Planetary and meteoritic Mg/Si and d30Si variations inherited from solar nebula chemistry

TL;DR

This study investigates the variations in Mg/Si and δ30Si inherited from solar nebula chemistry, using isotope analysis of meteorites and planetary materials to refine models of planetary composition and core formation.

Contribution

It demonstrates that nebular isotopic fractionation, not core processes, explains Si isotope variations, leading to revised estimates of Earth's core composition and planetary Mg/Si ratios.

Findings

Nebular fractionation explains Si isotope variations in meteorites and planetary mantles.

Revised Earth's core Si concentration is approximately 3.6 wt%.

Correlated Mg/Si and δ30Si variations follow a slope of 1 in meteorites.

Abstract

The bulk chemical compositions of planets are uncertain, even for major elements such as Mg and Si. This is due to the fact that the samples available for study all originate from relatively shallow depths. Comparison of the stable isotope compositions of planets and meteorites can help overcome this limitation. Specifically, the non-chondritic Si isotope composition of the Earth's mantle was interpreted to reflect the presence of Si in the core, which can also explain its low density relative to pure Fe-Ni alloy. However, we have found that angrite meteorites display a heavy Si isotope composition similar to the lunar and terrestrial mantles. Because core formation in the angrite parent-body (APB) occurred under oxidizing conditions at relatively low pressure and temperature, significant incorporation of Si in the core is ruled out as an explanation for this heavy Si isotope signature. Instead, we show that equilibrium isotopic fractionation between gaseous SiO and solid forsterite at 1370 K in the solar nebula could have produced the observed Si isotope variations. Nebular fractionation of forsterite should be accompanied by correlated variations between the Si isotopic composition and Mg/Si ratio following a slope of 1, which is observed in meteorites. Consideration of this nebular process leads to a revised Si concentration in the Earth's core of 3.6 (+6.0/-3.6) wt% and provides estimates of Mg/Si ratios of bulk planetary bodies.