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 similar to 5-25% of garnet fractionation and relatively low pressures 1<8 GPa) and low temperatures (similar to 1300-1500 degrees C) are required to produce Al-depleted Barberton-type komatiites (> 3.5 Ga) ill 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. (C) 2000 Elsevier Science B.V. All rights reserved.