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

Paolo A Sossi,Oliver Nebel,Mahesh Anand,Franck Poitrasson

doi:10.1016/j.epsl.2016.05.030

Abstract

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.

Highlights

How planets in the inner solar system accreted and differentiated is a question fundamental to understanding the chemical differences that exist between them today
Measured δ57Fe values are inversely correlated with whole-rock MgO contents (Fig. 1a), mimicking trends observed in terrestrial mafic rocks (Sossi et al, 2012; Teng et al, 2008)
A similar correlation is found with iron isotopes and CaO (Fig. 1b), with the nakhlites displaced towards higher CaO values as a result of clinopyroxene accumulation

Summary

Introduction

How planets in the inner solar system accreted and differentiated is a question fundamental to understanding the chemical differences that exist between them today. Iron offers a unique opportunity to address this question Is it abundant in each of the major planetary reservoirs, the crust, mantle and core, it exists in three different oxidation states – Fe3+, Fe2+ and Fe0 – the relative proportions of which are constrained by local redox conditions. Despite the range of iron oxidation states, its isotopes are remarkably constant in chondritic meteorites, identical to the IRMM-014 reference material (Schoenberg and von Blanckenburg, 2006; Craddock and Dauphas, 2011; Needham et al, 2009; Theis et al, 2008; Wang et al, 2013); i.e., ((57Fe/54Fe)chondrite/ (57Fe/54Fe)IRMM-014 − 1)∗1000 = δ57Fe = 0h This constancy is independent of chondrite class, if planets accreted from chondritic material, deviations from 0h should reflect post-nebular processes.

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Results

Conclusion