The iron isotope composition of enstatite meteorites: Implications for their origin and the metal/sulfide Fe isotopic fractionation factor

Abstract

Despite their unusual chemical composition, it is often proposed that the enstatite chondrites represent a significant component of Earth’s building materials, based on their terrestrial similarity for numerous isotope systems. In order to investigate a possible genetic relationship between the Fe isotope composition of enstatite chondrites and the Earth, we have analyzed 22 samples from different subgroups of the enstatite meteorites, including EH and EL chondrites, aubrites (main group and Shallowater) and the Happy Canyon impact melt. We have also analyzed the Fe isotopic compositions of separated (magnetic and non-magnetic) phases from both enstatite chondrites and achondrites.On average, EH3–5 chondrites (δ56Fe=0.003±0.042‰; 2 standard deviation; n=9; including previous literature data) as well as EL3 chondrites (δ56Fe=0.030±0.038‰; 2SD; n=2) have identical and homogeneous Fe isotopic compositions, indistinguishable from those of the carbonaceous chondrites and average terrestrial peridotite. In contrast, EL6 chondrites display a larger range of isotopic compositions (−0.180‰<δ56Fe<0.181‰; n=11), a result of mixing between isotopically distinct mineral phases (metal, sulfide and silicate). The large Fe isotopic heterogeneity of EL6 is best explained by chemical/mineralogical fragmentation and brecciation during the complex impact history of the EL parent body.Enstatite achondrites (aubrites) also exhibit a relatively large range of Fe isotope compositions: all main group aubrites are enriched in the light Fe isotopes (δ56Fe=−0.170±0.189‰; 2SD; n=6), while Shallowater is, isotopically, relatively heavy (δ56Fe=0.045±0.101‰; 2SD; n=4; number of chips). We take this variation to suggest that the main group aubrite parent body formed a discreet heavy Fe isotope-enriched core, whilst the Shallowater meteorite is most likely from a different parent body where core and silicate material remixed. This could be due to intensive impact-induced shearing stress, or the ultimate destruction of the Shallowater parent body.Analysis of separated enstatite meteorite mineral phases show that the magnetic phase (Fe metal) is systematically enriched in the heavier Fe isotopes when compared to non-magnetic phases (Fe hosted in troilite), which agrees with previous experimental observations and theoretical calculations. The difference between magnetic and non-magnetic phases from enstatite achondrites provides an equilibrium metal–sulfide Fe isotopic fractionation factor of Δ56Femetal–troilite=δ56Femetal−δ56Fetroilite of 0.129±0.060‰ (2SE) at 1060±80K, which confirms the predictions of previous theoretical calculations.