To understand accretion and differentiation of Mars, we report high-precision mass-dependent Ni (siderophile and chalcophile) isotope data of 37 bulk Martian meteorites. Large δ60/58Ni variations observed among these Martian meteorites are attributed primarily to magmatism and Ni diffusion in zoned olivine and sulfide. Shergottites show systematically higher Mg# and lower δ60/58Ni values relative to nakhlites, which can be caused by olivine crystallization, consistent with the Ni isotope fractionation factor between olivine and melt. Two Ni-rich chassignites (Martian dunites) provide the best current estimate of the upper limit of δ60/58Ni of bulk silicate Mars (BSM): 0.110 ± 0.031 ‰, since olivine crystallization causes Ni isotope fractionation. Subtracting a presumably chondritic contribution by late accretion, the proto-BSM should possess a δ60/58Ni of ≤ 0.074 ‰ that is lower than the average of chondrites (∼0.24 ‰). This sub-chondritic value of Martian mantle suggests the sulfur-rich core formation has not caused Ni isotope fractionation, because the sulfide and Martian sulfur-rich core is believed to enrich in light Ni isotopes. Instead, Ni isotope differences between Earth, Mars, Vesta, and the ureilites can be inherited from non-bulk chondritic precursor materials.
Zhu et al. (Thu,) studied this question.
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