Early differentiation processes on Mars inferred from silicon isotopes

Accretion of terrestrial planets involved partial or global melting events such as magma oceans or magma ponds. Mars experienced large-scale differentiation very early in its history, as shown by its Sm-146-Nd-142 and Hf-182-W-182 record. The broad variations in epsilon Nd-142 and epsilon W-142 of SNC meteorites highlight the presence of mantle sources that must have remained isolated, at least partly, after the crystallization of a global magma ocean. In this study, we have investigated whether the crystallization of the martian magma ocean could have generated mantle reservoirs characterized by different silicon isotope signatures, as the fractionation of Si isotopes between minerals and melts is known to depend on pressure. Thus, the goal of this study was to investigate whether there were any relationships between magma ocean crystallisation and possible variations in the Si isotope record of SNC meteorites. High resolution silicon isotope measurements were performed on twelve meteorites from the Shergottite, Nakhlite and Chassignite groups using a Neptune Plus MC-ICP-MS in dry plasma mode. The delta Si-30 values are in good agreement with previous studies but display a narrower range of variations with a mean value at -0.46 parts per thousand +/- 0.07 (2SD). A magma ocean crystallization model shows that the range of delta Si-30 in SNCs is consistent with that generated by magma ocean crystallisation. In particular, there is a correlation between calculated Sm-147/Nd-144 for the moderately depleted mantle sources with delta Si-30 values; this correlation is consistent with the crystallization model if one includes trapped melt in the cumulates. In contrast, enriched shergottites displayed a very homogenous composition in Sm/Nd ratios, despite significant variability in delta Si-30. This observation could be related to either fluid-rock interactions or redox effect during magma differentiation. Altogether, silicon isotope compositions of SNC provide new constraints about magma ocean crystallization processes in Mars. (C) 2022 The Author(s). Published by Elsevier Ltd.

Automated summary

The accretion of terrestrial planets, such as Mars, involved melting events like magma oceans. This study investigates the relationship between the crystallization of Mars' magma ocean and variations in the silicon isotope record of SNC meteorites. The results suggest that different mantle reservoirs with distinct silicon isotope signatures could have been generated during the crystallization of the martian magma ocean.