Origin of moderately volatile elements in Earth inferred from mass-dependent Ge isotope variations among chondrites

TL;DR

This study uses Ge isotope variations in chondrites to trace Earth's volatile element origins, suggesting a mixture of volatile-rich and -poor materials contributed to Earth's formation, with implications for planetary accretion processes.

Contribution

It provides new Ge isotope data for chondrites and interprets these in terms of Earth's building blocks, revealing a specific mixing ratio and the volatile-rich nature of accreting bodies.

Findings

Ge isotope variations reflect mixing of volatile-rich and -poor materials.

Earth's Ge isotope composition can be explained by a ~2:1 mixture of CI and enstatite chondrites.

Implication that accreting bodies were volatile-rich, possibly undifferentiated or volatile-rich objects.

Abstract

The bulk silicate Earth (BSE) is depleted in moderately volatile elements, indicating Earth formed from a mixture of volatile-rich and -poor materials. To better constrain the origin and nature of Earth's volatile-rich building blocks, we determined the mass-dependent isotope compositions of Ge in carbonaceous (CC) and enstatite chondrites. We find that, similar to other moderately volatile elements, the Ge isotope variations among the chondrites reflect mixing between volatile-rich, isotopically heavy matrix and volatile-poor, isotopically light chondrules. The Ge isotope composition of the BSE is within the chondritic range and can be accounted for as a ~2:1 mixture of CI and enstatite chondrite-derived Ge. This mixing ratio appears to be distinct from the ~1:2 ratio inferred for Zn, reflecting the different geochemical behavior of Ge (siderophile) and Zn (lithophile), and suggesting the late-stage addition of volatile-rich CC materials to Earth. On dynamical grounds it has been argued that Earth accreted CC material through a few Moon-sized embryos, in which case the Ge isotope results imply that these objects were volatile-rich, presumably because they were either undifferentiated or accreted volatile-rich objects themselves before being accreted by Earth.