Melting of a Hydrous Mantle: III. Phase Relations of Garnet Websterite+H2O at High Pressures and Temperatures

A garnet websterite nodule from the Honolulu volcanic series, Oahu, Hawaii, has been melted in the presence of nearly pure H2O. The solidus is intermediate between that of peridotite and gabbro. The curve displays a temperature minimum around 20 kb reflecting the breakdown of plagioclase. The liquidus is between 1130 °C and 1150°C between 10 and 20 kb vapor pressure. Amphibole (pargasitic hornblende) has an extensive stability field, reaching a maximum temperature about 20 °C below the garnet websterite liquidus at 15 kb and a maximum pressure of 27-5 kb at 950 °C. The amphibole-out curve intersects the solidus with a positive slope. Liquids formed by partial melting of garnet websterite are quartz-normative within the stability field of amphibole, but become olivine-normative (tholeiitic) with increasing temperature. Amphibole and clinopyroxene are enriched in Tschermak's molecule at higher temperatures, pargasite content of amphibole increases with increasing pressure. A garnet websterite-rich upper mantle containing modal olivine yields quartz-normative (13-16 per cent), aluminous (21-4 wt. per cent AI2O3) melts at 17 S: P ^ 10 kb and in the presence of nearly pure H2O. However, the presence of amphibole controls the liquid com-position, a situation not found for liquids formed from wet peridotite. In contrast to many basalt liquids, liquid of garnet websterite composition cannot fractionate to andesite by pre-cipitation of amphibole, as amphibole is not a liquidus phase