Water-saturated and undersaturated melting relations in a Paricutin andesite and an estimate of water content in the natural magma

Abstract

Phase relations have been determined for a Paricutin Volcano andesite at pressures to 10 kilobars and for H2O contents in the melt of 2 to 10 weight percent. Runs were made under H2O-saturated and undersaturated conditions. In undersaturated runs a H2O-CO2 fluid phase was always present. Fugacity of H2O in melt, which is directly related to H2O content in the melt, was calculated from thermodynamic data. Plagioclase was found to be the liquidus phase when H2O contents in melt were less than about two percent. With more H2O, orthopyroxene, in some cases joined by olivine, assumes the liquidus. Clinopyroxene crystallizes near the liquidus only for H2O contents greater than five percent. The upper temperature stability limit of hornblende is about 950° C, well below the other silicate liquidi except at H2O-saturated conditions above 5 kb.