High Sulfur in Primitive Arc Magmas, Its Origin and Implications

Sulfur contents in 98.5% of melt inclusions (MI) from calc-alkaline subduction basalts do not exceed 4000 ppm, whereas experimentally established limits of sulfur solubility in basaltic melts with high fO2 (characteristic of subduction zones, e.g., QFM + 2) surpass 14,000 ppm. Here we show that primitive (Mg# 62-64) subduction melts may contain high sulfur, approaching the experimental limit of sulfur solubility. Up to 11,700 ppm S was measured in olivine-hosted MI from primitive arc basalt from the 1941 eruption of the Tolbachik volcano, Kamchatka. These MI often contain magmatic sulfide globules (occasionally enriched in Cu, Ni, and platinum-group elements) and anhydrite enclosed within a brown, oxidized glass. We conclude that the ubiquitous low sulfur contents in MI may originate either from insufficient availability of sulfur in the magma generation zone or early magma degassing prior to inclusion entrapment. Our findings extend the measured range of sulfur concentrations in primitive calc-alkaline basaltic melts and demonstrate that no fundamental limit of 4000 ppm S exists for relatively oxidized subduction basalts, where the maximum sulfur content may approach the solubility limit determined by crystallization of magmatic anhydrite.

The sulfur solubility minimum and maximum in silicate melt

The behaviour of sulfur in magmas is complex because it dissolves as both sulfide (S2-) and sulfate (S6+) in silicate melt. An interesting aspect in the behaviour of sulfur is the solubility minima (SSmin) and maxima (SSmax) with varying oxygen fugacity (fO2). We use a simple ternary model (silicate–S2–O2) to explore the varying fO2 paths where these phenomena occur. Both SSmin and SSmax occur when S2- and S6+ are present in the silicate melt in similar quantities due to the differing solubility mechanism of these species. At constant T, a minimum in dissolved total S content (wmST) in vapour-saturated silicate melt occurs along paths of increasing fO2 and either constant fS2 or P; for paths on which wmST is held constant with increasing fO2, the SSmin is expressed as a maximum in P. However, the SSmin is not encountered during closed-system depressurisation in the simple system we modelled. The SSmax occurs when the silicate melt is multiply-saturated with vapour, sulfide melt, and anhydrite. The SSmin and SSmax influence processes throughout the magmatic system, such as mantle melting, magma mixing and degassing, and SO2 emissions; and calculations of the pressures of vapour-saturation, fO2, and SO2 emissions using melt inclusions.

Experimental Study on the Elemental Sulfur Solubility in Sour Gas Mixtures

The investigation of elemental sulfur solubility plays critical roles on sour gas reservoir development. In this paper, the solubility of elemental sulfur was directly measured by static method with gas samples from well M1 of a sour gas reservoir in Sichuan Basin. The results show that the solubility of elemental sulfur ranges from 0.001 g/cm3 to 0.968 g/cm3 at 40–98.9 MPa and 15–49.8 MPa. The elemental sulfur solubility increases with increasing temperature and pressure, especially when the pressure is greater than 30 MPa. Moreover, the H2S content in sour gas mixtures is also an important factor affecting elemental sulfur solubility. The elemental sulfur solubility increases with increasing H2S content of the sour gas mixtures. The experimental data in this paper display a consistent trend with the reported experimental data. Based on the experimental results, the chrastil-type model, Robert’s model and Hu’s model were investigated and compared. The results show that the chrastil-type model by fitting experimental data in this paper has less error and higher accuracy in calculating elemental sulfur solubility in M gas reservoir. The chrastil-type models proposed in the literature, meanwhile, are only based on the regression of specific gas components and experimental conditions, which lead to a large error in the calculation of elemental sulfur solubility of sour gas samples in this research. The research results provide important basic data and technical support for the development of M gas reservoir.