Origin of cratonic peridotite and komatiite: Evidence for melting in the wet archean mantle

We have conducted melting experiments of pyrolite containing 1 and 2 wt. % of water at 6.5 GPa by a multiànvil high pressure apparatus. We observed melting at temperatures above 1550°C and 1700°C for pyrolite+2%H₂O and pyrolite+1%H₂O respectively at 6.5 GPa. A small amount of water reduces the solidus temperature by 250-100°C. Another important observation is expansion of the stability field of orthopyroxene above the solidus, and reduction of the stability field of olivine+liquid. The present observation of expansion of the orthopyroxene stability field above the solidus at 6.5 GPa implies that the harzburgite residue can be formed by partial melting of the primitive mantle under the hydrous conditions at the depth of around 200km. Hydrous melting is a key process to fill the missing link between the cratonic peridotite and komatiite; i.e., cratonic peridotite was formed as a residue of the komatiite extraction in a wet Archean mantle.