relationships of H2, H2O, CO2, H2S, and SO2. We also executed model runs wherein we set the solubility of H2 in the silicate melt to zero in order to demonstrate how uncertainty in the speciation of H‐species in silicate melts (e.g., finite solubility [Hirschmann et al., 2012; Mysen et al., 2011] vs no solubility ([Newcombe et al., 2017]) propagates into uncertainty in degassing trajectories, particularly those at relatively low fO2. Among these simulations, only the scenario of an arc magma decompressing at QFM= 0 (i.e., H2O‐rich magma in equilibrium with a gas phase containing non‐negligible amounts of H2) was sensitive to this assumption (Fig. 3.5). All calculations are calculated as equilibrium (i.e., batch) isothermal decompression, at 1100 °C. The calculations intended to simulate MORB degassing were started at QFM and 1385 bar, with concentrations of volatiles similar to those calculated for globally representative primary MORB melts (Le Voyer et al., 2018) containing 0.2 wt.% H2O, 1100 ppm CO2, and 1425 ppm S. Increasing CO2 to several thousand ppm has no effect on the trajectories shown. The calculations intended to simulate OIB degassing were started at QFM +1.4 and 2115 bar, with concentrations of volatiles similar to those expected for undegassed Erebus melts (Mousallam et al., 2014) containing 1.5 wt% H2O, 1710 ppm CO2, and 2430 ppm S. The calculations intended to simulate arc degassing were started at QFM +1.5 and 2380 bar, with concentrations of volatiles similar to those observed in melt inclusions from Agrigan volcano, containing 4.5 wt.% H2O, 800 ppm CO2, and 2050 ppm S (e.g., Kelley & Cottrell, 2012). Melt chemistry (including fO2) and gas phase compositions were calculated in 1 bar increments and stopped at 5 bars (total pressure).
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