Volatile Element Depletion of Carbonaceous Chondrites-Insights from Mass-dependent Zinc, Cadmium, and Tellurium Isotope Variations

The origin of volatile depletion in the solar system remains a topic of intense debate. To further inform our understanding of the mechanisms involved, this study characterized the mass-dependent Zn, Cd, and Te isotope compositions and concentrations of a comprehensive suite of carbonaceous chondrites (CCs). In accord with previous studies, Zn and Te display covariations between light isotope enrichments and elemental depletions. Observed here for the first time, Cd shows a similar trend. These correlations are consistent with the interpretation that the primary volatile element budgets of CCs were established by mixing of a volatile-rich CI-like matrix and a volatile-depleted non-matrix endmember (NME) in the solar nebula. All three elements display minor isotopic variations in CI and CM chondrites, as a consequence of aqueous alteration at low temperatures. In contrast, Cd and Te isotope compositions and concentrations are highly variable in CV and CO (Cd) and CK chondrites (Te). This reflects mobilization of the elements during thermal metamorphism at distinct redox conditions. The data of this study show that the NME has Zn, Cd, and Te concentrations that are depleted to an identical level of 0.12 ± 0.03 × CI chondrites, and it is characterized by mass-dependent isotope compositions for all three elements that are fractionated to light isotope values relative to CIs by a similar extent. In conjunction with literature data, this suggests that the concentrations and isotope compositions of NME volatiles record the same depletion processes, and that the NME volatile inventory is likely hosted predominantly in chondrules.