Influence of redox processes on the germanium isotopic composition of ordinary chondrites
Guillaume Florin, Béatrice Luais, Tracy Rushmer, Olivier Alard
Geochimica et Cosmochimica Acta
In Press, Journal Pre-proof, Available online 6 November 2019
Ordinary chondrites (OCs) are classified into three groups, according to their oxidation state, which increases from the H to L to LL groups. This is demonstrated by the decrease in metal content (H=∼8 vol%, L=∼4 vol%, and LL=∼2 vol%), and by a positive correlation between Δ17O and %Fa through the OC sequence. Compared to other chondrites, OCs exhibit the largest variation in oxidation state but there is an ongoing debate on the processes that control this variation. To constrain the causes of the variations in the oxidation state with respect to the associated nebular versus parent bodies processes, we investigated the elemental and isotopic variations of germanium (moderately siderophile and volatile) in the bulk sample, as well as in the metal, silicate and sulfide phases, over a range of petrographic types for the H, L, and LL ordinary chondrites.
We found that δ74/70Gemetal is a proxy for the δ74/70Gebulk composition and that each OC group is distinguishable by their δ74/70Gemetal, which increases from –0.51±0.09‰ for H chondrites, –0.31±0.06‰ for L chondrites, and, finally, to –0.26±0.09‰ for LL chondrites (2σSD). Additionally, the OC sequence exhibited a positive correlation, from H to L to LL, between δ74/70Gemetal and %Fa, as well as oxygen isotopes (δ17O, δ18O and Δ17O), that was not a consequence of a “size sorting effect” on chondrules (i.e. chondrule mixing) or metamorphic processes in the parent bodies but, rather, was the result of nebular processes. We propose that the correlation between the δ74/70Ge values and %Fa, Δ17O, δ18O can be explained by an increasing proportion of accreted hydrated phyllosilicates, from the H, L to LL groups, with high δ74/70Ge and Δ17O. We found that 10 to 15% of phyllosilicates, with a composition of [Ge]=4–7 ppm and δ74/70Ge=3–2.5‰, is needed to change the δ74/70Ge from H to LL, which corresponds to a Δ17O≈8-7‰. This value agrees with the Δ17O≈7‰ composition of the accreted nebular component reported by Choi et al. (1998). During thermal metamorphism, phyllosilicates destabilize, liberating germanium that will be incorporated in the metal, then leading to its high δ74/70Ge signature.
High-temperature metamorphism can explain the lack of δ74/70Gemetal variation with the petrologic type in the OC, even for the type 3 chondrites (T≈675°C), implying a complete reaction even at low petrologic types. In addition, metal-silicate re-equilibration in response to thermal metamorphism results in a decrease in Δ74/70Gemetal-silicate from 0.33‰ to 0.06‰, within the H chondrite group, which is interpreted as the result of δ74/70Gesilicate variation. The mean positive Δ74/70Gemetal-silicate fractionation factor of +0.22±0.36‰ (error propagation on individual error) also displays a remarkable similarity to the direction of isotopic fractionation with other germanium isotopic metal-silicate datasets, such as the magmatic iron meteorites, the Earth silicate reservoirs. We propose that the Δ74/70Gemetal-silicate and the negative δ74/70Ge values of OCs are inherited from metal-silicate melting and partial exchange before planetesimal accretion in a light isotope-enriched gas. Finally, the δ74/70Gemetal-Δ17Osilicate correlation between the IIE iron meteorites and OCs, provides new evidence for the existence of a highly reduced HH group.
