Germanium isotopes reveal distinct processes of moderately volatile element depletion among planetesimals

The depletion of moderately volatile elements (MVEs) is a key characteristic of planetary materials, but the origins of these depletions are unclear. To investigate the relative importance of nebular vs. planetary volatile depletion processes, we studied the mass-dependent isotopic composition of the MVE Ge in chondrites and iron meteorites. Chondrites exhibit Ge isotope fractionations which correlate with matrix mass fraction and Ge depletion. This indicates mixing between volatile-rich, isotopically heavy matrix and a volatile-poor, isotopically light chondrule component. Despite much larger Ge depletions, iron meteorites exhibit a similar range of Ge isotope fractionations as chondrites. Specifically, the strongly depleted IVA/IVB irons have lighter Ge isotope compositions than CI chondrites, while there rarely are iron groups with heavier compositions. These observations indicate that the MVE depletions among irons cannot simply result from degassing of CI-like starting materials. Instead, these systematics likely reflect two stages of MVE depletion. The first stage took place in the solar nebula and, like for the chondrites, involved mixing of volatile-rich and volatile-poor precursor components. The second stage occurred on the iron parent bodies and likely involved degassing from molten iron cores, after collisional disruption of the parent bodies. While for most irons the Ge elemental depletion mostly occurred during this second stage, these losses do not appear to have induced large Ge isotope fractionations. As such, an iron parent bodies' Ge isotopic composition largely reflects that of its precursor material. Thus, both primordial nebular and secondary planetary volatile loss shaped the MVE budgets of differentiated planetesimals.