Metal – Silicate Fractionation of Iron Isotopes in Chondritic Meteorites

Paolo Sossi
Australian National University

Oliver Nebel
Australian National University

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     Last modified: July 30, 2011

Abstract
The Bulk Silicate Earth (BSE) shares an iron isotopic composition with ordinary and carbonaceous chondrites, with δ57Fe ≈ 0 (‰ relative to IRMM-014). However, the Bulk Earth (BE) potentially differs in its isotopic composition, as a result of metal-silicate fractionation during core-mantle segregation, with 35% of the terrestrial Fe residing in its metallic core. Equilibrium isotopic fractionation between Fe0 and Fe2+-bearing melt has not been resolved in high-temperature and -pressure experiments relevant to core formation (Roskosz et al., 2006; Poitrasson et al., 2009). However, δ57Femet correlates with the petrologic type of chondrites, where unmetamorphosed chondrites show relatively unfractionated δ57Fe ≈ 0‰ increasing up to strongly positive values (+0.55‰) in equilibrated meteorites, resulting in a metallic phase that is systematically heavier than its coexisting silicate fraction (Poitrasson et al., 2005; Theis et al., 2008; Needham et al., 2009). This implies that metal-silicate equilibration accompanying metamorphism results in the preferential incorporation of 57Fe-enriched iron into the metal phase. Here, we report iron isotope compositions of the H5 ordinary chondrite Gao-Guenie, and the silicated IAB NWA 968 that show δ57Femet up to +0.6‰ higher than δ57Fesil, and bulk compositions that reach ≈ +0.15‰. This is in contrast to earlier studies, which report tight values between -0.2 and +0.05‰ for ordinary chondrites. The determination of the metal-silicate fractionation factor is crucial for discerning between a chondritic or non-chondritic Earth model, and for comparison between Earth and other rocky planets. It may also be employed to provide constraints on the temperatures and oxygen fugacity at which these late-stage metamorphic processes in meteorites occur.