Experimental constraints on the differentiation of CM-like planetary embryos

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 h, to investigate the onset of differentiation of hydrous (ice-bearing) planetary embryos in the early Solar System. Heating of the CM chondrite at near-solidus conditions results in the thermal decomposition of the hydrated minerals that comprise the matrix of the meteorite, and their metamorphic recrystallization into 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 less than 5 area% at 1050–1200°C to 16 area% at 1400°C. A sulfur-rich phase is also observed. The exceptionally high abundance of olivine can be interpreted as the combination of the initially high Mg content in phyllosilicates, correlated to the extent of aqueous alteration and the reducing oxygen fugacity conditions regulated by the devolatilization of the chondrite matrix. This latter process 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.