Investigating volatile depletion processes using Te isotope fractionations in iron meteorites

Magmatic iron meteorites sample the metal cores of differentiated planetesimals, and are classified into distinct chemical groups by their markedly different contents of the moderately volatile elements (MVE) Ge and Ga. The variable MVE depletions may be inherited from an iron parent body’s precursor materials or are the result of secondary losses associated with parent body accretion, differentiation, and disruption. To better understand the origin of these MVE depletions, we obtained mass-dependent Te isotopic data, using a ¹²³Te-¹²⁵Te double-spike and MC-ICP-MS, for metal and troilite samples from several magmatic irons (IC, IIF, IIIAB, and IVA). The Te concentrations vary by two orders of magnitude among the investigated samples and follow the volatile depletion trends defined by the less volatile Ge and Ga. In all samples, Te is predominantly hosted in troilite, with troilite-metal partitioning coefficients of ≈100. As such, the Te isotope composition of troilite can be taken as a proxy for that of the bulk iron meteorite and, by inference, the bulk core. Despite large differences in Te concentration, the troilites investigated in this study show only limited mass-dependent Te isotope variations and fall within the range of compositions observed among carbonaceous chondrites. Of note, all troilites investigated thus far are isotopically lighter than CI chondrites. Together these observations suggest that any MVE loss from the iron parent bodies did not induce large Te isotope fractionations and that the precursor material of these bodies was already somewhat MVE-depleted compared to CI chondrites.