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

Jinia Sikdar,Vinai K Rai

doi:10.1038/s41598-020-57635-1

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 δ30Si 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 (δ30Si ≥ −6.94 ± 0.09‰, Mg/Si = ~0.001) whereas its silicate phases are isotopically heavier (Av. δ30SiEC-silicates = −0.33 ± 0.11‰, Mg/Si = ~1.01) and closer to BSE (δ30SiBSE = −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 δ30Si 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.

Highlights

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
Our results reveal considerable Si isotope heterogeneity among phase separates of EH3 chondrites with the metal-enriched phases being characterized by significantly light Si isotope composition (δ30SiEC-metals ranges from −6.94 ± 0.09‰ to −4.08 ± 0.05‰)
The correlated variation between δ30Si and Mg/Si ratio across diverse meteorite types has been explained by equilibrium isotope fractionation between gaseous SiO and high temperature RLE rich condensate in the solar nebula[35]

Summary

Introduction

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. The relatively larger δ30Si offset between BSE and EC (Δ30SiBSE-EC = δ30SiBSE − δ30SiBulk EC = ~0.34‰) demands ~26 wt% Si in the core for enstatite chondrite Earth model[23], which is unrealistically high and cannot explain core’s geophysical properties Based on these estimates, it has been concluded that a major portion of the Earth is made up of a combination of ordinary-carbonaceous chondrites (LL, CI, CO) and a maximum of 15% enstatite chondrites[23]. It has been concluded that a major portion of the Earth is made up of a combination of ordinary-carbonaceous chondrites (LL, CI, CO) and a maximum of 15% enstatite chondrites[23] Such mass balance calculations to determine Si content of the core assume that different proportions of light and heavy Si isotopes were accumulated in the core and mantle of the Earth during high temperature-pressure metal and silicate equilibration processs generated by core formation above ~2500 K. The most influential cause for the observed δ30Si variations among planetary-scale objects has remained debated, which needs to be first addressed before making any concluding remarks on the building blocks of Earth from the non-chondritic Si isotope composition of BSE

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Conclusion