The effect of H2O on the crystallization of orthopyroxene in a high-Mg andesitic melt

Near-liquidus crystallization experiments were performed on a high-Mg andesite from the Izu-Bonin-Mariana forearc to study the effect of H2O on the orthopyroxene crystallization temperature. Experiments were conducted at 200 and 500 MPa in an internally heated pressure vessel at temperatures ranging from 1260 to 1075 °C. Orthopyroxene was the liquidus phase at both investigated pressures. The H2O content in quenched glasses was quantified by Raman spectroscopy. The following equation describes the liquidus depression for orthopyroxene at 200 MPa: ΔTdry liq=26.02(±1.84)·CH2O0.933(±0.050), where ΔTdry liq is the difference between the anhydrous liquidus temperature (in °C) and the liquidus at a specific melt water concentration CH2O (in wt%). A linear equation can describe the liquidus depression curve at 500 MPa: ΔTdry liq=25.72(±0.20)·CH2O. The experimental data set shows that the orthopyroxene liquidus depression is independent of absolute melt H2O concentration at constant pressure. Thermodynamic and petrological models predict a more pronounced nonlinear behavior and generally overestimate the role of H2O in the depression of the orthopyroxene liquidus at H2O contents below 5 wt%, while partly underestimating it at higher H2O contents. Comparing our data set with experimental results from the literature for a high-Mg basaltic andesite and different boninites confirms the validity of our orthopyroxene liquidus parameterization for a wider range of melt compositions. A comparison with previous experimental data obtained for olivine and plagioclase shows that the effect of H2O on the depression of the liquidus temperature of orthopyroxene is slightly lower than for olivine, while the plagioclase liquidus temperature is depressed more significantly in the presence of H2O. Our experimental data can be used to predict liquidus temperatures for hydrous melts in which orthopyroxene is the liquidus phase. Moreover, the empirical parameterization can be incorporated into petrological models to improve the modeling of crystallization paths in hydrous magmas.