Garnet fractionation in a hydrous magma ocean and the origin of Al-depleted komatiites: melting experiments of hydrous pyrolite with REEs at high pressure

Melting experiments of a representative mantle composition doped with some key trace elements were conducted under hydrous conditions at pressures to 20 GPa. The results demonstrate that only ∼5–25% of garnet fractionation and relatively low pressures (<8 GPa) and low temperatures (∼1300–1500°C) are required to produce Al-depleted Barberton-type komatiites (>3.5 Ga) in the solidifying hydrous magma ocean. This suggests that the Archean mantle was much cooler than so far believed from the origin of the komatiites based on anhydrous experiments.