The effect of pressure on sulphur speciation in mid‐ to deep‐crustal arc magmas and implications for the formation of porphyry copper deposits

Piston cylinder experiments are used to investigate the effect of oxygen fugacity (ƒO₂) on sulphur speciation and phase relations in arc magmas at 0.5–1.5 GPa and 840–950 °C. The experimental starting composition is a synthetic trachyandesite containing 6.0 wt% H₂O, 2880 ppm S, 1500 ppm Cl and 3800 ppm C. Redox conditions ranging from 1.7 log units below the Ni–NiO buffer (NNO − 1.7) to NNO + 4.7 were imposed by solid-state buffers: Co–CoO, Ni–NiO, Re–ReO₂ and haematite–magnetite. All experiments are saturated with a COH fluid. Experiments produced crystal-bearing trachydacitic melts (SiO₂ from 60 to 69 wt%) for which major and volatile element concentrations were measured. Experimental results demonstrate a powerful effect of oxidation state on phase relations. For example, plagioclase was stable above NNO, but absent at more reduced conditions. Suppression of plagioclase stability produces higher Al₂O₃ and CaO melts. The solid sulphur-bearing phases and sulphur speciation in the melt are strong functions of ƒO₂, as expected, but also of pressure. At 0.5 GPa, the anhydrite stability field is intersected at NNO ≥ +2, but at 1.0 and 1.5 GPa, experiments at the same ƒO₂ produce sulphides and the stability field of sulphate moves towards higher ƒO₂ by ~1 log unit at 1.0 GPa and ~1.5 log units at 1.5 GPa. As a result, models that appeal to high oxidation state as an important control on the mobility of Cu (and other chalcophiles) during crustal differentiation must also consider the enhanced stability of sulphide in deep- to mid-crustal cumulates even for relatively oxidized (NNO + 2) magmas. Experimental glasses reproduce the commonly observed minimum in sulphur solubility between the S²⁻ and S⁶⁺ stability fields. The solubility minimum is not related to the Fe content (Fe²⁺/Fe³⁺ or total) of the melt. Instead, we propose this minimum results from an unidentified, but relatively insoluble, S-species of intermediate oxidation state.