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

摘要

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-ICPMS. Silicates in chondrules show variations in delta Si-30 (% variations of Si-30/Si-28 relative to NBS-28) ranges from -1.06 +/- 0.13 parts per thousand (2 S.E.) to -0.38 +/- 0.11 parts per thousand. delta Si-30 in matrix silicates ranges from -0.96 +/- 0.18 parts per thousand to -0.22 +/- 0.12 parts per thousand. The delta Si-30-value of silicate phases varies independently of Mg/Si, ruling out simple equilibrium condensation from nebular gas. Some silicates in both enstatite chondrites have delta Si-30-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.