pilbara Craton, Australia. These data shed light on sulfur reservoirs and sulfide precipitation processes and provide clues for the contribution of sulfur–cycling microbes to sulfidization.Sulfur isotope signatures derived from mass dependent fractionation (MDF; monitored by δ34S) and mass independent fractionation (MIF; here Δ33S and Δ36S) of pyrite in stromatolites, and of microscopic pyrite within associated barite, allow for the identification of distinctive sulfur sources: i) magmatic–hydrothermal sulfide (H2S) with δ34S and Δ33S ~ 0%; ii) magmatic–hydrothermal sulfate (SO42−) with a MDF signature (MDF–SO42−; δ34S ~ 10‰ and Δ33S ~ 0‰; iii) photochemically–derived sulfate with a MIF signature (MIF–SO4; δ34S ~ −6‰ and Δ33S ~ −3.0‰); iv) photochemically–derived elemental sulfur (S0) with δ34S ≪ 0 and Δ33S ≫ 0‰. The sulfur isotope data suggest that sulfidization was largely driven by reduction of intermixed MDF–SO42− and MIF–SO42− (bulk signature of δ34S ~ 5‰ and Δ33S ~ −1.4‰), and dilution of produced H2S (δ34S ~ −12‰ and Δ33S ~ −1.4‰) by native H2S in magmatic–hydrothermal fluids. The δ34S shifts (up to ~17‰) generated by sulfate reduction are consistent with both thermochemical reactions and influence of sulfate–cycling microbes, the latter which may have facilitated rapid pyrite precipitation and preservation of microbial remains that are entombed within the petrogenetically earliest pyrite generation of stromatolites.Collectively, our data are consistent with ancient stromatolite growth in proximity to shallow marine hydrothermal vents, where hydrothermal fluids contributed to sulfidization that may have been further influenced by sulfur–cycling microbes.”

Paleomagnetism indicates that primary magnetite in zircon records a strong Hadean geodynamo

John A. Tarduno, Rory D. Cottrell, Richard K. Bono, Hirokuni Oda, William J. Davis, Mostafa Fayek, Olaf van ’t Erve, Francis Nimmo, Wentao Huang, Eric R. Thern, Sebastian Fearn, Gautam Mitra, Aleksey V. Smirnov, and Eric G. Blackman

PNAS, first published January 21, 2020

“Determining the age of the geomagnetic field is of paramount importance for understanding the evolution of the planet because the field shields the atmosphere from erosion by the solar wind. The absence or presence of the geomagnetic field also provides a unique gauge of early core conditions. Evidence for a geomagnetic field 4.2 billion-year (Gy) old, just a few hundred million years after the lunar-forming giant impact, has come from paleomagnetic analyses of zircons of the Jack Hills (Western Australia). Herein, we provide new paleomagnetic and electron microscope analyses that attest to the presence of a primary magnetic remanence carried by magnetite in these zircons and new geochemical data indicating that select Hadean zircons have escaped magnetic resetting since their formation. New paleointensity and Pb-Pb radiometric age data from additional zircons meeting robust selection criteria provide further evidence for the fidelity of the magnetic record and suggest a period of high geomagnetic field strength at 4.1 to 4.0 billion years ago (Ga) that may represent efficient convection related to chemical precipitation in Earth’s Hadean liquid iron core.”

Origins of high δ18O in 3.7–3.6 Ga crust: A zircon and garnet record in Isua clastic metasedimentary rocks

Laure Gauthiez-Putallaz, Allen Nutman, Vickie Bennett, Daniela Rubatto

Chemical Geology
In Press, Journal Pre-proof, Available online 17 January 2020

“Elevated δ18O is used as a marker for the presence of continents and surficial alteration in the Eoarchean and Hadean. This study establishes a timeline for δ18O enrichment in Eoarchean metasedimentary rocks of the Isua supracrustal belt in Greenland. The source-rocks for the protolith of these metasedimentary rocks are mafic to intermediate magmatic rocks of ≥3709 ± 4 Ma, based on the age of zircons found in volcanogenic layers. The δ18O of +5.4 ± 0.4‰ of the zircon crystals indicate that the sources had a primary mantle-derived signature. However, garnet in two metasediments yields higher δ18O values of +8.7 to +9.7‰, in equilibrium with a whole-rock of +11 to +12‰ at 500–600 °C. This requires that the mafic protolith was weathered at surficial conditions, in agreement with previous conclusions based on major element geochemistry. The garnet grains with high δ18O record four growth zones, assigned to I) arc-building thermal metamorphism, II–III) terrane assembly at medium to high-pressure conditions, estimated to occur at 3660–3690 Ma and IV) late-Archaean overprint likely at ca. 2690 Ma. This shows that material with originally mantle-like δ18O was altered at low temperature (near-surface) to generate elevated oxygen isotope signatures and then recycled to middle-crustal conditions within 10–50 million years of crystallization in the Eoarchean. We propose that melting of such rocks could produce the zircon crystals with high δ18O that are found in the detrital and magmatic record in the Archean.”

A comparison between zircons from the Acasta Gneiss Complex and the Jack Hills region

Jesse R. Reimink, Joshua H.F.L. Davies, Ann M. Bauer, Thomas Chacko

Earth and Planetary Science Letters
In Press, Corrected Proof, Available online 4 December 2019


• Processes that formed the earliest continental crust are hotly contested.
• The oldest zircon-bearing rocks are 4.02 Ga, while the oldest zircons are 4.36 Ga.
• We compare zircon trace-element geochemistry between these two samples.
• Zircons from each location are chemically identical, indicating a similar process.
• The oldest crust we know of was formed by a transition from shallow-to-deep melting.”

“The composition and origin of the earliest continental crust is intensely debated. The understanding of its composition relies heavily on the oldest vestiges of continental crust, which includes detrital zircon grains that are up to 4.36 billion-years-old (Ga) and <4.03 Ga zircon-bearing rocks. However, the interpretation of these two sample suites has thus far remained largely separate. Here, we demonstrate that the trace-element compositions of magmatic zircons from the Acasta Gneiss Complex compare favorably with those of ancient detrital zircons from western Australia. We combine these new data with existing oxygen and hafnium isotope datasets to show that the petrological processes that formed the Acasta Gneiss Complex, namely partial melting of mafic crust at various depths, are an appropriate analogue for the formation of the Hadean crust parental to the Jack Hills zircon grains. We also suggest that a transition from shallow-, to deep-crustal melting occurred in the JH source region ca. 3.8 Ga.”

Accumulation of Transition Metals and Metalloids in Sulfidized Stromatolites of the 3.48 Billion–year–old Dresser Formation, Pilbara Craton

Raphael J. Baumgartner, Martin J. Van Kranendonk, Anais Pagès, Marco L. Fiorentini, David Wacey, Chris Ryan

Precambrian Research
17 November 2019, 105534


• Primary accumulations of transition metals and metalloids in Archean stromatolites.
• Element distributions and concentrations correlate with microbial growth fabrics.
• Accumulation owing to binding to organic matter of microbial communities.
• Accumulation driven by microbial utilization.”

“Stromatolites of the 3.48 billion–year–old Dresser Formation (Pilbara Craton, Western Australia) provide some of the oldest convincing evidence of life on Earth. Here, we augment previous evidence with a detailed investigation of the concentrations and distributions of transition metals (Cr, Mn, Co, Ni, Cu, Zn, Mo, Se, Ag, Sn, Au, Hg, and Pb) and metalloids (As, Sb, and Te) in unweathered samples of strongly sulfidized stromatolites from drill cores. High–resolution elemental mapping and in situ compositional analysis of sulfides (pyrite and sphalerite) show that these sedimentary and hydrothermally sourced elements are strongly concentrated in texturally distinctive, nano–porous pyrite enriched in autochthonous organic matter, which forms the major, petrogenetically earliest, component of wrinkly laminated and digitate growth fabrics within the stromatolites. Repeated cyclic alternations of various transition metals and metalloids (most importantly Ni and Zn), plus the presence of disconformities and overgrowth relationships between wrinkly stromatolite laminae, suggest that these element accumulations were primarily established by depositional processes during continuous stromatolite formation. Because transition metals and metalloids generally have strong affinities for organic matter, and can play active roles in biochemical processes, we interpret these element accumulations in the Dresser Formation stromatolites to be the result of binding to organic matter of living microbial communities and/or dead biomass, and perhaps also microbial utilization. Collectively, our results show that the precise characterization of transition metal–metalloid concentrations and distributions can unveil element enrichment patterns suggestive of biological activity, even in some of Earth’s oldest stromatolites.”

Disturbances in the Sm–Nd isotope system of the Acasta Gneiss Complex—Implications for the Nd isotope record of the early Earth

Christopher M. Fisher, Ann M. Bauer, Jeffrey D. Vervoort

Earth and Planetary Science Letters
In Press, Corrected Proof, Available online 28 October 2019


• We present LASS U–Pb and Sm–Nd isotope data from the Acasta Gneiss Complex, Canada.
• Sm–Nd redistribution occurred at 3.3 Ga and again during the 1.89 Ga Wopmay orogen.
• Sm–Nd redistribution is a common feature of ancient poly-metamorphic terranes.
• Sm–Nd mobility may compromise the initial Nd isotope composition of ancient rocks.
• Analysis of Nd-rich minerals proves useful for assessing bulk-rock Nd isotope data.”

“The whole rock 143Nd isotope record of the Acasta Gneiss Complex (AGC) has been used to argue for the development of both depleted mantle and enriched crustal reservoirs during the Hadean. The 147Sm–143Nd isotope compositions of rocks from AGC, however, fall on an array with an errorchron date of ∼3.3 Ga, despite the majority of samples yielding substantially older zircon U–Pb ages. This has led to the suggestion of a major Sm–Nd “resetting” event at this time in the AGC. To better understand the Sm–Nd bulk-rock systematics of the AGC, we investigate the U–Pb age and Sm–Nd isotope compositions of apatite and titanite from the AGC, using the laser ablation split stream (LASS) method. Apatite from all samples yield broadly similar U–Pb dates of ∼1.89 to 1.83 Ga, in agreement with resetting during the 1.9 Ga Wopmay orogen. The Sm–Nd isotope compositions of the apatite, however, lie off the WR errorchron and show a high degree of initial Nd isotope heterogeneity. Conversely, titanite Sm–Nd systematics are consistent with the ∼3.3 Ga whole rock errorchron further supporting Sm–Nd modification at ∼3.3 Ga. Taken together, these show that the oldest rocks in the AGC underwent at least two post-crystallization events (at 3.3 and 1.9 Ga) that modified the Sm–Nd isotope system. These results also demonstrate the relative mobility of the Sm–Nd isotope system during orogenic events, which may compromise the ability to determine robust bulk-rock 143Nd isotope signatures.”

The accumulation of non-formula elements in zircons during weathering: Ancient zircons from the Jack Hills Western Australia

R.T. Pidgeon, A.A Nemchin, M.P. Roberts, M.J. Whitehouse, J.J. Bellucci

Chemical Geology
Available online 14 October 2019, 119310

“In this contribution we describe the influx of non-formula elements ( Fe, Ca, Al, Y, U and Th) into fractures and selected zone lamellae in zircons from Jack Hills during recent weathering and discuss the effects of this on overlapping SIMS U-Th-Pb and oxygen isotope analyses. Previous research has recognised the importance of fractures in the generation of anomalous U-TH-Pb and oxygen isotope systems. In this report we show that besides fractures specific zones in euhedrally zoned zircon can act as pathways for the influx of weathering solutions and contain a similar range of trace element materials as do the fractures. Whereas zero-age discordant U-Pb systems of Jack Hills zircons have been explained by many authors in terms of Pb loss, present results confirm conclusions of our previous study that the main discordance mechanism of Jack Hills zircons is U-Th gain, due to overlap of SIMS analyses with mineralized fractures and zone lamellae with excess weathering-fluid-deposited U and Th. We explain the anomalously light and heavy oxygen isotopes and significant OH in SIMS analyses that overlap fractures and mineralized zones as due to the presence in the fractures of Ca, Fe, Al oxides and hydroxides with complexly fractionated oxygen isotopic systems. There is a suggestion in some of the elemental maps that there has been minor dispersion of trace elements away from fractures. But SIMS U-Th-Pb and oxygen isotope analyses on parts of the zircon away from fractures and mineralized zones show no evidence of interaction with weathering-fluid, indicating that penetration of weathering fluids into the body of the zircon at the location of the SIMS spots has not occurred. Results of this study have implications for other SIMS U-Th-Pb and oxygen isotope studies of zircons from rocks that have been subjected to weathering and also for early TIMS U-Pb measurements of bulk zircon samples that show zero Ma U-Pb discordance.”

Nano−porous pyrite and organic matter in 3.5-billion-year-old stromatolites record primordial life

Raphael J. Baumgartner, Martin J. Van Kranendonk, David Wacey, Marco L Fiorentini, Martin Saunders, Stefano Caruso, Anais Pages, Martin Homann, Paul Guagliardo

Geology (2019)

“Stromatolites of the ~3.5 billion-year-old Dresser Formation (Pilbara Craton, Western Australia) are considered to be some of Earth’s earliest convincing evidence of life. However, uniquely biogenic interpretations based on surface outcrops are precluded by weathering, which has altered primary mineralogy and inhibited the preservation of microbial remains. Here, we report on exceptionally preserved, strongly sulfidized stromatolites obtained by diamond drilling from below the weathering profile. These stromatolites lie within undeformed hydrothermal-sedimentary strata and show textural features that are indicative of biogenic origins, including upward-broadening and/or upward-branching digitate forms, wavy to wrinkly laminae, and finely laminated columns that show a thickening of laminae over flexure crests. High-resolution textural, mineralogical, and chemical analysis reveals that the stromatolites are dominated by petrographically earliest, nano-porous pyrite that contains thermally mature, N-bearing organic matter (OM). This nano-porous pyrite is consistent with a formation via sulfidization of an originally OM-dominated matrix. Evidence for its relationship with microbial communities are entombed OM strands and filaments, whose microtexture and chemistry are consistent with an origin as mineralized biofilm remains, and carbon isotope data of extracted OM (δ13COM = –29.6‰ ± 0.3‰ VPDB [Vienna Peedee belemnite]), which lie within the range of biological matter. Collectively, our findings provide exceptional evidence for the biogenicity of some of Earth’s oldest stromatolites through preservation of OM, including microbial remains, by sulfidization.”

Lack of late-accreted material as the origin of 182W excesses in the Archean mantle: Evidence from the Pilbara Craton, Western Australia

Gregory J. Archer, Gregory A. Brennecka, Philipp Gleißner, Andreas Stracke, Harry Becker, Thorsten Kleine

Earth and Planetary Science Letters
Volume 528, 15 December 2019, 115841

“We report 182W and 142Nd isotopic compositions, 187Re–187Os systematics, and abundances of highly siderophile elements (HSE: Re, Os, Ir, Ru, Rh, Pt, Pd, and Au) for a suite of komatiites and basalts from the Ruth Well Formation and the Warrawoona Group of the Pilbara Craton, Western Australia. The 182W compositions from all samples are indistinguishable from each other, and more radiogenic than modern bulk silicate Earth, with a mean μ182W value of (2SD). By contrast, the 142Nd values for all samples are indistinguishable from each other and terrestrial standards, with a mean μ142Nd value of (2SD). The 146Sm–142Nd and 187Re–187Os systematics are consistent with chondritic Sm/Nd and Re/Os ratios in the mantle source during the lifetime of 182Hf, and the observed 182W excesses therefore cannot be accounted for by early Hf–W fractionation by magma ocean processes, neither by silicate liquid-crystal fractionation nor by high P–T metal-silicate equilibration. The estimated abundances of HSE in the mantle source, however, are significantly lower than modern bulk silicate Earth, with only (1SD) of modern bulk silicate Earth abundances. These results are consistent with a partial lack of late-accreted material within the Pilbara source at to account for the 182W excesses. Further, widespread 182W excesses of similar magnitude in other Archean mantle-derived rocks worldwide strongly suggests that a common process, most likely incomplete addition of late-accreted material, was responsible. The apparent mismatch between late-accreted 182W–HSE systematics for some other localities likely reflects either the inherent difficulties associated with estimating source HSE abundances, and/or dissociation of W and HSE by mantle processes. Finally, the combined average 182W–HSE systematics of Archean samples indicate that the pre-late accretion BSE likely had a μ182W value similar to that of the lunar mantle, which strongly suggests post-giant impact Earth–Moon equilibration and indicates that the Moon formed after 182Hf extinction.”

The Eoarchean legacy of Isua (Greenland) worth preserving for future generations

Allen P. Nutman, Vickie C. Bennett, Clark R.L. Friend, Martin Van Kranendonk

Earth-Science Reviews
Available online 7 August 2019, 102923

“The Eoarchean (>3600 Ma, or millions of years ago) folded and metamorphosed Isua supracrustal belt and the adjacent orthogneiss exposures of Greenland contain rare low deformation lenses that display some uniquely-preserved components of Earth’s oldest rock record. These include world’s oldest (but contested) stromatolites in dolomitic carbonates, conglomerates, pillow basalts demonstrating submarine eruption, slivers of upper mantle rocks, formation of earliest continental crust by multistage tonalite + diorite emplacement followed by intracrustal granite production. All these diverse occurrences are keys to establish early Earth’s processes at the start of the geological record. Although some of these features are preserved at several localities, other critical ones are exposed on only a few m2 of rock at single localities or are of historical significance. None of these sites are currently protected, and there is a reliance on responsible sampling to keep them intact for future generations. Given the high interest in the Archean Eon, combined with the increased ease of fieldwork in remote localities, many significant ‘Deep Time’ localities in not only Greenland but worldwide are in danger of eradication. Here, five key Isua area geological sites are presented, with an explanation of their significance and worthiness for initially reliance on already-collected samples, but hopefully ultimately government protection. This highlights an increasing problem of destruction of in situ evidence of Earth’s unique early geological heritage and the need for collaboration in protecting and archiving of these key scientific resources.”

A new 3.59 Ga magmatic suite and a chondritic source to the east Pilbara Craton

Andreas Petersson, Anthony I.S. Kemp, Arthur H. Hickman, Martin J. Whitehouse, Laure Martin , Chris M. Gray

Chemical Geology
Volume 511, 20 April 2019, Pages 51-70

“The Pilbara Craton, Western Australia hosts one of the best-preserved Paleoarchean granite-greenstone terrains on Earth, and is inferred to have developed on an older (>3.8 Ga), possibly Hadean, continental substrate. Such ancient crust has, however, never been identified in outcrop. Here, we show that metamorphosed gabbroic, leucogabbroic and anorthositic rocks of the South Daltons area, in the western part of the Shaw Granitic Complex, formed at 3.59–3.58 Ga and were intruded by granitic magma at 3.44 Ga. The 3.59–3.58 Ga gabbroic rocks, here named the Mount Webber Gabbro, represent the oldest, unambiguous igneous rock emplacement in the Pilbara Craton and significantly predate the oldest volcanic activity of the 3.53–3.23 Ga Pilbara Supergroup within the East Pilbara Terrane. We interpret the Mount Webber Gabbro samples to represent fragments of a dismembered layered mafic intrusion. Mantle-like zircon δ18O and Hf isotope signatures indicate derivation from a chondritic to near chondritic mantle at ~3.59 Ga, and do not support the existence of a >3.8 Ga basement to the East Pilbara Terrane. These results strengthen the notion of an approximately chondritic >3.5 Ga mantle beneath the Pilbara Craton, and provide further evidence that recent estimates of Archean stabilised continental volumes, based on the assumption of crust extraction from a global, convecting depleted mantle reservoir, may be overestimated.”

Reassessing evidence of life in 3,700-million-year-old rocks of Greenland

Abigail C. Allwood, Minik T. Rosing, David T. Flannery, Joel A. Hurowitz & Christopher M. Heirwegh

Published: 17 October 2018

“The Palaeoarchean supracrustal belts in Greenland contain Earth’s oldest rocks and are a prime target in the search for the earliest evidence of life on Earth. However, metamorphism has largely obliterated original rock textures and compositions, posing a challenge to the preservation of biological signatures. A recent study of 3,700-million-year-old rocks of the Isua supracrustal belt in Greenland described a rare zone in which low deformation and a closed metamorphic system allowed preservation of primary sedimentary features, including putative conical and domical stromatolites1 (laminated accretionary structures formed by microbially mediated sedimentation). The morphology, layering, mineralogy, chemistry and geological context of the structures were attributed to the formation of microbial mats in a shallow marine environment by 3,700 million years ago, at the start of Earth’s rock record. Here we report new research that shows a non-biological, post-depositional origin for the structures. Three-dimensional analysis of the morphology and orientation of the structures within the context of host rock fabrics, combined with texture-specific analyses of major and trace element chemistry, show that the ‘stromatolites’ are more plausibly interpreted as part of an assemblage of deformation structures formed in carbonate-altered metasediments long after burial. The investigation of the structures of the Isua supracrustal belt serves as a cautionary tale in the search for signs of past life on Mars, highlighting the importance of three-dimensional, integrated analysis of morphology, rock fabrics and geochemistry at appropriate scales.”

see also: Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures (Nature, Sept. 2016)

Evidence for evolved Hadean crust from Sr isotopes in apatite within Eoarchean zircon from the Acasta Gneiss Complex

Robert B. Emo, Matthijs A. Smit, Melanie Schmitt, Ellen Kooijman, Erik E. Scherer, Peter Sprung, Wouter Bleeker, Klaus Mezger

Geochimica et Cosmochimica Acta
Available online 5 June 2018

“Current models for the properties of Hadean-Eoarchean crust encompass a full range of possibilities, involving crust that is anywhere from thick and differentiated to thin and mafic. New data are needed to test and refine these models, and, ultimately, to determine how continents were first formed. The Rb-Sr system provides a potentially powerful proxy for crustal evolution and composition. However, this system has thus far been underutilized in studies on early crustal evolution due to its susceptibility to re-equilibration. Overcoming this issue requires new analytical approaches to micro-sample ancient Sr-rich mineral relics that may retain primary Rb-Sr systematics, allowing for the precise and accurate determination of initial 87Sr/86Sr values. In this study, we used a novel application of laser-ablation multi-collector inductively coupled plasma mass spectrometry to determine the Sr isotope composition of apatite inclusions in >3.7 Ga zircon grains from Eoarchean granodiorite gneisses of the Acasta Gneiss Complex, Slave Province, Canada. The 87Rb-corrected 87Sr/86Sr values of these inclusions are largely identical and are distinctly different from values obtained from altered matrix apatite. The inclusion data provide the first direct estimate of initial 87Sr/86Sr for these ancient rocks. Combining this result with information on the protolith and source-extraction age yields estimates for the range of Rb/Sr, and by extension composition, that the source of these rocks may have had. The data indicate that continental crust containing over 60 wt% of SiO2 was present in the ca. 4.2 Ga source of the Acasta Gneiss Complex. Thus vestiges of evolved crust must have existed within the primitive proto-continents that were present on the Hadean Earth.”

Petrogenesis and tectonics of the Acasta Gneiss Complex derived from integrated petrology and 142Nd and 182W extinct nuclide-geochemistry

Jesse R. Reimink, Thomas Chacko, Richard W. Carlson, Steven B. Shirey, Jingao Liu, Richard A. Stern, Ann M. Bauer, D. Graham Pearson, Larry M. Heaman

Earth and Planetary Science Letters
Volume 494, 15 July 2018, Pages 12–22


• Rocks from the AGC came from distinct sources throughout their >1 Ga history.
• AGC rocks from 4.02 to 3.4 Ga have homogeneous positive μ182W of +10 ppm.
• >3.75 Ga rocks were formed by melting of Hadean mafic crust at low pressure.
• ∼3.6 Ga rocks were formed from more juvenile mafic source rocks.”

“The timing and mechanisms of continental crust formation represent major outstanding questions in the Earth sciences. Extinct-nuclide radioactive systems offer the potential to evaluate the temporal relations of a variety of differentiation processes on the early Earth, including crust formation. Here, we investigate the whole-rock 182W/184W and 142Nd/144Nd ratios and zircon Δ17O values of a suite of well-studied and lithologically-homogeneous meta-igneous rocks from the Acasta Gneiss Complex, Northwest Territories, Canada, including the oldest-known zircon-bearing rocks on Earth. In the context of previously published geochemical data and petrogenetic models, the new 142Nd/144Nd data indicate that formation of the Hadean–Eoarchean Acasta crust was ultimately derived from variable sources, both in age and composition. Although 4.02 Ga crust was extracted from a nearly bulk-Earth source, heterogeneous μ142Nd signatures indicate that Eoarchean rocks of the Acasta Gneiss Complex were formed by partial melting of hydrated, Hadean-age mafic crust at depths shallower than the garnet stability field. By ∼3.6 Ga, granodioritic–granitic rocks were formed by partial melting of Archean hydrated mafic crust that was melted at greater depth, well into the garnet stability field. Our 182W results indicate that the sources to the Acasta Gneiss Complex had homogeneous, high-μ182W on the order of +10 ppm—a signature ubiquitous in other Eoarchean terranes. No significant deviation from the terrestrial mass fractionation line was found in the triple oxygen isotope (16O–17O–18O) compositions of Acasta zircons, confirming homogeneous oxygen isotope compositions in Earth’s mantle by 4.02 Ga.”

Earth’s oldest stable crust in the Pilbara Craton formed by cyclic gravitational overturns

Daniel Wiemer, Christoph E. Schrank, David T. Murphy, Lana Wenham & Charlotte M. Allen

Nature Geoscience (2018)

“During the early Archaean, the Earth was too hot to sustain rigid lithospheric plates subject to Wilson Cycle-style plate tectonics. Yet by that time, up to 50% of the present-day continental crust was generated. Preserved continental fragments from the early Archaean have distinct granite-dome/greenstone-keel crust that is interpreted to be the result of a gravitationally unstable stratification of felsic proto-crust overlain by denser mafic volcanic rocks, subject to reorganization by Rayleigh–Taylor flow. Here we provide age constraints on the duration of gravitational overturn in the East Pilbara Terrane. Our U–Pb ages indicate the emplacement of ~3,600–3,460-million-year-old granitoid rocks, and their uplift during an overturn event ceasing about 3,413 million years ago. Exhumation and erosion of this felsic proto-crust accompanied crustal reorganization. Petrology and thermodynamic modelling suggest that the early felsic magmas were derived from the base of thick (~43 km) basaltic proto-crust. Combining our data with regional geochronological studies unveils characteristic growth cycles on the order of 100 million years. We propose that maturation of the early crust over three of these cycles was required before a stable, differentiated continent emerged with sufficient rigidity for plate-like behaviour.”

Most Ancient Evidence for Life in the Barberton Greenstone Belt: Microbial Mats and Biofabrics of the ∼3.47 Ga Middle Marker Horizon

Keyron Hickman-Lewis, Barbara Cavalazzi, Frédéric Foucher, Frances Westall

Precambrian Research
In Press, Accepted Manuscript, Available online 9 April 2018

“The Middle Marker, or horizon H1 of the Hooggenoeg Formation, is the oldest sedimentary horizon in the Barberton greenstone belt and one of the oldest sedimentary horizons on Earth. Herein, we describe a range of carbonaceous microstructures in this unit which bear resemblance to phototrophic microbial biofilms, biosedimentary structures, and interpreted microfossils in contemporaneous greenstone belts from the Early Archaean. Post-depositional iron-rich fluid cycling through these sediments has resulted in the precipitation of pseudo-laminated structures, which also bear resemblance, at the micron-scale, to certain microbial mat-like structures, although are certainly abiogenic. Poor preservation of multiple putative microbial horizons due to coarse volcaniclastic sedimentation and synsedimentary fragmentation by hydrothermal fluid also makes a conclusive assessment of biogenicity challenging. Nonetheless, several laminated morphologies within volcaniclastic sandstones and siltstones and coarse-grained volcaniclastic sandstones are recognisable as syngenetic phototrophic microbial biofilms and microbially induced sedimentary structures, therefore, the Middle Marker preserves the oldest evidence for life in the Barberton greenstone belt. Among these biosignatures are fine, crinkly, micro-tufted, laminated biofilms, pseudo-tufted laminations and wisp-like carbonaceous fragments interpreted as either partially formed biofilms or their erosional products. In the same sediments, lenticular objects, which have previously been interpreted as bona fide microfossils, are rare but recurrent finds whose biogenicity we question. The Middle Marker preserves an ancient record of epibenthic microbial communities flourishing, struggling and perishing in parallel with a waning volcanic cycle, an environment upon which they depended and through which they endured. Direct comparisons can be made between environment-level characters of the Middle Marker and other Early Archaean cherts, suggesting that shallow-water, platformal, volcanogenic-hydrothermal biocoenoses were major microbial habitats throughout the Archaean.”

Ideas and perspectives: hydrothermally driven redistribution and sequestration of early Archaean biomass – the “hydrothermal pump hypothesis”

Duda, Jan-Peter et al.

Biogeosciences, 15, 1535-1548, 2018
Published: 15 Mar 2018

“Archaean hydrothermal chert veins commonly contain abundant organic carbon of uncertain origin (abiotic vs. biotic). In this study, we analysed kerogen contained in a hydrothermal chert vein from the ca. 3.5 Ga Dresser Formation (Pilbara Craton, Western Australia). Catalytic hydropyrolysis (HyPy) of this kerogen yielded n-alkanes up to n-C22, with a sharp decrease in abundance beyond n-C18. This distribution ( ≤  n-C18) is very similar to that observed in HyPy products of recent bacterial biomass, which was used as reference material, whereas it differs markedly from the unimodal distribution of abiotic compounds experimentally formed via Fischer–Tropsch-type synthesis. We therefore propose that the organic matter in the Archaean chert veins has a primarily microbial origin. The microbially derived organic matter accumulated in anoxic aquatic (surface and/or subsurface) environments and was then assimilated, redistributed and sequestered by the hydrothermal fluids (“hydrothermal pump hypothesis”).”

Dr. Jan-Peter Duda

HCN production via Impact Ejecta Reentry during the Late Heavy Bombardment

Parkos D., A. Pikus, A. Alexeenko, and H.J. Melosh

Journal of Geophysical Research, 123, doi:10.1002/2017JE005393.
Accepted manuscript online: 1 March 2018

“Major impact events have shaped the Earth as we know it. The Late Heavy Bombardment is of particular interest because it immediately precedes the first evidence of life. The reentry of impact ejecta creates numerous chemical byproducts, including biotic precursors such as HCN. This work examines the production of HCN during the Late Heavy Bombardment in more detail. We stochastically simulate the range of impacts on the early Earth, and use models developed from existing studies to predict the corresponding ejecta properties. Using multi-phase flow methods and finite rate equilibrium chemistry, we then find the HCN production due to the resulting atmospheric heating. We use DSMC to develop a correction factor to account for increased yields due to thermochemical nonequilibrium. We then model 1D atmospheric turbulent diffusion to find the time-accurate transport of HCN to lower altitudes and ultimately surface water. Existing works estimate the necessary HCN molarity threshold to promote polymerization is 0.01 M. For a mixing depth of 100 m, we find that the Late Heavy Bombardment will produce at least 1 impact event above this threshold with probability 24.1% for an oxidized atmosphere and 56.3% for a partially reduced atmosphere. For a mixing depth of 10 m, the probability is 79.5% for an oxidized atmosphere and 96.9% for a partially reduced atmosphere. Therefore, LHB impact ejecta is likely an HCN source sufficient for polymerization in shallow bodies of water, particularly if the atmosphere were in a partially reduced state.”

Secondary magnetic inclusions in detrital zircons from the Jack Hills, Western Australia, and implications for the origin of the geodynamo

Benjamin P. Weiss, Roger R. Fu, Joshua F. Einsle, David. R. Glenn, Pauli Kehayias, Elizabeth A. Bell, Jeff Gelb, Jefferson F.D.F. Araujo, Eduardo A. Lima, Cauê S. Borlina, Patrick Boehnke, Duncan N. Johnstone, T. Mark Harrison, Richard J. Harrison, Ronald L. Walsworth

Geology (2018)
Published:March 01, 2018

“The time of origin of Earth’s dynamo is unknown. Detrital zircon crystals containing ferromagnetic inclusions from the Jack Hills of Western Australia have the potential to contain the oldest records of the geodynamo. It has recently been argued that magnetization in these zircons indicates that an active dynamo existed as far back as 4.2 Ga. However, the ages of ferromagnetic inclusions in the zircons are unknown. Here we present the first detailed characterization of the mineralogy and spatial distribution of ferromagnetic minerals in Jack Hills detrital zircons. We demonstrate that ferromagnetic minerals in most Jack Hills zircons are commonly located in cracks and on the zircons’ exteriors. Hematite is observed to dominate the magnetization of many zircons, while other zircons also contain significant quantities of magnetite and goethite. This indicates that the magnetization of most zircons is likely to be dominantly carried by secondary minerals that could be hundreds of millions to billions of years younger than the zircons’ crystallization ages. We conclude that the existence of the geodynamo prior to 3.5 Ga has yet to be established.”

The origin of the Moon within a terrestrial synestia

Simon J. Lock, Sarah T. Stewart, Michail I. Petaev, Zoë M. Leinhardt, Mia T. Mace, Stein B. Jacobsen, Matija Ćuk

Journal of Geophysical Research: Planets, 123, doi:10.1002/2017JE005333.

“The giant impact hypothesis remains the leading theory for lunar origin. However, current models struggle to explain the Moon’s composition and isotopic similarity with Earth. Here we present a new lunar origin model. High-energy, high-angular momentum giant impacts can create a post-impact structure that exceeds the corotation limit (CoRoL), which defines the hottest thermal state and angular momentum possible for a corotating body. In a typical super-CoRoL body, traditional definitions of mantle, atmosphere and disk are not appropriate, and the body forms a new type of planetary structure, named a synestia. Using simulations of cooling synestias combined with dynamic, thermodynamic and geochemical calculations, we show that satellite formation from a synestia can produce the main features of our Moon. We find that cooling drives mixing of the structure, and condensation generates moonlets that orbit within the synestia, surrounded by tens of bars of bulk silicate Earth (BSE) vapor. The moonlets and growing moon are heated by the vapor until the first major element (Si) begins to vaporize and buffer the temperature. Moonlets equilibrate with BSE vapor at the temperature of silicate vaporization and the pressure of the structure, establishing the lunar isotopic composition and pattern of moderately volatile elements. Eventually, the cooling synestia recedes within the lunar orbit, terminating the main stage of lunar accretion. Our model shifts the paradigm for lunar origin from specifying a certain impact scenario to achieving a Moon-forming synestia. Giant impacts that produce potential Moon-forming synestias were common at the end of terrestrial planet formation.”

Halogen Chemistry and Hydrogen Isotopes of Apatite from the >3.7 Ga Isua Supracrustal Belt, SW Greenland

Alicja Wudarska, Michael Wiedenbeck, Ewa Słaby, Aivo Lepland, Łukasz Birski, Klaus Simon

Precambrian Research
Available online 26 February 2018
In Press, Accepted Manuscript


• H isotopes in apatite from Archean rocks have been investigated for the first time.
• δD of apatite from Isua have been overprinted during its long geologic history.
• δD of Isua apatite do not indicate the origin of volatiles on the Early Earth.
• Secondary processes which altered rocks of the Isua belt were of local importance.”

“The origin and evolution of volatiles, particularly water, in the abiotic early Earth environment have been intensively studied, and this is a topic of high relevance when considering the timing and conditions of life’s emergence. Investigation of hydrogen isotopes in the oldest crystals of minerals from the apatite group Ca10(PO4)6(F,Cl,OH)2 should bring new insight to this topic as the D/H ratio of apatite has proved useful for establishing the evolution of volatiles in other solar system bodies. Apatite crystals from metasedimentary and metavolcanic rocks collected from a low-strain domain of the Eoarchean Isua supracrustal belt have been investigated for their D/H signatures using secondary ion mass spectrometry and for major and trace element abundances using EPMA and laser ablation ICP-MS. Apatite crystals from Isua have a broad range in volatile compositions, both within samples from the same lithology (e.g., banded iron formation; BIF) and between different lithologies (BIF, metacarbonate, mafic dyke). Most of the crystals are chlorine-rich, which is in contrast to typical modern sedimentary apatite that is predominantly carbonate fluorapatite. We have used the REE distribution patterns from Isua apatite to distinguish those crystals having sedimentary, metasomatic and magmatic origins. The D/H ratios for all samples were determined to be very low with δDVSMOW values falling within the range from -93 and -56 ‰. δD values of Isua apatite do not correlate with REE signatures of crystals nor could we relate δD to the host lithologies. We conclude that this geochemical system has been reset during long and multi-phase geologic history, making it difficult, if not impossible, to use hydrogen isotopes for assessment of the primary signatures of the Archean hydrosphere.”

Remarkably preserved tephra from the 3430 Ma Strelley Pool Formation, Western Australia: Implications for the interpretation of Precambrian microfossils

David Wacey, Martin Saunders, Charlie Kong

Earth and Planetary Science Letters
Volume 487, 1 April 2018, Pages 33–43

“The ∼3430 Ma Strelley Pool Formation (SPF), Pilbara, Western Australia contains some of the most diverse microfossil evidence for early life on Earth. Here we report an assemblage of tephra (scoria, tubular pumice, plus vesicular and non-vesicular volcanic glass shards) from two stratigraphic levels in the SPF, including morphotypes that closely resemble previously described microfossils from this unit and elsewhere.Clasts of scoria are characterised by numerous spheroidal vesicles, with subordinate eye- and lens-shaped morphotypes, commonly lined with anatase (TiO2) and small amounts of organic material. Their diameters range from 5–180 μm with 80% in the 10–50 μm range. Fragments of tubular pumice are also lined with anatase +/− carbon and have tube diameters of 5–15 μm. Other volcanic ejecta particles include a multitude of sub-angular shard particles with or without vesicles, plus more rounded vase-shaped, eye-shaped, and hair-like morphologies; once again, most of these are coated by anatase +/− carbon and are several tens of micrometres in size. Many of the tephra fragments are now entirely silicified with no compositional difference between the former volcanic glass, the vesicle infill and the clast matrix. However, some examples retain a partial aluminosilicate composition, either as a vesicle infilling phase or as isolated lath-like grains within the formerly glassy groundmass.Isolated occurrences of some of these tephra morphotypes strongly resemble simple microbial morphologies including pairs and clusters of cells (cf. scoria), filamentous microbes (cf. tubular pumice) and larger sheaths/cysts (cf. sub-rounded glass shards). Furthermore, some tephra-containing clasts occur in a SPF sandstone unit that hosts previously described microfossils, while others are interbedded with chert layers from which microfossils have also been described. In light of our new volcanogenic data, we evaluate the robustness of previous microfossil evidence from the SPF in the East Strelley greenstone belt. We find that the majority of previously illustrated microfossils from this greenstone belt possess multiple features that are consistent with a biological interpretation and are unlikely to be volcanogenic, but at least one previously illustrated specimen is here reinterpreted as volcanic in origin.The importance of this work is that it serves to highlight the common occurrence of volcanogenic microstructures resembling biological fossils (i.e. pseudo-fossils) in Archean environments that are habitable for life. Such structures have until now been largely overlooked in the assessment of putative Precambrian microfossils. Our data show that tephra-derived microstructures should be considered as a null hypothesis in future evaluations of potential signs of life on the early Earth, or on other planets.”

A 4463 Ma apparent zircon age from the Jack Hills (Western Australia) resulting from ancient Pb mobilization

Rongfeng Ge, Simon A. Wilde, Alexander A. Nemchin, Martin J. Whitehouse, Jeremy J. Bellucci, Timmons M. Erickson, Adam Frew, Eric R. Thern

Geology (2018)
Published: February 01, 2018

“Hadean (≥4.0 Ga) zircon grains provide the only direct record of the first half-billion years of Earth’s history. Determining accurate and precise crystallization ages of these ancient zircons is a prerequisite for any interpretation of crustal evolution, surface environment, and geodynamics on the early Earth, but this may be compromised by mobilization of radiogenic Pb due to subsequent thermal overprinting. Here we report a detrital zircon from the Jack Hills (Western Australia) with 4486–4425 Ma concordant ion microprobe ages that yield a concordia age of 4463 ± 17 Ma (2σ), the oldest zircon age recorded from Earth. However, scanning ion imaging reveals that this >4.4 Ga apparent age resulted from incorporation of micrometer-scale patches of unsupported radiogenic Pb with extremely high 207Pb/206Pb ratios and >4.5 Ga 207Pb/206Pb ages. Isotopic modeling demonstrates that these patches likely resulted from redistribution of radiogenic Pb in a ca. 4.3 Ga zircon during a ca. 3.8 Ga or older event. This highlights that even a concordia age can be spurious and should be carefully evaluated before being interpreted as the crystallization age of ancient zircon.”

Cluster analysis on a sphere: Application to magnetizations from metasediments of the Jack Hills, Western Australia

Richard K. Bono, John A. Tarduno, Matthew S. Dare, Gautam Mitra, Rory D. Cottrell

Earth and Planetary Science Letters
Pages 67-80


• A new approach identifies direction subsets in paleomagnetic conglomerate test data.
• Spherical contouring/cluster analysis can detect overprints in high dispersion data.
• Applied to Jack Hills (JH) cobbles, there is no evidence for a 1 Ga remagnetization.
• An intermediate temperature ∼2.6 Ga metamorphic overprint is identified in JH cobbles.
• Analyses of higher temperature data support preservation of primary JH magnetizations.”

“Metasediments of the Jack Hills contain the oldest known terrestrial minerals in the form of zircons nearly 4.4 billion years old. Paleointensity data from these zircons provide evidence for a Hadean geodynamo as old as 4.2 billion years old. Given the importance of these zircons for constraining the earliest history of the core, it is vital to understand the fidelity of the zircon record. A fundamental aspect providing context for the preservation of primary magnetic signals is the nature of overprints predicted to have been imparted on rocks of the Jack Hills due to Archean to Proterozoic metamorphic events. To be viable magnetic records of a Hadean geodynamo, zircon magnetization directions should differ from these secondary magnetizations. To evaluate these secondary magnetizations, we report paleomagnetic analyses of a comprehensive sampling of 68 quartzite cobble-sized clasts from the Jack Hills metasediments ∼0.5 to 1.0 km from the Discovery Site (which has yielded the oldest zircons and paleofield estimates). While application of standard paleomagnetic tests suggests that the ensemble of cobble directions cannot be distinguished from those drawn from a random distribution, a new cluster analysis of directions on a sphere and non-parametric resampling approaches reveal significant directions amongst subsets of the data. One, isolated at the lowest temperature analyzed [200 to 300 °C, Declination (Dec.) = 316.8°, Inclination (Inc.) =−51.1°=−51.1°] appears to be dominated by the present day field. Another, isolated at higher (but still relatively low unblocking temperatures that we call “intermediate”, of ∼350–500 °C, Dec. = 243.8°, Inc. = 9.5°) agrees with a magnetic overprint isolated from the secondary Cr–Fe mica fuchsite isolated from the Jack Hills Discovery site, passing a field test at the 80% confidence level. No evidence is found in our data, or in the data of others collected on similar Jack Hills lithologies, for a widespread 1 Ga remagnetization event. Instead, we interpret the most prevalent secondary magnetization of the quartzite (i.e., intermediate unblocking) and the fuchsite characteristic remanent magnetization to be ∼2.65 Ga in age, coincident with peak metamorphism (as high as ca. 475 °C) of the Jack Hills. The presence of this distinct secondary magnetization, its difference from that recorded by Jack Hills zircons at high unblocking temperatures, and the lack of a dominant remagnetization direction at high unblocking temperatures in the cobble data (the expected result for a primary magnetization), lends further support to the fidelity of the Hadean geomagnetic record. The presence of the secondary magnetization also lends support to the conclusion that most of the Jack Hills metasediments were deposited in the Archean, with only minor reworking and potential tectonic interleaving of Proterozoic components. Overall, the application of the new directional cluster analysis presented here has the potential to reveal magnetic directions in highly scattered data sets, typical of weakly magnetized coarse-grained sedimentary rocks”

SIMS analyses of the oldest known assemblage of microfossils document their taxon-correlated carbon isotope compositions

J. William Schopf, Kouki Kitajima, Michael J. Spicuzza, Anatoliy B. Kudryavtsev, and John W. Valley

PNAS2017 ; published ahead of print December 18, 2017, doi:10.1073/pnas.1718063115


Although the existence of the Archaea (one of three all-encompassing domains of life) in the Archean Eon (4,000 to 2,500 million years ago) has been inferred from carbon isotopes in bulk samples of ancient rocks, their cellular fossils have been unknown. We here present carbon isotope analyses of 11 microbial fossils from the ∼3,465-million-year-old Western Australian Apex chert from which we infer that two of the five species studied were primitive photosynthesizers, one was an Archaeal methane producer, and two others were methane consumers. This discovery of Archaea in the Archean is consistent with the rRNA “tree of life,” confirms the earlier disputed biogenicity of the Apex fossils, and suggests that methane-cycling methanogen−methanotroph communities were a significant component of Earth’s early biosphere.”

“Analyses by secondary ion mass spectroscopy (SIMS) of 11 specimens of five taxa of prokaryotic filamentous kerogenous cellular microfossils permineralized in a petrographic thin section of the ∼3,465 Ma Apex chert of northwestern Western Australia, prepared from the same rock sample from which this earliest known assemblage of cellular fossils was described more than two decades ago, show their δ13C compositions to vary systematically taxon to taxon from −31‰ to −39‰. These morphospecies-correlated carbon isotope compositions confirm the biogenicity of the Apex fossils and validate their morphology-based taxonomic assignments. Perhaps most significantly, the δ13C values of each of the five taxa are lower than those of bulk samples of Apex kerogen (−27‰), those of SIMS-measured fossil-associated dispersed particulate kerogen (−27.6‰), and those typical of modern prokaryotic phototrophs (−25 ± 10‰). The SIMS data for the two highest δ13C Apex taxa are consistent with those of extant phototrophic bacteria; those for a somewhat lower δ13C taxon, with nonbacterial methane-producing Archaea; and those for the two lowest δ13C taxa, with methane-metabolizing γ-proteobacteria. Although the existence of both methanogens and methanotrophs has been inferred from bulk analyses of the carbon isotopic compositions of pre-2,500 Ma kerogens, these in situ SIMS analyses of individual microfossils present data interpretable as evidencing the cellular preservation of such microorganisms and are consistent with the near-basal position of the Archaea in rRNA phylogenies.”

Geochronology of Hadean zircon grains from the Jack Hills, Western Australia constrained by quantitative scanning ion imaging

J.J. Bellucci, A.A. Nemchin, M.J. Whitehouse, R.B. Kielman, J.F. Snape, R.T. Pidgeon

Chemical Geology
In Press, Accepted Manuscript, Available online 2 December 2017

“Five Hadean (> 4 Ga) aged zircon grains from the Jack Hills metasedimentary belt have been investigated by a secondary ion mass spectrometry scanning ion image technique. This technique has the ability to obtain accurate and precise full U-Pb systematics on a scale < 5 μm, as well as document the spatial distribution of U, Th and Pb. All five of the grains investigated here have complex cathodoluminescence patterns that correlate to different U, Th, and Pb concentration domains. The age determinations for these different chemical zones indicate multiple reworking events that are preserved in each grain and have affected the primary crystalized zircon on the scale of < 10 μm, smaller than conventional ion microprobe spot analyses. In comparison to the spot analyses performed on these grains, these new scanning ion images and age determinations indicate that almost half of the spot analyses have intersected several age and chemical domains in both fractured and unfractured parts of the individual crystals. Some of these unfractured, mixed domain spot analyses have concordant ages that are inaccurate. Thus, if the frequency of spot analyses intersecting mixed domains here is even close to representative of all other studies of the Jack Hills zircon population, it makes the interpretation of any trace element, Hf, or O isotopic data present in the literature tenuous. Lastly, all of the grains analysed here preserve at least two distinguishable 207Pb/206Pb ages. These ages are preserved in core-rim and/or complex internal textural relationships in unfractured domains. These secondary events took place at ca. 4.3, 4.2, 4.1, 4.0, 3.7, and 2.9 Ga, which are coincident with previously determined statistically robust age peaks present in this zircon population.”

Spatially-resolved isotopic study of carbon trapped in ∼3.43 Ga Strelley Pool Formation stromatolites

David T. Flannery, Abigail C. Allwood, Roger E. Summons, Kenneth H. Williford, William Abbey, Emily D. Matys, Nicola Ferralis

Geochimica et Cosmochimica Acta

“The large isotopic fractionation of carbon associated with enzymatic carbon assimilation allows evidence for life’s antiquity, and potentially the early operation of several extant metabolic pathways, to be derived from the stable carbon isotope record of sedimentary rocks. Earth’s organic carbon isotope record extends to the Late Eoarchean-Early Paleoarchean: the age of the oldest known sedimentary rocks. However, complementary inorganic carbon reservoirs are poorly represented in the oldest units, and commonly reported bulk organic carbon isotope measurements do not capture the micro-scale isotopic heterogeneities that are increasingly reported from younger rocks. Here, we investigated the isotopic composition of the oldest paired occurrences of sedimentary carbonate and organic matter, which are preserved as dolomite and kerogen within textural biosignatures of the ∼3.43 Ga Strelley Pool Formation. We targeted least-altered carbonate phases in situ using microsampling techniques guided by non-destructive elemental mapping. Organic carbon isotope values were measured by spatially-resolved bulk analyses, and in situ using secondary ion mass spectrometry to target microscale domains of organic material trapped within inorganic carbon matrixes. Total observed fractionation of 13C ranges from -29 to -45 ‰. Our data are consistent with studies of younger Archean rocks that host biogenic stromatolites and organic-inorganic carbon pairs showing greater fractionation than expected for Rubisco fixation alone. We conclude that organic matter was fixed and/or remobilized by at least one metabolism in addition to the CBB cycle, possibly by the Wood-Ljungdahl pathway or methanogenesis-methanotrophy, in a shallow-water marine environment during the Paleoarchean.”

SIMS Microanalysis of the Strelley Pool Formation Cherts and the Implications for the Secular-Temporal Oxygen-isotope Trend of Cherts

J.N. Cammack, M.J. Spicuzza, A.J. Cavosie, M.J. Van Kranendonk, A.H. Hickman, R. Kozdon, I.J. Orland, K. Kitajima, J.W. Valley

Precambrian Research
In Press, Accepted Manuscript, Available online 14 November 2017


• Petrographic analysis correlates quartz textures to µm-scale SIMS δ18O(Qz) data.
• New petrogenetic information for the Strelley Pool Formation cherts.
• Controversial secular-temporal chert δ18O(Qz) trend related to sampling bias?”

“The significance of oxygen isotope ratios in Archean chert has long been debated. Cherts from the c. 3.4 Ga Strelley Pool Formation (SPF) (Pilbara Craton, Western Australia) host some of the oldest stromatolite and microfossil evidence for life, but the genesis and timing of silica cements has been unclear. Field relations, petrography and a combination of laser fluorination and in-situ SIMS measurements of δ18O in quartz show that bedded cherts of the SPF were originally precipitated as carbonates and were later widely replaced by quartz. Three localities were studied and analyzed for δ18O(Qz) in chert: 1) Camel Creek: foliated, metamorphosed, bedded cherts and meter-scale black chert veins; 2) Unconformity Ridge and ABDP8 drill core: stromatolitic and bedded chert overlying basal detrital quartz sandstone; and 3) the Trendall locality: “bedded” stromatolitic chert replacing original dolomite, low temperature hydrothermal quartz, and mm- to decimeter-scale chert-quartz veins. Laser fluorination (mm-scale) values of δ18O(Qz) range from: 14.2 to 18.2‰ VSMOW at Camel Creek; 9.3 to 18.9‰ at Unconformity Ridge; and 13.7 to 25.7‰ at Trendall. Values of δ18O(Qz) in cm to decimeter-scale hydrothermal chert veins cutting bedded carbonates at Trendall range from ca. 15 to 16‰, whereas “bedded cherts” are 17 to 26‰. These laser data include the highest δ18O values reported for cherts in Paleoarchean sediments and are up to 4‰ higher than the upper limit of ∼22‰ reported in other studies, in apparent contrast to the long-standing secular-temporal trend which shows such high δ18O only in younger chert. However, analysis by laser fluorination at the 1-mm scale cannot resolve microtextures seen petrographically. In contrast, in-situ SIMS analyses can resolve petrographic microtextures and show δ18O(Qz) at 10-μm scale have an even greater range of 7 to 31‰ in “bedded” cherts at the Trendall locality, up to 9‰ above the secular-temporal trend.
Textures observed optically at the Trendall locality were classified as: microquartz, mesoquartz, chalcedony, megaquartz veins, and cavity megaquartz. SEM-CL imaging shows two generations of meso- and megaquartz; bright CL with well-developed growth zoning, and dark CL with massive or mottled texture. Microquartz is the earliest textural generation of quartz and has a maximum δ18O(Qz) of ∼ 22‰ by SIMS. Dark-CL mesoquartz has similar δ18O to microquartz and is interpreted to also be early. Bright CL mesoquartz, which formed post-Archean, has even higher δ18O, up to 29‰. Vein megaquartz crosscuts most quartz generations and has a restricted range of δ18O, mostly from 16 to 19‰. Chalcedony pseudomorphs rhombic cavities and fractures, lines the edges of veins, and has similar δ18O to veins (16 to 19‰). Late cavity megaquartz is bright and zoned by CL, grows into late open cavities, and has the highest δ18O(Qz) values reported from the Pilbara, up to 31.3‰. Thus, the highest-δ18O quartz cements at the Trendall locality are the youngest and may be related to weathering. Early silicification and the formation of microquartz, chalcedony and low δ18O mesoquartz occurred during low temperature hydrothermal activity in the Archean.

None of the SPF quartz examined is interpreted to have formed as a direct precipitate from Paleoarchean seawater. Thus, values of δ18O(Qz) do not record either water chemistry or temperature of Archean oceans. In-situ SIMS analysis shows that high-δ18O(Qz) values above 22‰ are only found in late-forming cavity megaquartz and high δ18O mesoquartz at the Trendall locality.

The SPF results from our sample suite demonstrate the ability to resolve complex history using detailed petrography and SIMS analysis. Similar studies may show equal complexity of δ18O(Qz) data for other localities that are interpreted to show secular-temporal trends for chert. The apparent increase of δ18O(Qz) through time may reflect differences in diagenesis, and/or an inherent and previously unrecognized sampling bias that compares fundamentally different populations of quartz, such as Archean hydrothermal chert from volcanic greenstone belts, with unrelated Phanerozoic biogenic quartz.”

Earth’s volatile contents established by melting and vaporization

C. Ashley Norris & Bernard J. Wood

Nature 549, 507–510
(28 September 2017)

“The silicate Earth is strongly depleted in moderately volatile elements (such as lead, zinc, indium and alkali elements) relative to CI chondrites, the meteorites that compositionally most closely resemble the Sun1. This depletion may be explained qualitatively by accretion of 10 to 20 per cent of a volatile-rich body to a reduced volatile-free proto-Earth2, 3, followed by partial extraction of some elements to the core1. However, there are several unanswered questions regarding the sources of Earth’s volatiles4, 5, notably the overabundance of indium in the silicate Earth. Here we examine the melting processes that occurred during accretion on Earth and precursor bodies and report vaporization experiments under conditions of fixed temperature and oxygen fugacity. We find that the pattern of volatile element depletion in the silicate Earth is consistent with partial melting and vaporization rather than with simple accretion of a volatile-rich chondrite-like body. We argue that melting and vaporization on precursor bodies and possibly during the giant Moon-forming impact6, 7, 8 were responsible for establishing the observed abundances of moderately volatile elements in Earth.”

Magnesium isotope evidence that accretional vapour loss shapes planetary compositions

Remco C. Hin, Christopher D. Coath, Philip J. Carter, Francis Nimmo, Yi-Jen Lai, Philip A. E. Pogge von Strandmann, Matthias Willbold, Zoë M. Leinhardt, Michael J. Walter, Tim Elliott

Nature 549, 511–515
(28 September 2017)

“It has long been recognized that Earth and other differentiated planetary bodies are chemically fractionated compared to primitive, chondritic meteorites and, by inference, the primordial disk from which they formed. However, it is not known whether the notable volatile depletions of planetary bodies are a consequence of accretion1 or inherited from prior nebular fractionation2. The isotopic compositions of the main constituents of planetary bodies can contribute to this debate3, 4, 5, 6. Here we develop an analytical approach that corrects a major cause of measurement inaccuracy inherent in conventional methods, and show that all differentiated bodies have isotopically heavier magnesium compositions than chondritic meteorites. We argue that possible magnesium isotope fractionation during condensation of the solar nebula, core formation and silicate differentiation cannot explain these observations. However, isotopic fractionation between liquid and vapour, followed by vapour escape during accretionary growth of planetesimals, generates appropriate residual compositions. Our modelling implies that the isotopic compositions of magnesium, silicon and iron, and the relative abundances of the major elements of Earth and other planetary bodies, are a natural consequence of substantial (about 40 per cent by mass) vapour loss from growing planetesimals by this mechanism.”

Early trace of life from 3.95 Ga sedimentary rocks in Labrador, Canada

Takayuki Tashiro, Akizumi Ishida,Masako Hori,Motoko Igisu,Mizuho Koike,Pauline Méjean,Naoto Takahata,Yuji Sano & Tsuyoshi Komiya

Nature 549, 516–518 (28 September 2017)

“The vestiges of life in Eoarchean rocks have the potential to elucidate the origin of life. However, gathering evidence from many terrains is not always possible1, 2, 3, and biogenic graphite has thus far been found only in the 3.7–3.8 Ga (gigayears ago) Isua supracrustal belt4, 5, 6, 7. Here we present the total organic carbon contents and carbon isotope values of graphite (δ13Corg) and carbonate (δ13Ccarb) in the oldest metasedimentary rocks from northern Labrador8, 9. Some pelitic rocks have low δ13Corg values of −28.2, comparable to the lowest value in younger rocks. The consistency between crystallization temperatures of the graphite and metamorphic temperature of the host rocks establishes that the graphite does not originate from later contamination. A clear correlation between the δ13Corg values and metamorphic grade indicates that variations in the δ13Corg values are due to metamorphism, and that the pre-metamorphic value was lower than the minimum value. We concluded that the large fractionation between the δ13Ccarb and δ13Corg values, up to 25‰, indicates the oldest evidence of organisms greater than 3.95 Ga. The discovery of the biogenic graphite enables geochemical study of the biogenic materials themselves, and will provide insight into early life not only on Earth but also on other planets.”

Impact-driven subduction on the Hadean Earth

C. O’Neill, S. Marchi, S. Zhang & W. Bottke

Nature Geoscience (2017). DOI: 10.1038/ngeo3029

“Impact cratering was a dominant geologic process in the early Solar System that probably played an active role in the crustal evolution of the young terrestrial planets. The Earth’s interior during the Hadean, 4.56 to 4 billion years ago, may have been too hot to sustain plate tectonics. However, whether large impacts could have triggered tectonism on the early Earth remains unclear. Here we conduct global-scale tectonic simulations of the evolution of the Earth through the Hadean eon under variable impact fluxes. Our simulations show that the thermal anomalies produced by large impacts induce mantle upwellings that are capable of driving transient subduction events. Furthermore, we find that moderate-sized impacts can act as subduction triggers by causing localized lithospheric thinning and mantle upwelling, and modulate tectonic activity. In contrast to contemporary subduction, the simulated localized subduction events are relatively short-lived (less than 10 Myr) with relatively thin, weak plates. We suggest that resurgence in subduction activity induced by an increased impact flux between 4.1 and 4.0 billion years ago may explain the coincident increase in palaeointensity of the magnetic field. We further suggest that transient impact-driven subduction reconciles evidence from Hadean zircons for tectonic activity with other lines of evidence consistent with an Earth that was largely tectonically stagnant from the Hadean into the Archaean.”

Cubic zirconia in >2370 °C impact melt records Earth’s hottest crust

Nicholas E. Timms, Timmons M. Erickson, Michael R. Zanetti, Mark A. Pearce, Cyril Cayron, Aaron J. Cavosie, Steven M. Reddy, Axel Wittmann, Paul K. Carpenter

Earth and Planetary Science Letters
Volume 477, 1 November 2017, Pages 52–58

• Zircon has partially dissociated in impact melt rock from a Canadian impact crater.
• Former presence of cubic ZrO2 is crystallographically encoded in reaction rims.
• Cubic zirconia required >2370 °C melt, which is hottest recorded on Earth’s surface.
• Such superheated melt susceptible to devolatilization resulting in dry rigid crust.
• Potential global effects for crustal evolution during bombardment of early Earth.”

“Bolide impacts influence primordial evolution of planetary bodies because they can cause instantaneous melting and vaporization of both crust and impactors. Temperatures reached by impact-generated silicate melts are unknown because meteorite impacts are ephemeral, and established mineral and rock thermometers have limited temperature ranges. Consequently, impact melt temperatures in global bombardment models of the early Earth and Moon are poorly constrained, and may not accurately predict the survival, stabilization, geochemical evolution and cooling of early crustal materials. Here we show geological evidence for the transformation of zircon to cubic zirconia plus silica in impact melt from the 28 km diameter Mistastin Lake crater, Canada, which requires super-heating in excess of 2370 °C. This new temperature determination is the highest recorded from any crustal rock. Our phase heritage approach extends the thermometry range for impact melts by several hundred degrees, more closely bridging the gap between nature and theory. Profusion of >2370 °C superheated impact melt during high intensity bombardment of Hadean Earth likely facilitated consumption of early-formed crustal rocks and minerals, widespread volatilization of various species, including hydrates, and formation of dry, rigid, refractory crust.”

Elements of Eoarchean life trapped in mineral inclusions

T. Hassenkam, M. P. Andersson, K. N. Dalby, D. M. A. Mackenzie & M. T. Rosing

Nature (2017)
Published online 24 July 2017

“Metasedimentary rocks from Isua, West Greenland (over 3,700 million years old) contain 13C-depleted carbonaceous compounds, with isotopic ratios that are compatible with a biogenic origin1, 2, 3. Metamorphic garnet crystals in these rocks contain trails of carbonaceous inclusions that are contiguous with carbon-rich sedimentary beds in the host rock, where carbon is fully graphitized. Previous studies4, 5 have not been able to document other elements of life (mainly hydrogen, oxygen, nitrogen and phosphorus) structurally bound to this carbonaceous material. Here we study carbonaceous inclusions armoured within garnet porphyroblasts, by in situ infrared absorption on approximately 10−21 m3 domains within these inclusions. We show that the absorption spectra are consistent with carbon bonded to nitrogen and oxygen, and probably also to phosphate. The levels of C–H or O–H bonds were found to be low. These results are consistent with biogenic organic material isolated for billions of years and thermally matured at temperatures of around 500 °C. They therefore provide spatial characterization for potentially the oldest biogenic carbon relics in Earth’s geological record. The preservation of Eoarchean organic residues within sedimentary material corroborates earlier claims2, 6 for the biogenic origins of carbon in Isua metasediments.”

CO2 fluid inclusions in Jack Hills zircons

Martina Menneken, Thorsten GeislerAlexander, A. Nemchin, Martin J. Whitehouse, Simon A. Wilde, Biliana Gasharova, Robert T. Pidgeon

Contributions to Mineralogy and Petrology
August 2017, 172:66

“The discovery of Hadean to Paleoarchean zircons in a metaconglomerate from Jack Hills, Western Australia, has catalyzed intensive study of these zircons and their mineral inclusions, as they represent unique geochemical archives that can be used to unravel the geological evolution of early Earth. Here, we report the occurrence and physical properties of previously undetected CO2 inclusions that were identified in 3.36–3.47 Ga and 3.80–4.13 Ga zircon grains by confocal micro-Raman spectroscopy. Minimum P–T conditions of zircon formation were determined from the highest density of the inclusions, determined from the density-dependence of the Fermi diad splitting in the Raman spectrum and Ti-in-zircon thermometry. For both age periods, the CO2 densities and Ti-in-zircon temperatures correspond to high-grade metamorphic conditions (≥5 to ≥7 kbar/~670 to 770 °C) that are typical of mid-crustal regional metamorphism throughout Earth’s history. In addition, fully enclosed, highly disordered graphitic carbon inclusions were identified in two zircon grains from the older population that also contained CO2 inclusions. Transmission electron microscopy on one of these inclusions revealed that carbon forms a thin amorphous film on the inclusion wall, whereas the rest of the volume was probably occupied by CO2 prior to analysis. This indicates a close relationship between CO2 and the reduced carbon inclusions and, in particular that the carbon precipitated from a CO2-rich fluid, which is inconsistent with the recently proposed biogenic origin of carbon inclusions found in Hadean zircons from Jack Hills.”

Processes of Crust Formation in the Early Earth Imaged through Hf isotopes from the East Pilbara Terrane

Nicholas J. Gardiner, Arthur H. Hickman, Christopher L. Kirkland, Yongjun Lu, Tim Johnson, Jian-Xin Zhao

Precambrian Research
In Press, Accepted Manuscript, Available online 23 May 2017

• We present new Hf and Nd data from the Mount Edgar Dome, East Pilbara Terrane.
• These chart the Palaeo-Mesoarchaean magmatic evolution of a single granite complex.
• Reworking of existing TTG crust dominated late Palaeoarchaean magmatism.
• This trend supports a vertical tectonic geodynamic regime >3.2 Ga for the Pilbara.
• The data further support a cryptic >3.5Ga protocrust of unknown extent.”

“The Pilbara Craton, Western Australia, is one of the best preserved Palaeo- to Mesoarchaean terrains on Earth. The East Pilbara Terrane is the archetypical granite-greenstone belt, the dome-like complexes of which were formed through three major magmatic events. These granite domes are comprised of metamorphosed granitic igneous rocks that exhibit a magmatic evolution from early tonalite-trondhjemite-granodiorite (TTG) rocks towards K-rich granites over the period 3.53–2.83 Ga. Accordingly, East Pilbara has been a focus for workers seeking to constrain early Archaean geodynamic processes. One way to inform on this debate is to interrogate successive igneous supersuites using tools sensitive to magmatic source. We present new zircon Hf and whole-rock Nd isotope data from four major supersuites of the Mount Edgar Dome, of the East Pilbara Terrane. Early ca. 3.45 Ga TTGs exhibit isotopic signatures that imply their partial derivation from existing crust, with addition of some juvenile material. Subsequent Palaeoarchaean magmatic events show a secular trend towards more evolved isotopic signatures, interpreted as a dominance of increasing reworking of existing crust, with only minor addition of new juvenile crust. The implication of this is that these later Palaeoarchaean supersuites were largely derived from the melting of older granitic crust, with mass balance modelling suggesting an input of ca. 20% juvenile (depleted mantle) material. The limited addition of juvenile material and increased reworking of existing crust with time, does not support a model of modern-style subduction, but is consistent with vertical tectonic processes in a volcanic plateau-type setting for the East Pilbara Terrane > 3.2 Ga. All Palaeoarchaean Mount Edgar samples resolve to two-stage Hf model ages of ca. 3.7 Ga and Nd model ages of ca. 3.6 Ga. These Eoarchaean model ages support the existence of a cryptic pre-3.5 Ga protocrust, albeit of unknown extent. Analysis of late Mesoarchaean granites yields highly evolved Hf isotope signatures, consistent with a ca. < 3.2 Ga switch within the Mount Edgar Granitic Complex from dominantly sodic TTG type magmatism towards more K-rich granites as the craton stabilized.”

Earliest signs of life on land preserved in ca. 3.5 Ga hot spring deposits

Tara Djokic, Martin J. Van Kranendonk, Kathleen A. Campbell, Malcolm R. Walter & Colin R. Ward

Nature Communications 8, Article number: 15263 (2017)

“The ca. 3.48 Ga Dresser Formation, Pilbara Craton, Western Australia, is well known for hosting some of Earth’s earliest convincing evidence of life (stromatolites, fractionated sulfur/carbon isotopes, microfossils) within a dynamic, low-eruptive volcanic caldera affected by voluminous hydrothermal fluid circulation. However, missing from the caldera model were surface manifestations of the volcanic-hydrothermal system (hot springs, geysers) and their unequivocal link with life. Here we present new discoveries of hot spring deposits including geyserite, sinter terracettes and mineralized remnants of hot spring pools/vents, all of which preserve a suite of microbial biosignatures indicative of the earliest life on land. These include stromatolites, newly observed microbial palisade fabric and gas bubbles preserved in inferred mineralized, exopolymeric substance. These findings extend the known geological record of inhabited terrestrial hot springs on Earth by ∼3 billion years and offer an analogue in the search for potential fossil life in ancient Martian hot springs.”

Interview with Tara Djokic
on Quirks and Quarks (CBC Radio Canada, 12 May 2017)
starting at 8:28 min. running time

Formation of Hadean granites by melting of igneous crust

A. D. Burnham & A. J. Berry

Nature Geoscience(2017)
Published online 08 May 2017

“The oldest known samples of Earth, with ages of up to 4.4 Gyr, are detrital zircon grains in meta-sedimentary rocks of the Jack Hills in Australia. These zircons offer insights into the magmas from which they crystallized, and, by implication, igneous activity and tectonics in the first 500 million years of Earth’s history, the Hadean eon. However, the compositions of these magmas and the relative contributions of igneous and sedimentary components to their sources have not yet been resolved. Here we compare the trace element concentrations of the Jack Hills zircons to those of zircons from the locality where igneous (I-) and sedimentary (S-) type granites were first distinguished. We show that the Hadean zircons crystallized predominantly from I-type magmas formed by melting of a reduced, garnet-bearing igneous crust. Further, we propose that both the phosphorus content of zircon and the ratio of phosphorus to rare earth elements can be used to distinguish between detrital zircon grains from I- and S-type sources. These elemental discriminants provide a new geochemical tool to assess the relative contributions of primeval magmatism and melting of recycled sediments to the continents over geological time.”

Large and robust lenticular microorganisms on the young Earth

Dorothy Z. Oehler, Maud M. Walsh, Kenichiro Sugitani, Ming-Chang Liu, Christopher H. House

Precambrian Research
In Press, Accepted Manuscript, Available online 26 April 2017

• Lenticular forms (Kromberg Fm) of S. Africa are bona fide ∼ 3.4 Ga microfossils.
• They are related to ∼3.4 to 3.0 Ga lenticular forms in the Pilbara of Australia.
• All represent organisms that were robust, and likely autotrophic and planktonic.
• Their success may be due, in part, to their robustness and planktonic habit.
• Results support the concept that autotrophy developed early on the young Earth.

“In recent years, remarkable organic microfossils have been reported from Archean deposits in the Pilbara craton of Australia. The structures are set apart from other ancient microfossils by their complex lenticular morphology combined with their large size and robust, unusually thick walls. Potentially similar forms were reported in 1992 from the ∼ 3.4 Ga Kromberg Formation (KF) of the Kaapvaal craton, South Africa, but their origin has remained uncertain. Here we report the first determination of in situ carbon isotopic composition (δ13C) of the lenticular structures in the KF (obtained with Secondary Ion Mass Spectrometry [SIMS]) as well as the first comparison of these structures to those from the Pilbara, using morphological, isotopic, and sedimentological criteria.
Our results support interpretations that the KF forms are bona fide, organic Archean microfossils and represent some of the oldest morphologically preserved organisms on Earth. The combination of morphology, occurrence, and δ 13C values argues that the lenticular forms represent microbes that had planktonic stages to their life cycles. The similarity in morphology, δ 13C, and facies associations among specimens from Australia and South Africa suggests that the lenticular microfossils on the two continents represent related organisms. The biological success these organisms is demonstrated by their abundance, widespread distribution, and the fact that, as a group, they appear to have been present at least 400 million years. This success may be due in part to their robust structure and planktonic habit, features that may have contributed to survival on a young planet. Isotopic results further suggest that the lenticular organisms were autotrophs, an interpretation supporting the view that autotrophic metabolisms developed early on the young Earth.”

Tungsten-182 heterogeneity in modern ocean island basalts

Andrea Mundl, Mathieu Touboul, Matthew G. Jackson, James M. D. Day, Mark D. Kurz, Vedran Lekic, Rosalind T. Helz, Richard J. Walker

Science 07 Apr 2017:
Vol. 356, Issue 6333, pp. 66-69
DOI: 10.1126/science.aal4179

“New tungsten isotope data for modern ocean island basalts (OIB) from Hawaii, Samoa, and Iceland reveal variable 182W/184W, ranging from that of the ambient upper mantle to ratios as much as 18 parts per million lower. The tungsten isotopic data negatively correlate with 3He/4He. These data indicate that each OIB system accesses domains within Earth that formed within the first 60 million years of solar system history. Combined isotopic and chemical characteristics projected for these ancient domains indicate that they contain metal and are repositories of noble gases. We suggest that the most likely source candidates are mega–ultralow-velocity zones, which lie beneath Hawaii, Samoa, and Iceland but not beneath hot spots whose OIB yield normal 182W and homogeneously low 3He/4He.”

Evidence for early life in Earth’s oldest hydrothermal vent precipitates

Matthew S. Dodd,Dominic Papineau,Tor Grenne,John F. Slack,Martin Rittner,Franco Pirajno,Jonathan O’Neil & Crispin T. S. Little

Nature 543,60–64
02 March 2017

“Although it is not known when or where life on Earth began, some of the earliest habitable environments may have been submarine-hydrothermal vents. Here we describe putative fossilized microorganisms that are at least 3,770 million and possibly 4,280 million years old in ferruginous sedimentary rocks, interpreted as seafloor-hydrothermal vent-related precipitates, from the Nuvvuagittuq belt in Quebec, Canada. These structures occur as micrometre-scale haematite tubes and filaments with morphologies and mineral assemblages similar to those of filamentous microorganisms from modern hydrothermal vent precipitates and analogous microfossils in younger rocks. The Nuvvuagittuq rocks contain isotopically light carbon in carbonate and carbonaceous material, which occurs as graphitic inclusions in diagenetic carbonate rosettes, apatite blades intergrown among carbonate rosettes and magnetite–haematite granules, and is associated with carbonate in direct contact with the putative microfossils. Collectively, these observations are consistent with an oxidized biomass and provide evidence for biological activity in submarine-hydrothermal environments more than 3,770 million years ago.”

Haematite tubes from the Nuvvuagittuq Supracrustal Belt (NSB) hydrothermal vent deposits. Photo: Matthew Dodd

Layer-deflecting bright red concretion of haematitic chert (an iron-rich and silica-rich rock), which contains tubular and filamentous microfossils. This co-called jasper is in contact with a dark green volcanic rock in the top right and represent hydrothermal vent precipitates on the seafloor. Nuvvuagittuq Supracrustal Belt, Quebec, Canada. Photo: Dominic Papineau

Earth’s first stable continents did not form by subduction

Tim E. Johnson, Michael Brown, Nicholas J. Gardiner, Christopher L. Kirkland & R. Hugh Smithies

Nature (2017)
Published online 27 February

“The geodynamic environment in which Earth’s first continents formed and were stabilized remains controversial1. Most exposed continental crust that can be dated back to the Archaean eon (4 billion to 2.5 billion years ago) comprises tonalite–trondhjemite–granodiorite rocks (TTGs) that were formed through partial melting of hydrated low-magnesium basaltic rocks2; notably, these TTGs have ‘arc-like’ signatures of trace elements and thus resemble the continental crust produced in modern subduction settings3. In the East Pilbara Terrane, Western Australia, low-magnesium basalts of the Coucal Formation at the base of the Pilbara Supergroup have trace-element compositions that are consistent with these being source rocks for TTGs. These basalts may be the remnants of a thick (more than 35 kilometres thick), ancient (more than 3.5 billion years old) basaltic crust4, 5 that is predicted to have existed if Archaean mantle temperatures were much hotter than today’s6, 7, 8. Here, using phase equilibria modelling of the Coucal basalts, we confirm their suitability as TTG ‘parents’, and suggest that TTGs were produced by around 20 per cent to 30 per cent melting of the Coucal basalts along high geothermal gradients (of more than 700 degrees Celsius per gigapascal). We also analyse the trace-element composition of the Coucal basalts, and propose that these rocks were themselves derived from an earlier generation of high-magnesium basaltic rocks, suggesting that the arc-like signature in Archaean TTGs was inherited from an ancestral source lineage. This protracted, multistage process for the production and stabilization of the first continents—coupled with the high geothermal gradients—is incompatible with modern-style plate tectonics, and favours instead the formation of TTGs near the base of thick, plateau-like basaltic crust9. Thus subduction was not required to produce TTGs in the early Archaean eon.”

SHRIMP U–Pb zircon geochronology establishes that banded iron formations are not chronostratigraphic markers across Archean greenstone belts of the Pilbara Craton

Stephen Sheppard, Bryan Krapež, Jian-Wei Zi, Birger Rasmussen, Ian Fletcher

Precambrian Research, Available online 11 February 2017

• The c. 3020 Ma Cleaverville Formation is the oldest unit correlated across the Pilbara Craton.
• However, the formation in the eastern part of the craton was deposited at c. 3105 Ma.
• The Cleaverville Formation is not a chronostratigraphic marker.
• The Mesoarchean amalgamation history of the craton may need to be revised.”

“Banded iron formation (BIF) of the c. 3020 Ma Cleaverville Formation is correlated across the Pilbara Craton and is considered to be the first stratigraphic unit common to the West Pilbara Superterrane and the East Pilbara Terrane. New field mapping and geochronology from the Shay Gap and Goldsworthy belts on the northeastern margin of the East Pilbara Terrane test that correlation. Along that margin, the Farrel Quartzite at the base of the Gorge Creek Group nonconformably overlies granite and unconformably overlies metavolcanic rocks. The Farrel Quartzite is conformably overlain by what is interpreted to be Cleaverville Formation, which is divided into a lower BIF member, a middle mudstone member and an upper BIF member. We have dated a tuff from the middle mudstone member by the Sensitive High-Resolution Ion Microprobe (SHRIMP) U-Pb zircon method at 3104 ± 16 Ma. The underlying Farrel Quartzite has a maximum depositional age of 3295 ± 5 Ma and no zircon population at c. 3105 Ma. Therefore, the SHRIMP date for the tuff does not represent reworking of underlying sedimentary rocks and is interpreted to be a depositional age. This new date refutes correlation of the Cleaverville Formation across the craton, and it implies that BIFs should not be used as chronostratigraphic markers across Archean greenstone terrains. We suggest reinstating the name Nimingarra Iron Formation for BIFs that crop out along the northeastern margin of the Pilbara Craton, to distinguish them from the Cleaverville Formation. If the Cleaverville Formation, as defined in the West Pilbara Superterrane, is not the oldest stratigraphic unit common to the craton, then our understanding of the Mesoarchean evolution of the Pilbara Craton needs to be revised.”

The Hunt for Shocked Zircon in the Jack Hills: 21,000 and Counting…

M. A. Cox, A. J. Cavosie, S. M. Reddy, P. A. Bland, J. W. Valley
48th Lunar and Planetary Science Conference (2017), Abstract #1402

Initiation of plate tectonics in the Hadean: Eclogitization triggered by the ABEL Bombardment
S. Maruyama, M. Santosh, S. Azuma

Geoscience Frontiers
Available online 9 December 2016


• Presence of water is the most critical factor for plate tectonics
• Bombardment of carbonaceous chondrites delivered water on completely dry Earth
• Eclogitization provided slab-pull force to initiate plate tectonics
• Stagnant lid tectonics shifted to plate tectonics by the ABEL Bombardment”

“When plate tectonics began on the Earth has been long debated and here we argue this topic based on the records of Earth-Moon geology and asteroid belt to conclude that the onset of plate tectonics was during the middle Hadean (between 4.37–4.20 Ga). The trigger of the initiation of plate tectonics is the ABEL Bombardment, which delivered oceanic and atmospheric components on a completely dry reductive Earth, originally comprised of enstatite chondrite-like materials. Through the accretion of volatiles, shock metamorphism processed with vaporization of both CI chondrite and supracrustal rocks at the bombarded location, and significant recrystallization went through under wet conditions, caused considerable eclogitization in the primordial continents composed of felsic upper crust of 21 km thick anorthosite, and 50 km or even thicker KREEP lower crust. Eclogitization must have yielded a powerful slab-pull force to initiate plate tectonics in the middle Hadean. Another important factor is the size of the bombardment. By creating Pacific Ocean class crater by 1000 km across impactor, rigid plate operating stagnant lid tectonics since the early Hadean was severely destroyed, and oceanic lithosphere was generated to have bi-modal lithosphere on the Earth to enable the operation of plate tectonics. Considering the importance of the ABEL Bombardment event which initiated plate tectonics including the appearance of ocean and atmosphere, we propose that the Hadean Eon can be subdivided into three periods: (1) early Hadean (4.57–4.37 Ga), (2) middle Hadean (4.37–4.20 Ga), and (3) late Hadean (4.20–4.00 Ga).”

Highly siderophile element and 182W evidence for a partial late veneer in the source of 3.8 Ga rocks from Isua, Greenland

Christopher W. Dale, Thomas S. Kruijer, Kevin W. Burton

Earth and Planetary Science Letters
In Press, Corrected Proof, Available online 22 November 2016

• HSE and 182W give mutually consistent estimate of late veneer proportion in Isua source.
• The >3.7 Ga Isua source contains around 55–60% of Earth’s full late veneer.
• Late veneer component in Moon < Isua source < bulk silicate Earth.
• Isua HSE-182W supports disproportional late accretion to the Earth and Moon.”

“The higher-than-expected concentrations of highly siderophile elements (HSE) in Earth’s mantle most likely indicate that Earth received a small amount of late accreted mass after core formation had ceased, known as the ‘late veneer’. Small 182W excesses in the Moon and in some Archaean rocks – such as the source of 3.8 billion-year-old Isua magmatics – also appear consistent with the late veneer hypothesis, with a lower proportion received. However, 182W anomalies can also relate to other processes, including early mantle differentiation. To better assess the origin of these W isotope anomalies – and specifically whether they relate to the late veneer – we have determined the HSE abundances and 182W compositions of a suite of mafic to ultramafic rocks from Isua, from which we estimate HSE abundances in the source mantle and ultimately constrain the 182W composition of the pre-late veneer mantle.

Our data suggest that the Isua source mantle had HSE abundances at around 50–65% of the present-day mantle, consistent with partial, but not complete, isolation from the late veneer. These data also indicate that at least part of the late veneer had been added and mixed into the mantle at the time the Isua source formed, prior to 3.8 Ga. For the same Isua samples we obtained a 13±4 ppm13±4 ppm182W excess, compared to the modern terrestrial mantle, in excellent agreement with previous data. Using combined 182W and HSE data we show that the Moon, Isua, and the present-day bulk silicate Earth (BSE) produce a well-defined co-variation between 182W composition and the mass fraction of late-accreted mass, as inferred from HSE abundances. This co-variation is consistent with the calculated effects of various late accretion compositions on the HSE and 182W signatures of Earth’s mantle. The empirical relationship, therefore, implies that the Moon, Isua source and BSE received increasing proportions of late-accreted mass, supporting the idea of disproportional late accretion to the Earth and Moon, and consistent with the interpretation that the lunar 182W value of 27±4 ppm27±4 ppm represents the composition of Earth’s mantle before the late veneer was added. In this case, the Isua source can represent ambient mantle after the giant moon-forming impact, into which only a part of Earth’s full late veneer was mixed, rather than an isotopically distinct mantle domain produced by early differentiation, which would probably require survival through the giant Moon-forming impact.”

Iron and oxygen isotope fractionation during iron UV photo-oxidation: Implications for early Earth and Mars

Nicole X. Nie, Nicolas Dauphas, Richard C. Greenwood

Earth and Planetary Science Letters
Available online 10 November 2016

“Banded iron formations (BIFs) contain appreciable amounts of ferric iron (Fe3+). The mechanism by which ferrous iron (Fe2+) was oxidized into Fe3+ in an atmosphere that was globally anoxic is highly debated. Of the three scenarios that have been proposed to explain BIF formation, photo-oxidation by UV photons is the only one that does not involve life (the other two are oxidation by O2 produced by photosynthesis, and anoxygenic photosynthesis whereby Fe2+ is directly used as electron donor in place of water). We experimentally investigated iron and oxygen isotope fractionation imparted by iron photo-oxidation at a pH of 7.3. The iron isotope fractionation between precipitated Fe3+-bearing lepidocrocite and dissolved Fe2+ follows a Rayleigh distillation with an instantaneous 56Fe/54Fe fractionation factor of +1.2‰+1.2‰. Such enrichment in the heavy isotopes of iron is consistent with the values measured in BIFs. We also investigated the nature of the mass-fractionation law that governs iron isotope fractionation in the photo-oxidation experiments (i.e., the slope of the δ56Fe–δ57Fe relationship). The experimental run products follow a mass-dependent law corresponding to the high-T equilibrium limit. The fact that a ∼3.8 Gyr old BIF sample (IF-G) from Isua (Greenland) falls on the same fractionation line confirms that iron photo-oxidation in the surface layers of the oceans was a viable pathway to BIF formation in the Archean, when the atmosphere was largely transparent to UV photons.

Our experiments allow us to estimate the quantum yield of the photo-oxidation process (∼0.07 iron atom oxidized per photon absorbed). This yield is used to model iron oxidation on early Mars. As the photo-oxidation proceeds, the aqueous medium becomes more acidic, which slows down the reaction by changing the speciation of iron to species that are less efficient at absorbing UV-photons. Iron photo-oxidation in centimeter to meter-deep water ponds would take months to years to complete. Oxidation by O2 in acidic conditions would be slower. Iron photo-oxidation is thus likely responsible for the formation of jarosite–hematite deposits on Mars, provided that shallow standing water bodies could persist for extended periods of time.

The oxygen isotopic composition of lepidocrocite precipitated from the photo-oxidation experiment was measured and it is related to the composition of water by mass-dependent fractionation. The precipitate-fluid 18O/16O isotope fractionation of ∼+6‰∼+6‰ is consistent with previous determinations of oxygen equilibrium fraction factors between iron oxyhydroxides and water.”

Coupled zircon Lu–Hf and U–Pb isotopic analyses of the oldest terrestrial crust, the >4.03 Ga Acasta Gneiss Complex

Ann M. Bauer, Christopher M. Fisher, Jeffrey D. Vervoort, Samuel A. Bowring

Earth and Planetary Science Letters
In Press, Corrected Proof, Available online 8 November 2016

“The Acasta Gneiss Complex of the Northwest Territories, Canada, contains some of the earliest terrestrial continental crust and thus provides a critical sample set for characterization of crust-forming processes on the early Earth. Here we report the results of a combined Lu–Hf and U–Pb isotopic study of zircons from predominantly felsic orthogneisses from the Acasta Gneiss Complex that crystallized between ∼4.0 and 2.9 Ga, many of which contain complex zoning and therefore require an analytical treatment suited to distinguish amongst compositionally distinct age and Hf isotopic domains. To ensure the reliability of the analyses and of subsequent geologic interpretations, we employed the laser ablation split-stream (LASS) technique to concurrently measure the Lu–Hf and U–Pb isotopic systems in zircon. Our results confirm prior findings of precursor Hadean crust (>4.0 Ga) in the source of these rocks and the continued involvement of this reservoir until ∼3.6 Ga. We present evidence for the input of relatively more juvenile material at ∼3.6 Ga, which we suggest corresponds to a fundamental change in the source of the magmas. This study extends the lower bound of the published Acasta Hf isotopic record from 3.6 Ga to 2.9 Ga and demonstrates that the ∼3.6 Ga–2.9 Ga interval is largely represented by reworking of relatively juvenile ∼3.6 Ga crust and the diminution of the >4.0 Ga crustal signal. Significantly, there is no evidence that rocks within the Acasta Gneiss Complex were derived from a strongly depleted mantle.”

Sluggish Hadean geodynamics: Evidence from coupled 146,147Sm–142,143Nd systematics in Eoarchean supracrustal rocks of the Inukjuak domain (Québec)

G. Caro, P. Morino, S.J. Mojzsis, N.L. Cates, W. Bleeker

Earth and Planetary Science Letters
Available online 22 October 2016

“The discovery of deficits in 142Nd/144Nd in mafic rocks of the Nuvvuagittuq supracrustal belt (NSB) has triggered a debate about the possible preservation of Hadean (pre-3.85 Ga) crustal remnants in the little-known but areally extensive Innuksuac complex (3.6–3.8 Ga, Inukjuak domain, Northeast Superior Province, Canada). Geochronological investigations in the NSB, however, are hampered by the poor preservation and highly disturbed isotopic record of various mafic (amphibolite) lithologies that host the 142Nd anomalies. Here we present 146Sm–142Nd and 147Sm–143Nd data for rocks of extrusive magmatic and sedimentary protoliths from the Ukaliq supracrustal belt, a newly discovered volcano-sedimentary enclave enclosed in granitoid gneisses of the Inukjuak domain. Our study also includes the first 146Sm–142Nd data for quartz-magnetite rocks (banded iron-formation; BIF) of the NSB and the Eoarchean Isua supracrustal belt (ISB) in southern West Greenland. We show that Ukaliq amphibolites carry variably negative 142Nd anomalies, ranging from 0 to −10 ppm, which are positively correlated with their Sm/Nd ratio. If considered as an isochron relationship, the 146Sm–142Nd array yields an apparent Hadean emplacement age of View the MathML source4215−76+50 Ma. The negative 142Nd anomalies, however, appear to be mainly restricted to amphibolites with boninitic affinities, likely reflecting inheritance from an enriched mantle source. In contrast, tholeiitic and ultramafic lavas have normal μ142Nd regardless of their Sm/Nd ratio. Furthermore, BIF from Ukaliq and Nuvvuagittuq lack the negative 142Nd anomalies that should have been produced by in situ decay of 146Sm had these sediments been deposited prior to ca. 4.1 Ga. Instead, they exhibit μ142Nd identical to that measured in Isua BIF. Collectively, our results suggest that the 146Sm–142Nd array characterizing mafic lithologies of Ukaliq and Nuvvuagittuq is an inherited signature with doubtful chronological significance. We interpret the volcanic protoliths of the Innuksuac complex to have been produced by metasomatically triggered melting of a variably enriched Eoarchean mantle, following addition of felsic melts and/or fluids derived from a foundering Hadean mafic crust. Application of coupled 146,147Sm–142,143Nd chronometry to Ukaliq lavas yields a model age of differentiation of View the MathML source4.36−0.06+0.05 Ga for this Hadean precursor. This is similar to late-stage crystallization ages inferred for the lunar and terrestrial magma oceans. The long-term preservation of Earth’s primordial crust points to subdued lithospheric recycling in the post-magma ocean Earth.”

The effect of weathering on U-Th-Pb and oxygen isotope systems of ancient zircons from the Jack Hills, Western Australia

R.T. Pidgeon, A.A. Nemchin, M.J. Whitehouse

Geochimica et Cosmochimica Acta
In Press, Accepted Manuscript, Available online 21 October 2016

“We report the result of a SIMS U-Th-Pb and O-OH study of 44 ancient zircons from the Jack Hills in Western Australia with ages ranging from 4.3 Ga to 3.3 Ga. We have investigated the behaviour of oxygen isotopes and water in the grains by determining δ18O and OH values at a number of locations on the polished surfaces of each grain. We have divided the zircons into five groups on the basis of their U-Th-Pb and OH-oxygen isotopic behaviour. The first group has concordant U-Th-Pb ages, minimal common Pb, δ18O values consistent with zircons derived from mantle source rocks and no detectable OH content. U-Th-Pb systems in zircons from Groups 2, 3 and 4 vary from concordant to extremely discordant where influenced by cracks. Discordia intercepts with concordia at approximately zero Ma age are interpreted as disturbance of the zircon U-Th-Pb systems by weathering solutions during the extensive, deep weathering that has affected the Archean Yilgarn Craton of Western Australia since at least the Permian. Weathering solutions entering cracks have resulted in an influx of Th and U. δ18O values of Group 2 grains fall approximately within the “mantle “ range and OH is within background levels or slightly elevated. δ18O values of Group 3 grains are characterised by an initial trend of decreasing δ18O with increasing OH content. With further increase in OH this trend reverses and δ18O becomes heavier with increasing OH. Group 4 grains have a distinct trend of increasing δ18O with increasing OH. These trends are explained in terms of the reaction of percolating water with the metamict zircon structure and appear to be independent of analytical overlap with cracks. Group five zircons are characterised by U-Pb systems that appear to consist of more than one age but show only minor U-Pb discordance. Nevertheless trends in δ18O versus OH in this group of grains resemble trends seen in the other groups. The observed trends of δ18O with OH in the Jack Hills zircons are similar to those reported in a previous study of zircons from an Archean granite from south-western Australia.”

No evidence for Hadean continental crust within Earth’s oldest evolved rock unit

J.R.Reimink, J.H.F.L.Davies, T.Chacko, R.A.Stern, L. M. Heaman, C. Sarkar, U. Schaltegger, R. A. Creaser & D.G.Pearson

Nature Geoscience(2016)
Published online 19 September 2016

“Due to the acute scarcity of very ancient rocks, the composition of Earth’s embryonic crust during the Hadean eon (>4.0 billion years ago) is a critical unknown in our search to understand how the earliest continents evolved. Whether the Hadean Earth was dominated by mafic-composition crust, similar to today’s oceanic crust1, 2, 3, 4, or included significant amounts of continental crust5, 6, 7, 8 remains an unsolved question that carries major implications for the earliest atmosphere, the origin of life, and the geochemical evolution of the crust–mantle system. Here we present new U–Pb and Hf isotope data on zircons from the only precisely dated Hadean rock unit on Earth—a 4,019.6 ± 1.8 Myr tonalitic gneiss unit in the Acasta Gneiss Complex, Canada. Combined zircon and whole-rock geochemical data from this ancient unit shows no indication of derivation from, or interaction with, older Hadean continental crust. Instead, the data provide the first direct evidence that the oldest known evolved crust on Earth was generated from an older ultramafic or mafic reservoir that probably surfaced the early Earth.”

Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures

Allen P. Nutman, Vickie C. Bennett, Clark R. L. Friend, Martin J.Van Kranendonk & Allan R. Chivas

Nature (2016)
Published online 31 August 2016


“Biological activity is a major factor in Earth’s chemical cycles, including facilitating CO2 sequestration and providing climate feedbacks. Thus a key question in Earth’s evolution is when did life arise and impact hydrosphere–atmosphere–lithosphere chemical cycles? Until now, evidence for the oldest life on Earth focused on debated stable isotopic signatures of 3,800–3,700 million year (Myr)-old metamorphosed sedimentary rocks and minerals1, 2 from the Isua supracrustal belt (ISB), southwest Greenland3. Here we report evidence for ancient life from a newly exposed outcrop of 3,700-Myr-old metacarbonate rocks in the ISB that contain 1–4-cm-high stromatolites—macroscopically layered structures produced by microbial communities. The ISB stromatolites grew in a shallow marine environment, as indicated by seawater-like rare-earth element plus yttrium trace element signatures of the metacarbonates, and by interlayered detrital sedimentary rocks with cross-lamination and storm-wave generated breccias. The ISB stromatolites predate by 220 Myr the previous most convincing and generally accepted multidisciplinary evidence for oldest life remains in the 3,480-Myr-old Dresser Formation of the Pilbara Craton, Australia4, 5. The presence of the ISB stromatolites demonstrates the establishment of shallow marine carbonate production with biotic CO2 sequestration by 3,700 million years ago (Ma), near the start of Earth’s sedimentary record. A sophistication of life by 3,700 Ma is in accord with genetic molecular clock studies placing life’s origin in the Hadean eon (>4,000 Ma)”

see also: Reassessing evidence of life in 3,700-million-year-old rocks of Greenland (nature, 17 October 2018)

Petrology and geochemistry of mafic rocks in the Acasta Gneiss Complex: Implications for the oldest mafic rocks and their origin

Keiko Koshida, Akira Ishikawa, Hikaru Iwamori, Tsuyoshi Komiya

Precambrian Research
available online 30 July 2016


• Minor mafic rocks distributed over the Acasta Gneiss Complex were examined in detail.
• Major petrochemical varieties due to anatexis followed by amphibolite metamorphism.
• The least-modified rocks preserving basaltic magma compositions were identified.
• The source mantle has chondritic incompatible element ratios, except for Nb and Ta.
• Early Eoarchean Nb-deficit mantle probably originated from the Hadean core formation.”

“The Acasta Gneiss Complex, located in the western part of the Slave Province, Canada, is widely recognized as the oldest Eoarchean terrane. In addition to felsic gneisses with the ages of 3.6-4.0 Ga, minor mafic rocks occur as rounded to elliptical enclaves and inclusions within the felsic gneisses. Despite serving as potential sources of geochemical information on the Hadean mantle, the mafic rocks have received less attention in previous studies. Thus, we conducted a comprehensive geological petrological and geochemical investigation on the Acasta mafic rocks to constrain their petrogenesis and geodynamic setting.

The mafic rocks comprise massive to weakly foliated amphibolite, garnet amphibolite and hornblendite, with variable abundances of hornblende, plagioclase, chlorite and quartz and subordinate clinopyroxene, garnet and cummingtonite. They commonly underwent high-grade metamorphic recrystallization under amphibolite to upper-amphibolite facies conditions. The observed variations in mineral assemblages, abundances and compositions reflect large differences in whole-rock compositions, likely caused by crustal anatexis during the Eoarchean thermal events responsible for the generation of the surrounding felsic gneisses. Infiltration or extraction of felsic melts formed due to partial melting of precursor rocks can account for an overall negative correlation between Al2O3 and MgO contents and variable enrichments in the incompatible elements.

Despite the widespread influence of anatexis on the geochemistry of Acasta mafic rocks, we identified the precursor compositions of the least-modified amphibolites as basaltic magmas. They are characterized by intermediate Al2O3 and MgO contents on the observed array and by near chondritic patterns for incompatible trace elements, except for slightly negative Nb and Ta anomalies. We considered two scenarios to explain the origin of Eoarchean basaltic rocks with Nb-Ta anomalies: (1) generation of Nb-Ta deficient basaltic magma in a suprasubduction setting, analogous to modern arcs-derived magmas, and (2) generation of Nb-Ta deficient basaltic magma from the melting of a Nb-Ta deficient primitive mantle, possible if the core contains significant proportions of the Earth’s Nb and Ta budget. Although the operation of plate tectonics and the presence of subduction zones at the end of Hadean may be an attractive explanation for the observed Nb-Ta depletions, the chondritic relative proportions of other immobile trace elements for Acasta mafic rocks leave open the possibility of their formation from an Nb-Ta deficient primitive mantle.”

Recovering the primary geochemistry of Jack Hills zircons through quantitative estimates of chemical alteration

Elizabeth A. Bell, Patrick Boehnke, T. Mark Harrison

Geochimica et Cosmochimica Acta
In Press, Accepted Manuscript, Available online 19 July 2016

“Despite the robust nature of zircon in most crustal and surface environments, chemical alteration, especially associated with radiation damaged regions, can affect its geochemistry. This consideration is especially important when drawing inferences from the detrital record where the original rock context is missing. Typically, alteration is qualitatively diagnosed through inspection of zircon REE patterns and the style of zoning shown by cathodoluminescence imaging, since fluid-mediated alteration often causes a flat, high LREE pattern. Due to the much lower abundance of LREE in zircon relative both to other crustal materials and to the other REE, disturbance to the LREE pattern is the most likely first sign of disruption to zircon trace element contents. Using a database of 378 (148 new) trace element and 801 (201 new) oxygen isotope measurements on zircons from Jack Hills, Western Australia, we propose a quantitative framework for assessing chemical contamination and exchange with fluids in this population. The Light Rare Earth Element Index is scaled on the relative abundance of light to middle REE, or LREE-I = (Dy/Nd) + (Dy/Sm). LREE-I values vary systematically with other known contaminants (e.g., Fe, P) more faithfully than other suggested proxies for zircon alteration (Sm/La, various absolute concentrations of LREEs) and can be used to distinguish primary compositions when textural evidence for alteration is ambiguous. We find that zircon oxygen isotopes do not vary systematically with placement on or off cracks or with degree of LREE-related chemical alteration, suggesting an essentially primary signature. By omitting zircons affected by LREE-related alteration or contamination by mineral inclusions, we present the best estimate for the primary igneous geochemistry of the Jack Hills zircons. This approach increases the available dataset by allowing for discrimination of on-crack analyses (and analyses with ambiguous or no information on spot placement or zircon internal structures) that do not show evidence for chemical alteration. It distinguishes between altered and unaltered samples in ambiguous cases (e.g., relatively high Ti), identifying small groups with potentially differing provenance from the main Jack Hills population. Finally, filtering of the population using the LREE-I helps to more certainly define primary correlations among trace element variables, potentially relatable to magmatic compositional evolution.”

The birth of a cratonic nucleus: lithogeochemical evolution of the 4.02–2.94 Ga Acasta Gneiss Complex

Jesse R Reimink, Thomas Chacko, Richard A Stern, Larry M Heaman

Precambrian Research
In Press, Accepted Manuscript, Available online 14 June 2016

“Crust forming processes on the early Earth have long been debated and relatively few rock and mineral samples exist with which to evaluate many hotly contested themes. The Acasta Gneiss Complex contains rock units with crystallization ages exceeding 4.0 Ga, making them the oldest known evolved rock units in the world. However, the AGC has experienced a long and complex history with multiple periods of igneous intrusion, deformation and metamorphism. Indeed, previous workers have demonstrated that orthogneisses within the AGC have igneous ages ranging from ∼4.03 to ∼3.4 Ga. This large range in crystallization ages gives us the opportunity to investigate the evolution of Earth’s earliest known continental crust through a period of greater than 1 billion years.
Here we present an updated geologic map of key areas within the Acasta Gneiss Complex in which we delineate units based upon age as well as composition. Whole-rock geochemistry, zircon LA-ICPMS U-Pb geochronology and SIMS O-isotope analyses from a large suite of samples indicate a significant change in mode of crust formation over 400 million years. These data document a gradual change from a shallow crustal processes generating basaltic to andesitic compositions at 4.02 Ga to deep-seated partial melting of hydrated basalt, represented by voluminous Archean TTG-like intrusions at 3.6 Ga.
We find no evidence that classic Archean TTG-like rock units are present within the ACG prior to 3.6 Ga, suggesting a significantly different tectonic process at work prior to this time. We invoke an oceanic plateau-like model to describe the evolving nature of crust formation within the AGC, which forms a buoyant, evolved nucleus. This nucleus then initiates deep-seated partial melting of mafic crust forming voluminous TTG-like units at ∼3.6 Ga. This ultimately serves to stabilize the crust and forms a nucleus for later formation of the Slave craton.”

Preservation of Earth-forming events in the tungsten isotopic composition of modern flood basalts

Hanika Rizo, Richard J. Walker, Richard W. Carlson, Mary F. Horan, Sujoy Mukhopadhyay, Vicky Manthos, Don Francis, Matthew G. Jackson

Science 13 May 2016:
Vol. 352, Issue 6287, pp. 809-812
DOI: 10.1126/science.aad8563

“Isotopes isolated after impact

Details about how Earth formed are gleaned from the daughter products of certain short-lived radioactive isotopes found in rocks. Rizo et al. describe subtle tungsten isotope variations in rocks from the very deep mantle in Baffin Island and the Ontong Java Plateau (see the Perspective by Dahl). The results suggest that portions of Earth have remained unmixed since it formed. The unmixed deep mantle rocks also imply that Earth’s core formed from several large impact events.


How much of Earth’s compositional variation dates to processes that occurred during planet formation remains an unanswered question. High-precision tungsten isotopic data from rocks from two large igneous provinces, the North Atlantic Igneous Province and the Ontong Java Plateau, reveal preservation to the Phanerozoic of tungsten isotopic heterogeneities in the mantle. These heterogeneities, caused by the decay of hafnium-182 in mantle domains with high hafnium/tungsten ratios, were created during the first ~50 million years of solar system history, indicating that portions of the mantle that formed during Earth’s primary accretionary period have survived to the present.”

Identifying remnants of early Earth

Tais W. Dahl

Science 13 May 2016:
Vol. 352, Issue 6287, pp. 768-769
DOI: 10.1126/science.aaf2482

“The chemical composition of Earth’s mantle can tell us how our planet formed and how subsequent mantle dynamics have since homogenized the mantle through convective processes. Most terrestrial rocks have a similar tungsten (W) isotope composition (1), but some rocks that have been dated at 2.8 Ga (billion years old) (2), 3.8 Ga (3), and 3.96 Ga (4) have elevated 182W/184W ratios. This is reported as µ182W, in parts per million (ppm) deviation from the bulk silicate Earth. Until now, the outliers have included only these ancient rock samples with a small µ182W excess (≤15 ppm) that can be attributed to the final ∼0.5% of Earth’s mass that accreted late in its accretion history. On page 809 of this issue, Rizo et al. (5) report W isotope data from young mantle-derived rocks with µ182W excesses of 10 to 48 ppm. This result is spectacular because the range of µ182W values in mantle-derived rocks is larger than can be accommodated by late accretion; the implication is that remnants of Earth’s earliest mantle have been preserved over the entirety of Earth’s history.”

Geochemistry and Nd isotopic characteristics of Earth’s Hadean mantle and primitive crust

J. O’Neil, H. Rizo, M. Boyet, R.W. Carlson, M.T. Rosing

Earth and Planetary Science Letters
Available online 21 March 2016


“The Isua supracrustal belt (ISB) and the Nuvvuagittuq greenstone belt (NGB) are among the oldest suites of mafic volcanic rocks preserved on Earth and are the best candidates for representing its early crust. Despite the possible 500 Ma age difference between the belts, these mantle-derived rocks show compositional similarities, with features resembling rocks formed in subduction initiation environments. With the addition of new 142Nd data for the Garbenschiefer unit of the ISB reported here, high precision 142Nd data are now available for all the mafic lithologies from both belts. Mantle-derived rocks from both the ISB and NGB belts exhibit a range of 142Nd/144Nd ratios. The datasets for the two belts, however, are significantly different, suggesting a different origin for their 142Nd anomalies. Nearly all ISB samples have excesses in 142Nd, including the newly analyzed Garbenschiefer boninitic amphibolites (mean of +12 ppm+12 ppm). Excesses in 142Nd/144Nd compared to the Nd standard for all the ISB rocks range between +8 and +20 ppm+20 ppm, with a near Gaussian distribution around +12 ppm+12 ppm. This distribution could simply reflect the analytical error (±5 ppm) around a single 142Nd/144Nd ratio indicating that the samples formed after the extinction of 146Sm from a source with a nearly uniform 142Nd/144Nd ratio. In contrast, the NGB shows a range of 142Nd/144Nd ratios from +8 to −18 ppm−18 ppm relative to the modern Nd standard and displays a flat distribution of 142Nd/144Nd ratios. The ISB samples show no significant correlation between their 142Nd/144Nd and Sm/Nd ratios, consistent with their formation in the Eoarchean via melting of a Hadean depleted mantle. In contrast, all NGB samples display a 142Nd/144Nd vs. Sm/Nd correlation, consistent with their crystallization in the Hadean. The mantle sources for both the ISB and NGB mantle-derived rocks have a similar 142Nd/144Nd ratio at the possible age of formation of the NGB (∼4.3 Ga) suggesting the derivation of ISB and NGB rocks from a common early-formed depleted mantle source formed between 4.47 and 4.42 Ga with a 147Sm/144Nd ratio ∼0.218. This mantle appears to have been an important source component involved in the formation of the primitive crust during most of the Hadean and Eoarchean eons.”

Early Earth Differentiation Investigated Through 142Nd, 182W, and Highly Siderophile Element Abundances in Samples From Isua, Greenland

H. Rizo, R.J. Walker, R.W. Carlson, M. Touboul, M.F. Horan, I.S. Puchtel, M. Boyet, M.T. Rosing

Geochimica et Cosmochimica Acta


“We report new data for W concentrations, stable W isotopic compositions, high-precision 182W/184W ratios, highly siderophile element (HSE) abundances and 187Re-187Os isotopic systematics in a suite of 3.8 Ga to 3.3 Ga mafic and ultramafic rocks from the Isua supracrustal belt, and the Paleoarchean terrane in the northwestern part of the belt. These data are compared with published data for 146Sm-142Nd systematics in the same samples. The samples from the Isua supracrustal belt show well resolved excesses of 182W/184W of up to ∼21 ppm, consistent with previous W isotopic data reported by Willbold et al. (2011). While there is abundant evidence that W was mobilized in the crust accessed by the Isua supracrustal suite, the isotopic anomalies are interpreted to primarily reflect processes that affected the mantle precursors to these rocks. The origin of the 182W excesses in these rocks remains uncertain. The Isua mantle source could represent a portion of the post-core-formation mantle that was isolated from late accretionary additions (e.g., Willbold et al., 2011). However, the combined 182W, Re-Os isotopic systematics, and HSE abundances estimated for the source of the Isua basalts are difficult to reconcile with this interpretation. The W isotope variations were more likely produced as a result of fractionation of the Hf/W ratio in the mantle during the lifetime of 182Hf, i.e., during the first 50 Ma of Solar System history. This could have occurred as a result of differentiation in an early magma ocean. The Isua suite examined is also characterized by variable 142Nd/144Nd, but the variations do not correlate with the variations in 182W/184W. Further, samples with ages between 3.8 Ga and 3.3 Ga show gradual diminution of 142Nd anomalies until these are no longer resolved from the modern mantle isotopic composition. By contrast, there is no diminishment of 182W variability with time, suggesting different mechanisms of origin and retention of isotopic variations for these two extinct-radionuclide isotope systems. The presence of 182W isotopic anomalies in rocks as young as 3.3 Ga, implies that early-formed, high Hf/W domains survived for more than 1 Ga in the convective mantle.”

Excavation and Melting of the Hadean Continental Crust by Late Heavy Bombardment

Yuhito Shibaike, Takanori Sasaki, Shigeru Ida

accepted for publication in Icarus


“No Hadean rocks have ever been found on Earth’s surface except for zircons—evidence of continental crust, suggesting that Hadean continental crust existed but later disappeared. One hypothesis for the disappearance of the continental crust is excavation/melting by the Late Heavy Bombardment (LHB), a concentration of impacts in the last phase of the Hadean eon. In this paper, we calculate the effects of LHB on Hadean continental crust in order to investigate this hypothesis. Approximating the size-frequency distribution of the impacts by a power-law scaling with an exponent {\alpha} as a parameter, we have derived semi-analytical expressions for the effects of LHB impacts. We calculated the total excavation/melting volume and area affected by the LHB from two constraints of LHB on the moon, the size of the largest basin during LHB, and the density of craters larger than 20 km. We also investigated the effects of the value of {\alpha}. Our results show that LHB does not excavate/melt all of Hadean continental crust directly, but over 70% of the Earth’s surface area can be covered by subsequent melts in a broad range of {\alpha}. If there have been no overturns of the continental crust until today, LHB could be responsible for the absence of Hadean rocks because most of Hadean continental crust is not be exposed on the Earth’s surface in this case.”

Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon

Elizabeth A. Bell, Patrick Boehnke, T. Mark Harrison, and Wendy L. Mao

PNAS2015 ; published ahead of print October 19, 2015, doi:10.1073/pnas.1517557112

supporting information


Evidence for carbon cycling or biologic activity can be derived from carbon isotopes, because a high 12C/13C ratio is characteristic of biogenic carbon due to the large isotopic fractionation associated with enzymatic carbon fixation. The earliest materials measured for carbon isotopes at 3.8 Ga are isotopically light, and thus potentially biogenic. Because Earth’s known rock record extends only to ∼4 Ga, earlier periods of history are accessible only through mineral grains deposited in later sediments. We report 12C/13C of graphite preserved in 4.1-Ga zircon. Its complete encasement in crack-free, undisturbed zircon demonstrates that it is not contamination from more recent geologic processes. Its 12C-rich isotopic signature may be evidence for the origin of life on Earth by 4.1 Ga.


Evidence of life on Earth is manifestly preserved in the rock record. However, the microfossil record only extends to ∼3.5 billion years (Ga), the chemofossil record arguably to ∼3.8 Ga, and the rock record to 4.0 Ga. Detrital zircons from Jack Hills, Western Australia range in age up to nearly 4.4 Ga. From a population of over 10,000 Jack Hills zircons, we identified one >3.8-Ga zircon that contains primary graphite inclusions. Here, we report carbon isotopic measurements on these inclusions in a concordant, 4.10 ± 0.01-Ga zircon. We interpret these inclusions as primary due to their enclosure in a crack-free host as shown by transmission X-ray microscopy and their crystal habit. Their δ13CPDB of −24 ± 5‰ is consistent with a biogenic origin and may be evidence that a terrestrial biosphere had emerged by 4.1 Ga, or ∼300 My earlier than has been previously proposed.

Potentially biogenic carbon preserved in a 4.1 billion year old zircon by Elizabeth Bell (SETI Talks)
Tuesday, February 16 2016

Core formation and core composition from coupled geochemical and geophysical constraints

James Badro, John P. Brodholt, Hélène Piet, Julien Siebert, and Frederick J. Ryerson

PNAS2015 ; published ahead of print September 21, 2015, doi:10.1073/pnas.1505672112

We combine, for the first time to our knowledge, two approaches to study Earth’s core composition: a geochemical approach based on trace element depletion in the mantle and a geophysical approach based on a seismically lighter and faster (than pure iron−nickel) core. The joint approach allows making strong statements; first of all, as opposed to the current belief, Earth must have accreted material that is more oxidized than the present-day mantle, similar to that of planetesimals such as 4-Vesta, and got reduced to its present state during core formation. Secondly, core light-element concentrations in those conditions are 2.7% to 5% oxygen alongside 2% to 3.6% silicon; the oxygen concentrations in the core are higher than previously thought, and, conversely, silicon concentrations are lower than previous estimates.
The formation of Earth’s core left behind geophysical and geochemical signatures in both the core and mantle that remain to this day. Seismology requires that the core be lighter than pure iron and therefore must contain light elements, and the geochemistry of mantle-derived rocks reveals extensive siderophile element depletion and fractionation. Both features are inherited from metal−silicate differentiation in primitive Earth and depend upon the nature of physiochemical conditions that prevailed during core formation. To date, core formation models have only attempted to address the evolution of core and mantle compositional signatures separately, rather than seeking a joint solution. Here we combine experimental petrology, geochemistry, mineral physics and seismology to constrain a range of core formation conditions that satisfy both constraints. We find that core formation occurred in a hot (liquidus) yet moderately deep magma ocean not exceeding 1,800 km depth, under redox conditions more oxidized than present-day Earth. This new scenario, at odds with the current belief that core formation occurred under reducing conditions, proposes that Earth’s magma ocean started oxidized and has become reduced through time, by oxygen incorporation into the core. This core formation model produces a core that contains 2.7–5% oxygen along with 2–3.6% silicon, with densities and velocities in accord with radial seismic models, and leaves behind a silicate mantle that matches the observed mantle abundances of nickel, cobalt, chromium, and vanadium.”

Pervasive remagnetization of detrital zircon host rocks in the Jack Hills, Western Australia and implications for records of the early geodynamo

Benjamin P. Weiss, Adam C. Maloof, Nicholas Tailby, Jahandar Ramezani, Roger R. Fu, Veronica Hanus, Dustin Trail, E. Bruce Watson, T. Mark Harrison, Samuel A. Bowring, Joseph L. Kirschvink, Nicholas L. Swanson-Hysell, Robert S. Coe

Earth and Planetary Science Letters
Volume 430, 15 November 2015, Pages 115–128
Available online 28 August 2015

It currently is unknown when Earth’s dynamo magnetic field originated. Paleomagnetic studies indicate that a field with an intensity similar to that of the present day existed 3.5 billion years ago (Ga). Detrital zircon crystals found in the Jack Hills of Western Australia are some of the very few samples known to substantially predate this time. With crystallization ages ranging from 3.0–4.38 Ga, these zircons might preserve a record of the missing first billion years of Earth’s magnetic field history. However, a key unknown is the age and origin of magnetization in the Jack Hills zircons. The identification of >3.9 Ga (i.e., Hadean) field records requires first establishing that the zircons have avoided remagnetization since being deposited in quartz-rich conglomerates at 2.65–3.05 Ga. To address this issue, we have conducted paleomagnetic conglomerate, baked contact, and fold tests in combination with U–Pb geochronology to establish the timing of the metamorphic and alteration events and the peak temperatures experienced by the zircon host rocks. These tests include the first conglomerate test directly on the Hadean-zircon bearing conglomerate at Erawandoo Hill. Although we observed little evidence for remagnetization by recent lightning strikes, we found that the Hadean zircon-bearing rocks and surrounding region have been pervasively remagnetized, with the final major overprinting likely due to thermal and/or aqueous effects from the emplacement of the Warakurna large igneous province at ∼1070 million years ago (Ma). Although localized regions of the Jack Hills might have escaped complete remagnetization, there currently is no robust evidence for pre-depositional (>3.0 Ga) magnetization in the Jack Hills detrital zircons.

Comment on: Pervasive remagnetization of detrital zircon host rocks in the Jack Hills, Western Australia and implications for records of the early dynamo, by Weiss et al. (2015)
Richard K. Bono, John A. Tarduno, Rory D. Cottrell

Earth and Planetary Science Letters
In Press, Corrected Proof, Available online 4 July 2016

• No evidence for Hadean zircon host rock pervasive remagnetization at 1078–1070 Ma.
• Large paleosecular variation recording is inconsistent with remagnetization.
• Paleomagnetic uncertainties of 65–75° are inappropriate for interpretation.”

Reply to comment
by Benjamin P. Weiss, Adam C. Maloof, T. Mark Harrison, Nicholas L. Swanson-Hysell, Roger R. Fu, Joseph L. Kirschvink, E. Bruce Watson, Robert S. Coe, Sonia M. Tikoo, Jahandar Ramezani

Earth and Planetary Science Letters
In Press, Corrected Proof, Available online 19 July 2016


• All of Weiss et al. (2015)’s Jack Hills field tests failed or were inconclusive.
• Hadean zircon host rocks likely pervasively remagnetized at 1078–1070 Ma.
• The ages of the Jack Hills zircons’ magnetizations are essentially unknown.”