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Si-Mg isotopes in enstatite chondrites and accretion of reduced planetary bodies.

ABSTRACT: Among the primitive meteorite classes, Enstatite Chondrites (EC) are believed to share a common origin with the Earth due to its close similarity with terrestrial mantle (Bulk Silicate Earth, BSE) for numerous isotope systematics. Si isotopes are an exception to this trend and the large ?30Si difference of ~0.3‰ between bulk EC and BSE has been used to argue against any major contribution of EC like planetary materials in Earth's accretion. However, Si possess a bimodal distribution among silicate and metallic fractions of EC because of its formation under highly reducing conditions. Based on high precision Si isotope analyses in micro-milled phase separates of EH3 chondrites, here we report the presence of significantly light Si isotopes in EC-metals (?30Si???-6.94?±?0.09‰, Mg/Si?=?~0.001) whereas its silicate phases are isotopically heavier (Av. ?30SiEC-silicates?=?-0.33?±?0.11‰, Mg/Si?=?~1.01) and closer to BSE (?30SiBSE?=?-0.29?±?0.08‰). We discuss the origin of the observed Si isotope heterogeneity in terms of gas-solid interaction processes associated with metal-silicate condensation at high C/O environment (~0.83). Although the elevated ?30Si of BSE compared to chondrites is consistent with earlier conclusions that lighter Si has partitioned into Earth's metallic core, our results indicate that the super-chondritic Si isotope composition of BSE does not reflect the sole consequence of high temperature-pressure core and mantle equilibration in a deep magma-ocean. Instead, Si along with Mg isotope analyses carried out in the same aliquot of EC micro-phase separates suggest that processes such as metal-silicate Si isotope fractionation at reduced nebular environment and vapor loss of lighter Si isotopes during planetary volatilization were also influential in establishing the Si isotope composition of terrestrial mantle.

SUBMITTER: Sikdar J

PROVIDER: S-EPMC6985146 | biostudies-literature | 2020 Jan

REPOSITORIES: biostudies-literature

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Si-Mg isotopes in enstatite chondrites and accretion of reduced planetary bodies.

Sikdar Jinia J Rai Vinai K VK

Scientific reports 20200127 1

Among the primitive meteorite classes, Enstatite Chondrites (EC) are believed to share a common origin with the Earth due to its close similarity with terrestrial mantle (Bulk Silicate Earth, BSE) for numerous isotope systematics. Si isotopes are an exception to this trend and the large δ<sup>30</sup>Si difference of ~0.3‰ between bulk EC and BSE has been used to argue against any major contribution of EC like planetary materials in Earth's accretion. However, Si possess a bimodal distribution a ...[more]

PMID: 31988372

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Project description:A close relationship between CM and CO chondrites has been suggested by previous petrologic and isotopic studies, leading to the suggestion that they may originate from similar precursor materials or even a common parent body. In this study, we evaluate the genetic relationship between CM and CO chondrites using Ti, Cr, and O isotopes. We first provide additional constraints on the ranges of ε50Ti and ε54Cr values of bulk CM and CO chondrites by reporting the isotopic compositions of CM2 chondrites Murchison, Murray, and Aguas Zarcas and the CO3.8 chondrite Isna. We then report the ε50Ti and ε54Cr values for several ungrouped and anomalous carbonaceous chondrites that have been previously reported to exhibit similarities to the CM and/or CO chondrite groups, including Elephant Moraine (EET) 83226, EET 83355, Grosvenor Mountains (GRO) 95566, MacAlpine Hills (MAC) 87300, MAC 87301, MAC 88107, and Northwest Africa (NWA) 5958, and the O-isotope compositions of a subset of these samples. We additionally report the Ti, Cr, and O isotopic compositions of additional ungrouped chondrites LaPaz Ice Field (LAP) 04757, LAP 04773, Lewis Cliff (LEW) 85332, and Coolidge to assess their potential relationships with known carbonaceous and ordinary chondrite groups. LAP 04757 and LAP 04773 exhibit isotopic compositions indicating they are low-FeO ordinary chondrites. The isotopic compositions of Murchison, Murray, Aguas Zarcas, and Isna extend the compositional ranges defined by the CM and CO chondrites in ε50Ti versus ε54Cr space. The majority of the ungrouped carbonaceous chondrites with documented similarities to the CM and/or CO chondrites plot outside the CM and CO group fields in plots of ε50Ti versus ε54Cr, Δ17O versus ε50Ti, and Δ17O versus ε54Cr. Therefore, based on differences in their Ti, Cr, and O isotopic compositions, we conclude that the CM, CO, and ungrouped carbonaceous chondrites likely represent samples of multiple distinct parent bodies. We also infer that these parent bodies formed from precursor materials that shared similar isotopic compositions, which may indicate formation in regions of the protoplanetary disk that were in close proximity to each other.

Dataset Information

Si-Mg isotopes in enstatite chondrites and accretion of reduced planetary bodies.

Publications

Si-Mg isotopes in enstatite chondrites and accretion of reduced planetary bodies.

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