Abstract
Phase relations of a Mount Hood andesite, which has the composition of an average orogenic andesite, have been determined as a function of O2 fugacity at 1 atm and of H2O fugacity to pressures of 10 kb, at O2 fugacities of the quartz-fayalite-magnetite (QFM) buffer. All runs contained either a H2O or H2O–CO2 fluid phase; melts in runs with a H2O–CO2 fluid phase were H2O undersaturated. The H2O contents of the melts and H2O fugacities were calculated from NaAlSi3O8–H2O thermo-dynamic data on the assumption of ideal mixing in the system H2O–CO2.
One-atmosphere runs show that melting relations of silicates are little affected by fo2 but that both ilmenite- and magnetite-out temperatures are raised by higher fo2. Ilmenite precipitates at higher temperature than magnetite. In these runs and in all runs at high pressure with H2O and H2O–CO2 fluid phases, oxides were not stable at temperatures of the silicate liquidus. Oxides might be stable on the silicate liquidus if fo2 rose two or more log units above the Ni–NiO (NNO) buffer. However, calculations indicate that in natural magmas, those processes which might change fo2 —crystal-liquid equilibria or exchange of H2, or H2 and H2O with the wall rocks—cannot raise fo2 by that magnitude. Because differentiation of basalt melts to andesite must involve iron-rich oxide phase subtraction, such fractionation models appear unreasonable.
For the Mount Hood andesite composition, plagioclase is the liquidus phase under H2O–saturated conditions to 5 kb and under H2O–undersaturated conditions at 10 kb when the H2O content of the melt is less than 4.7 wt percent. For higher H2O contents, either orthopyroxene or, at H2O saturation at pressure greater than 8 kb, amphibole assumes the liquidus. In all cases, clinopyroxene crystallizes at lower temperature than orthopyroxene. Melting curves in the H2O–under-saturated region may be contoured either as percent H2O in melt or as PeH2O; in either case, the topology of the various silicate melting curves is different from the case of H2O–saturated melting. Therefore, melting relations determined at H2O–saturated conditions cannot be used successfully to predict melting relations in the H2O–undersaturated region.
Amphibole melting relations were studied isobarically at 5 kb as a function of temperature and fluid-phase composition. Amphibole has a maximum stability temperature of 940 ± 15°C for fluid compositions of 100 to 44 mole percent H2O; for fluids containing more CO2 than 56 percent (or, equivalently, less than 4.4 wt percent H2O in melt), the melting temperature is lower. The same relations would be seen if CO2 were not present and the melt were H2O undersaturated. These rather low melting temperatures, relative to other silicate phases, preclude andesite generation by basalt fractionation involving amphibole at pressures less than 10 kb.
