Volatile depletion in the early solar system: insights from Zn and Cd isotopes

Abstract

It has been argued by many authors that Earth acquired its volatile element budget during the main stage of accretion, alongside core formation. Others, however, argue that volatile elements were primarily delivered after core formation, by a large and volatile-rich late veneer. This study aims to address this debate through Zn and Cd stable isotope analyses of terrestrial and meteorite samples. Both Zn and Cd are moderately incompatible during mantle melting, with oceanic basalts displaying slight enrichments in higher concentrations and heavy isotopes compared to peridotites. Lherzolite data from this and previous studies define a 66Zn value of +0.20 ± 0.05‰ for the bulk silicate Earth (BSE), which would be consistent with volatile delivery to Earth by moderately volatile depleted carbonaceous or enstatite chondrites. In contrast, peridotite data presented here define an average 114Cd of -0.03 ± 0.16‰ for the BSE, which is lighter than previously analysed chondrites unaffected by secondary processes such as metamorphism. Metal-silicate partitioning experiments suggest that no significant Cd isotope fractionation took place during core formation and sulphide segregation. Instead, the results of mass balance accretion modelling show that Earth’s main stage accretion components had a Cd isotope composition lighter than known carbonaceous and enstatite chondrites, independent of the exact size of the late veneer. It is also concluded that Earth most likely accreted a smaller (≤2% ME) late veneer, as a larger late veneer requires main stage accretionary materials with 114Cd values increasingly removed from known chondrites. As such, Cd delivery primarily occurred before the late veneer. Mass balance calculations show that the Zn and Cd isotope compositions and concentrations of the Earth are consistent with mixing of average carbonaceous chondrites and volatile-depleted non-carbonaceous materials during main stage accretion. Earth’s non-carbonaceous component may resemble ordinary chondrite-like parent bodies not represented in current meteorite collections.