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, cratonic peridotites may have formed as residues of partial melting under various H2O contents. ¿ 1998 American Geophysical Union