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

Near-liquidus crystallization experiments were performed on a high-Mg andesite from the Izu-Bonin-Mariana fore-arc in order 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. H₂O contents in quenched glasses were quantified via Raman spectroscopy. The liquidus depression for orthopyroxene at 200 MPa can be described by the following equation: where ΔT^{dry liq} is the difference between the anhydrous liquidus temperature (in °C) and the liquidus at a specific melt water concentration $CH2O$ (in wt%). The liquidus depression curve at 500 MPa can be described by a linear equation: The experimental dataset demonstrates that the orthopyroxene liquidus depression shows no significant pressure dependence with respect to absolute melt H₂O concentrations. Thermodynamic and petrological models predict a more pronounced non-linear behavior and generally overestimate the role of H₂O on orthopyroxene liquidus depression at H₂O contents below 5 wt% while partly underestimating it at higher H₂O contents. Comparing our dataset with experimental results taken from literature obtained 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 for predicting liquidus temperatures for hydrous melts in which orthopyroxene appears as the liquidus phase. Moreover, the empirical parameterization can be incorporated in petrological models to improve modelling of crystallization paths of hydrous magmas.