Tin and zinc stable isotope characterisation of chondrites and implications for early Solar System evolution

Abstract

Moderately volatile elements show variable depletion in terrestrial planets compared to the Sun. Isotopic ratios can be used as a signature of the processes at the origin of this depletion. Using a new method, the Sn stable isotope composition and elemental abundance in 36 primitive meteorites (chondrites) have been characterised to high precision. Significant mass-dependent Sn isotope variations are found within chondrites. The widest isotopic range is observed for the ordinary chondrites (−1.1 ‰ to +0.5 ‰ in δ122/118Sn, representing the difference in the 122Sn/118Sn ratio of the sample relative to our in-house standard, Sn_IPGP), with the ordinary chondrite groups extending to lighter isotopic compositions in the order H > L > LL, while carbonaceous and enstatite chondrites are heavier and occupy narrower compositional ranges. Tin and Zn isotope and concentration data are strongly correlated, particularly in ordinary chondrites, from which both sets of data were obtained on the same rock powders. Given the difference in geochemical behaviour (Zn lithophile/chalcophile and Sn chalcophile/siderophile) of these elements, this suggests that the primary control on the isotope and abundance variations is volatility. Chondrite groups show variability increasing with petrographic types, suggesting a secondary control from parent-body metamorphism. The isotopic composition of the bulk silicate Earth (BSE; δ122/118Sn = 0.49 ± 0.11 ‰) overlaps with the carbonaceous chondrites (δ122/118Sn = 0.43 ± 0.12 ‰; excl. CR and CK). Despite isotopic similarities for almost all isotopic systems, EH chondrites have Sn isotope compositions that are distinct from the bulk silicate Earth (δ122/118Sn = 0.18 ± 0.21 ‰). Therefore, an enstatite chondrite-like bulk Earth requires that isotopically light Sn was lost from the silicate Earth, possibly into the metallic core or a sulphide matte, or by evaporative loss from Earth or its precursors.