The Sn isotope composition of chondrites: implications for volatile element depletion in the Solar System
Xueying Wang, Caroline Fitoussi, Bernard Bourdon, Kevin Righter, Quentin Amet
Geochimica et Cosmochimica Acta
Available online 19 August 2021
“The origin of volatile element depletion in terrestrial planets and meteorites relative to a solar composition represented by CI carbonaceous chondrites remains an unsolved problem. The isotope compositions of moderately volatile elements may offer the possibility to distinguish between the various processes that may have caused this depletion (e.g., partial condensation or partial evaporation). We report high precision Sn isotope measurements in carbonaceous chondrites and ordinary chondrites and the results are reported as δ124/116Sn. Four carbonaceous chondrites (Orgueil CI, Murchison CM2, NWA 5240 CV3 and Allende CV3) show a limited range in δ124/116Sn (–0.02‰ to 0.11‰) with an average value of 0.04±0.11‰ (2 s.d.) for a wide range of Sn concentrations (0.63 ppm to 1.57 ppm). The absence of Sn isotope fractionation among carbonaceous chondrites suggests that volatile depletion may have taken place under thermodynamic equilibrium conditions between solid and vapor in the Solar Nebula. Alternatively, the mixing of two components, a volatile-free component containing no Sn and a volatile-rich component could explain this trend. This latter hypothesis is consistent with the overall trace element pattern found in carbonaceous chondrites, showing a constant relative abundance when normalized to CI chondrites for the most volatile elements. In contrast with carbonaceous chondrites, ordinary chondrites exhibit a larger range of Sn isotope compositions (δ124/116Sn from–2.02‰ to 0.64‰), but neither the degree of metamorphism (3-6) nor the group (H, L, LL) is correlated with Sn isotopic variations, or with the Sn contents (range 0.20 to 1.44 ppm). Nineteen out of twenty-one ordinary chondrites are enriched in light Sn isotopes compared with carbonaceous chondrites and the bulk silicate Earth. The trace element patterns of volatile elements in ordinary chondrites suggest that equilibrated ordinary chondrites have been disturbed by parent body processes related to metamorphic or shock overprinting but also inherited isotope fractionation found in unequilibrated ordinary chondrites. Last, the isotope composition of the bulk silicate Earth (BSE) indicates that the volatile element depletion observed in the Earth took place in conditions perhaps similar to those of carbonaceous chondrites, as a simple model describing the effect of Earth’s core formation on Sn isotopes shows that the Sn isotope composition of the bulk Earth is identical to that of the BSE and of carbonaceous chondrites.”
