Rare meteorites common in the Ordovician periodOPEN ACCESS 

Philipp R. Heck, Birger Schmitz, William F. Bottke, Surya S. Rout, Noriko T. Kita, Anders Cronholm, Céline Defouilloy, Andrei Dronov & Fredrik Terfelt

Nature Astronomy 1
Article number: 0035 (2017)
Published online: 23 January 2017


“Most meteorites that fall today are H and L type ordinary chondrites, yet the main belt asteroids best positioned to deliver meteorites are LL chondrites 1,2 . This suggests that the current meteorite flux is dominated by fragments from recent asteroid breakup events 3,4 and therefore is not representative over longer (100-Myr) timescales. Here we present the first reconstruction of the composition of the background meteorite flux to Earth on such timescales. From limestone that formed about one million years before the breakup of the L-chondrite parent body 466 Myr ago, we have recovered relict minerals from coarse micrometeorites. By elemental and oxygen-isotopic analyses, we show that before 466 Myr ago, achondrites from different asteroidal sources had similar or higher abundances than ordinary chondrites. The primitive achondrites, such as lodranites and acapulcoites, together with related ungrouped achondrites, made up ~15–34% of the flux compared with only ~0.45% today. Another group of abundant achondrites may be linked to a 500-km cratering event on (4) Vesta that filled the inner main belt with basaltic fragments a billion years ago 5 . Our data show that the meteorite flux has varied over geological time as asteroid disruptions create new fragment populations that then slowly fade away from collisional and dynamical evolution. The current flux favours disruption events that are larger, younger and/or highly efficient at delivering material to Earth.
To investigate the past meteorite flux, we searched for relict chrome-spinel grains of coarse micrometeorites in condensed marine sediments in northwestern Russia, in a time window of ~10–100 kyr in the geological epoch of the Middle Ordovician period, which ranges from 470 to 458 Myr ago (Fig. 1; see Methods). Chrome spinels are the only minerals of meteorites and coarse micrometeorites that survived diagenesis in Ordovician limestone 6 . They retained their elemental and oxygen isotopic composition, enabling reliable classification based on single-grain microanalysis 7,8 . We also dissolved 32 meteorites of different types in HF or HCl acid to quantify their content of chrome-spinel grains. The sediment sample that we studied is about a million years older than the ~466-Myr-old sediments that contain the first collisional fragments from the L-chondrite parent body breakup (LCPB), the largest known asteroid disruption event in the past three billion years. The sampling level was chosen to exclude the extreme flux enhancement (more than two orders of magnitude 6,7 ) of L-chondritic fragments after the LCPB that obscures the background flux for more than 1 Myr (refs 7,​8,​9 ). The low, 50- to 100-kyr, cosmic-ray exposure ages of the oldest recovered fossil L chondrites 9 imply that any fragments from the LCPB that might have arrived on Earth before should have even shorter exposure ages. This indicates that a sample separation of one million years before the strata containing the first ­abundant L chondrites is large enough to assess the pre-LCPB flux. The ­interval sampled represents a time average of about 10 to 100 kyr and was selected with the aim of determining whether the ­composition of the meteorite flux to Earth was similar to or different from that of today. This is the first reconstruction of the background flux of the different meteorite types in a geological time perspective. Similar reconstructions are ongoing for other periods in the Earth’s ­geological past.

Figure 1: Micrometeorite-bearing limestone beds at the Lynna River section in northwestern Russia that were deposited around 466 million years ago.

The presence of surface-implanted solar-wind-derived helium and neon in sediment-dispersed extraterrestrial chrome ­spinels (SECs) that were recovered from similar sediments from ­several younger Ordovician beds from sites in Sweden, China and Russia is evidence that the SECs were parts of micrometeorites 11,​12,​13 . Because the abundance ratio of the two ordinary chondrite groups H and L chondrites in recently fallen coarse micrometeorites 14,15 is ­similar to this ratio in macroscopic meteorites, ­micrometeorites bearing coarse chromite grains can be used as a proxy for ­meteorites 7 . The same consistency between the composition of coarse ­micrometeorites and meteorites has been documented based on fossil material for the Ordovician period after the LCPB 8 . This relation is useful because of the much higher abundance of SECs compared with fossil meteorites 6 , allowing analyses of a larger number of samples 7 .

We recovered 46 chrome-spinel grains with diameters >63 μm, out of which 41 are extraterrestrial based on their oxygen isotopic and elemental composition (Table 1 and Supplementary Data 1; see Methods). We find a large diversity of micrometeorites that includes all three groups of ordinary chondrites and many types of achondrites in strikingly different proportions from today (Figs 2 and 3). Among the extraterrestrial grains, 23 originate from ordinary chondrites and 18 from achondrites. This corresponds to an ordinary chondrite/achondrite ratio of 1.3 compared with ~11 in today’s flux (Table 1). The proportions of the three ordinary chondrite groups H, L and LL are markedly different from the recent flux, and from the flux immediately after the LCPB. Today, L and H chondrites fall in about equal proportions and together dominate the flux, but in the Ordovician prior to the LCPB the same can instead be said about the LL and L types. (Table1″

Table 1: Classification of coarse micrometeorites in this study with their fractions of the total flux

Figure 2: Values of Δ17O and TiO2 of our data compared with compositions of different relevant meteorite groups.

Figure 3: Probability density functions (PDFs) of Δ17O values showing the distribution of different micrometeorite categories.

“Considering the variation in abundances of large chrome-spinel grains in recent meteorites, there are significant uncertainties when translating SEC grain abundances into estimates of Middle Ordovician meteorite flux (see Supplementary Data 2). Some first-order minimum estimates for the achondritic versus ordinary chondritic flux can be made, however, if we assume that the chrome-spinel grain contents of the achondrites were generally lower than or equal to those of the ordinary chondritic (mostly types 5 and 6) micrometeorites that contributed chrome-spinel grains to the ancient sea floor. With this extremely conservative approach, the achondrites were almost or as common as the ordinary chondrites, and the primitive achondrites and related ungrouped achondrites made up between 15 and 34% of all achondrites and ordinary chondrites, compared with around 0.45% today (Table 1 and Supplementary Information). The true achondrite fraction, however, may have been significantly higher. Although we have only studied 13 achondrites, our data for the chrome-spinel content indicate generally lower numbers than for the ordinary chondrites. If these numbers are accounted for in the palaeoflux estimates, achondrites dominated over ordinary chondrites.

The achondrite grains include one sample possibly from a rare Bocaiuva-type achondrite (category A; Figs 2 and 3). Based on the elemental and Δ17O compositions of Bocaiuva 16,17 and our category A grain, we argue that the grain may represent a piece of the missing mantle fraction of the Bocaiuva parent body, or of the surface if the Bocaiuva iron was impact-generated. One of our chrome-spinel grains (#105-05) appears to come from the Österplana 065 type of ungrouped achondrite. In Middle Ordovician sediments that formed after the LCPB, a 8-cm fossil achondrite, Österplana 065, was recently found that has a Cr and O isotopic composition different from all known recent meteorite types 18 . This single find of a new type of achondrite among about 100 fossil L-chondritic meteorites indicates that the assemblage of micrometeoritic chrome spinel in our study may also harbour grains from meteorites not known today. For a random find of a single meteorite on Earth today, the likelihood is much greater that it would belong to a common group than a very rare group of meteorites. Based on this reasoning, it is likely that Österplana 065 belongs to a type of meteorite that was common in the flux in the Ordovician. The data in this study support that this was the case. For Österplana 065, we know that the chrome-spinel content was rather low, at 50 grains per gram. If we use this number in the palaeoflux estimates, this would mean that meteorites of the type represented by the single grain #105-05 would represent 20–30 times the mass to which one of the ordinary chondritic grains corresponds. The unexpected high fraction of ungrouped and related primitive achondritic material in sediments pre-dating the LCPB is evidence that some partially differentiated asteroids had disrupted and were capable of producing a relatively high flux of meteoroids at that time. The fact that the same fraction is smaller today probably indicates that their source families were small enough that meteoroid production by a collisional cascade could not keep up with newer families much closer to their peak flux.

As regards howardites, eucrites and diogenites (HED) achondrites, today the HED/ordinary chondrites ratio is about 0.1. With a range of 4–12 possible HED micrometeorites among our 41 ­extraterrestrial grains (of which 23 are ordinary chondritic), this represents a (grain-to-grain) HED/ordinary chondrites ratio in the range 0.2 to 0.5, which is significantly higher than today. Considering also that the HED meteorites on average contain fewer chrome-­spinel grains in the >63-μm fraction than the equilibrated ordinary chondrites dominated by higher petrographic types (Supplementary Data 2), this gives additional support for HED meteorites being more ­abundant in the Middle Ordovician than today. This result is ­particularly interesting because the HEDs are believed to come from the Vesta family 19 that formed nearly 1 Gyr ago in the formation of the ~500-km Rheasilvia impact basin 5 . The collisional cascade for this family would have been just as capable, if not more so, of producing meteoroids ~467 Myr ago as today.

For our 23 samples with an unambiguously ordinary chondritic origin, the most significant difference from the recent flux composition is the high abundance of LL grains relative to H and L grains compared with post-LCPB and today (Table 1). Impact degassing ages of recent LL chondrite falls are sparse 20,21 , and the only degassing age that could date the same event is that of the Morokweng meteorite (625 ± 163 Ma) 22 ; others are mostly at or older than 1 Gyr, consistent with the dynamical age of the Flora asteroid family (950 +200/–170 Myr) 23 , a likely source of the LL chondrites 2 . The H chondrites in the Earth’s recent flux have impact degassing ages in the range of ~280–460 Myr, indicating one or a few younger events than the LCPB 466 Myr ago 21 . This could suggest that the primary source of today’s H chondrites had not yet disrupted, whereas the LL source had disrupted and was closer to its peak meteoroid flux than it is now.

The smaller size fraction (<100 μm) of today’s micrometeorites is dominated by carbonaceous chondritic material, reflecting the brittleness and fragmentation of such material on collision with Earth's atmosphere 24 . In the recent flux, the coarse micrometeorites that can contain 100-μm-sized unmelted spinel grans are dominated by ordinary chondritic material similar to the macrometeorite flux composition 24,25 . All previous studies on micrometeorites show that recent primitive achondrite-type micrometeorites are not a significant fraction of the present flux 26. The large diversity in our sample of coarse micrometeorites, representing many different types and origins, confirms that the studied sediments did not sample one event such as an atmospheric breakup, a terrestrial impact or even a breakup of a single type of asteroid in space, but rather represent a time-averaged sample of the extraterrestrial flux to the Earth over ~10–100 kyr. We predict that the same diversity and abundances of coarse micrometeorites should be preserved in sediments of the same age globally. Despite many uncertainties, using a conservative approach we have shown that the meteorite flux composition was fundamentally different ~467 Myr ago from today and varies on timescales of 10–100 Myr and longer. At that time, achondrites probably dominated over ordinary chondrites, and primitive achondrites and related ungrouped meteorites were at least one order and probably two orders of magnitude more abundant than today. This fits with the proposal that different asteroid families were dominating the meteorite flux at these times. Furthermore, it shows that only after the LCPB did L chondrites become the most significant type of coarse extraterrestrial matter that accreted to the Earth. These results confirm that the collisional cascade model of meteoroid delivery is reasonable and can help to tell us about the evolution of the asteroid belt. Studying different time windows will increase our knowledge of the variation of the flux of extraterrestrial material to the Earth in deep time and will provide new knowledge on the evolution of the asteroid belt from the Earth's sedimentary record"