Sulfide and sulfate saturation in hydrous silicate melts

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Abstract
Hydrothermal experiments have been conducted on a sulfur saturated dacitic melt over a range of pressure, temperature, oxygen fugacity, and melt FeO content in order to examine the effects of these variables on sulfur solubility in fractionated melts. Experiments done under both reducing (GCH, QFM buffers) and oxidizing (MNO, HM buffers) conditions indicate the solubility of sulfur increases with increasing total pressure ( = fluid pressure) over the pressure range 100 to 300 MPa. GCH buffered experiments (1025°C) with 18 to 30 wt % FeO show sulfur solubilities at sulfide saturation ranging from 0.1 to 0.5 wt % S; increasing pressure from 100 to 200 MPa increases sulfur solubility by 500 to 1000 ppm, with the greatest increase observed in more FeO‐rich melts. GCH buffered melts with <5 wt % FeO show no measurable change in sulfur solubility (300±150 ppm S) between 100 and 200 MPa at 1025°C. QFM buffered experiments (1025°C) with 10.0 to 12.5 wt % FeO show sulfur solubility increasing from 600 to 1000 ppm between 100 and 200 MPa. QFM experiments with lower FeO contents (2 to 8 wt %) showed no measurable (±200 ppm S) effect of temperature (912°C to 1025°C) or pressure (100 to 220 MPa) on sulfur solubility. Experiments done under oxidizing conditions of the HM and MNO buffers (1025°C) show sulfur solubilities in melts with 3 to 5 wt % FeO ranging from ∼1400 ppm at 100 MPa to ∼3000 ppm at 300 MPa. More importantly however, the change to more oxidizing conditions is accompanied by a change from sulfide (an FeS‐rich melt) to sulfate (crystalline CaS04) saturated conditions. Stabilization of anhydrite as a magmatic phase (1025°C, 100 to 300 MPa) is also accompanied by a significant increase in sulfur solubility relative to saturation values for more reduced melts with similar FeO contents. The results of this study show that upper crustal oxidation‐reduction and crystal fractionation processes may exert considerable influence on the amount of sulfur contained in magmas erupted at the surface. These results are of basic importance in understanding volatile transport and volcanic degasing processes on planets such as the Earth, Mars, and Venus.