Provenance and distribution of zinc in terrestrial planets

The nucleosynthetic Zn isotope compositions of planetary materials shed light on the sources of Zn and other similarly volatile elements during terrestrial planet formation. Previous results for Earth revealed that it acquired its Zn inventory primarily from primitive, undifferentiated bodies originating from both the inner and outer Solar System. To further investigate this, we obtained new nucleosynthetic Zn isotope data for six Martian meteorites and five Lunar samples. Our Martian data are consistent with previous findings which show that, despite being more volatile-rich than Earth, Mars’s Zn was derived almost entirely from inner Solar System sources. We incorporated these results into a comprehensive mixing model, including eight additional isotope systems and major element abundances. The model suggests that Mars accreted ~ 50% of its mass from primitive chondritic material that delivered ~ 90% of its Zn, while the remainder came from volatile-poor, differentiated bodies. This mirrors previous findings for Earth, where ~ 90% of Zn was delivered by primitive material, despite contributing only ~ 30% of Earth’s total mass. Together, these results imply that the volatile inventories of terrestrial planets depend more on the nature (primitive versus differentiated) than the origin (inner versus outer Solar System) of their building blocks. Finally, the nucleosynthetic Zn isotope compositions of five Lunar samples are indistinguishable, within uncertainty, from those of the bulk silicate Earth and non-carbonaceous meteorites. Further analyses are thus needed to more robustly constrain the origin of Lunar Zn.