Melting relations of hydrous and dry mantle compositions and the genesis of komatiites

The hydrous phase relations of primitive upper mantle model compositions in the CaO-MgO-Al₂O₃-SiO₂-H₂O system with 1, 2 and 5 wt.% H₂O have been investigated with a multianvil high pressure apparatus at 6.5 GPa and temperatures from 1550 °C to 2050 °C. Liquidus temperatures decrease by about 100 °C and the temperature interval to produce about 60% partial melting reduces to less than 50 °C in the 1-2 wt.% H₂O-bearing systems. At an H₂O content of about 5 wt.%, garnet is the first dissolved phase and the stability field of orthopyroxene expands. Aluminum undepleted komatiite magmas can be formed by melting at 200 km depth in a hydrous mantle at significantly lower temperatures than under dry conditions. Thus, komatiite magmas cannot constrain temperatures in the Archean mantle. In addition, crotonic peridotites may have formed as residues of partial melting under various H₂O contents.