The origin of unequilibrated EH chondrites – Constraints from in situ analysis of Si isotopes, major and trace elements in silicates and metal

Abstract

Various lines of evidence suggest that material isotopically similar to enstatite chondrites may have accreted to the terrestrial planets. However, the enrichment of light Si isotopes in bulk enstatite chondrites is not easy to reconcile with the heavy Si isotopic composition of the Bulk Silicate Earth (BSE). To investigate the origin of the light Si isotopic composition of enstatite chondrites, we have obtained in situ Si isotope data and simultaneously major- and trace element data in silicate and metal phases of chondrules, a metal-troilite spherule, and matrix from the enstatite chondrites Sahara 97072 (EH3) and Indarch (EH4) using laser ablation split stream-ICP-MS, which combines femto-second LA-MC-ICP-MS and Quadrupole-ICP-MS. Silicates in chondrules show variations in δ30Si (‰ variations of 30Si/28Si relative to NBS-28) ranges from −1.06 ± 0.13‰ (2 S.E.) to −0.38 ± 0.11‰. δ30Si in matrix silicates ranges from −0.96 ± 0.18‰ to −0.22 ± 0.12‰. The δ30Si-value of silicate phases varies independently of Mg/Si, ruling out simple equilibrium condensation from nebular gas. Some silicates in both enstatite chondrites have δ30Si-values like CI chondrites, whereas Si in other silicates is isotopically lighter, suggesting that the precursor materials of EH chondrites were already depleted in heavy Si isotopes.The metal phases in the matrix show average δ30Si of −6.0 ± 0.6‰. In spite of different metamorphic grades, the fractionation of Si isotopes between matrix metal and silicates in Sahara 97072 and Indarch shows no systematic differences, and thus no re-equilibration of Si isotopes occurred between silicates and metal at metamorphic temperatures below 900 K. The δ30Si-value of metal from a metal-troilite spherule from Sahara 97072 (−8.24 ± 0.12‰) is lower than that of matrix metals. These differences were likely inherited from different formation environments of matrix- and spherule metal. If metal formation occurred under equilibrium conditions, then matrix metal may have formed at higher temperatures than the MTS metal. or at similar temperatures but slightly lower oxygen fugacities, or the MTS metal equilibrated with gas or silicates which were not incorporated into EH chondrites because they were lost from the EH chondrite formation region. Alternatively, the differences in δ30Si of different metals could also reflect variable kinetic isotope fractionation during the formation of metal and exsolution of perryite.The considerably lower δ30Si-values of bulk EH chondrites compared to CI- and other chondrites partly reflects the presence of Si bearing metal and partly silicates with isotopically light Si. The latter indicate loss of a heavy Si-rich silicate component from the EH3 formation region, presumably together with refractory elements. Although the Si isotopic composition of bulk EH chondrites precludes that these represent major building material of the Earth, the combination of complementary heavy Si isotope- and refractory element-enriched reduced materials and carbonaceous or ordinary chondrites could provide a match for the heavy Si isotopic composition of Earth.