On the iron isotope composition of Mars and volatile depletion in the terrestrial planets

Iron is the most abundant multivalent element in planetary reservoirs, meaning its isotope composition (expressed as δ⁵⁷Fe) may record signatures of processes that occurred during the formation and subsequent differentiation of the terrestrial planets. Chondritic meteorites, putative constituents of the planets and remnants of undifferentiated inner solar system bodies, have δFe57≈0‰; an isotopic signature shared with the Martian Shergottite–Nakhlite–Chassignite (SNC) suite of meteorites. The silicate Earth and Moon, as represented by basaltic rocks, are distinctly heavier, δFe57≈+0.1‰. However, some authors have recently argued, on the basis of iron isotope measurements of abyssal peridotites, that the composition of the Earth's mantle is δFe57=+0.04±0.04‰, indistinguishable from the mean Martian value. To provide a more robust estimate for Mars, we present new high-precision iron isotope data on 17 SNC meteorites and 5 mineral separates. We find that the iron isotope compositions of Martian meteorites reflect igneous processes, with nakhlites and evolved shergottites displaying heavier δFe57(+0.05±0.03‰), whereas MgO-rich rocks are lighter (δFe57≈−0.01±0.02‰). These systematics are controlled by the fractionation of olivine and pyroxene, attested to by the lighter isotope composition of pyroxene compared to whole rock nakhlites. Extrapolation of the δFe57 SNC liquid line of descent to a putative Martian mantle yields a δ⁵⁷Fe value lighter than its terrestrial counterpart, but indistinguishable from chondrites. Iron isotopes in planetary basalts of the inner solar system correlate positively with Fe/Mn and silicon isotopes. While Mars and IV-Vesta are undepleted in iron and accordingly have chondritic δ⁵⁷Fe, the Earth experienced volatile depletion at low (1300 K) temperatures, likely at an early stage in the solar nebula, whereas additional post-nebular Fe loss is possible for the Moon and angrites.