Provenance and distribution of zinc in terrestrial planets

Analyses of Earth’s nucleosynthetic Zn isotope composition indicate that its inventory of this volatile element was derived from a mixture of materials originating in both the inner and outer regions of the Solar System. In contrast, the nucleosynthetic Zn isotope composition of Martian meteorites suggests that, despite being more volatile-rich than Earth, Mars received very limited Zn from outer Solar System sources. Modeling for Earth also indicates that, although only ~30% of its mass was supplied by chondritic material, such undifferentiated, primitive bodies contributed ~90% of its Zn. However, the total chondritic contribution to Mars, and its role in shaping the planet’s volatile budget, remain poorly constrained. Here, we present nucleosynthetic Zn isotope data for six Martian meteorites, confirming that Mars’ Zn was likely sourced exclusively from inner Solar System material. These data were incorporated into a comprehensive mixing model that includes constraints from eight additional isotope systems and major element abundances. The results are consistent with Mars having accreted ~50% of its mass from chondritic material, which also delivered ~90% of its Zn. Together, these findings suggest that the proportion of undifferentiated material accreted by a planet plays a more critical role in establishing its volatile budget than the specific provenance of the accreting sources. Finally, we report mass-independent Zn isotope compositions for five Lunar samples. The results are indistinguishable, within uncertainty, from values for the bulk silicate Earth (BSE) and non-carbonaceous (NC) meteorites. Further analyses are thus required to reliably constrain the origin of Lunar Zn.