Experimental constraints on the differentiation of CM-like planetary embryos

Abstract

This study investigates the onset of differentiation of hydrous (ice-bearing) planetary embryos in the early Solar System. To do so, we conducted high-pressure (1 GPa) melting experiments on a CM2 chondrite (MCY12002) across a temperature range of 1050-1400°C, with durations varying from 1 to 24 hours. Heating of the CM chondrite at near-solidus conditions results in the thermal decomposition of hydrated minerals composing the matrix of the meteorite and metamorphic recrystallization of olivine crystals. The resulting mineral assemblages are composed of abundant olivine and scarce Al-rich-low-Ca pyroxene and kirschsteinite. Silicate glass is present in all experiments, increasing from <5 area% at 1050-1200°C to 16 area% at 1400°C. A sulphur-rich phase is also observed. The abundance of olivine can be interpreted as the combination of: i) the initially high Mg content in phyllosilicates, correlated to the extent of aqueous alteration, and ii) the reduction in oxygen fugacity conditions regulated by the devolatilization (H2O and CO2) of the starting chondrite. This latter mechanism could explain the gap in oxygen fugacity between the most oxidized chondrites and the most oxidized achondrites, highlighting the impact of volatile loss during the early differentiation of CM-like bodies.