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 (⁠f_O₂). We use a simple ternary model (silicate–S₂–O₂) to explore the varying f_O₂ 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 mechanisms of these species. At constant T, a minimum in dissolved total S content in vapour-saturated silicate melt (⁠w^(m)_(S_T)⁠) occurs along paths of increasing f_O₂ and either constant f_S₂ or P. For paths on which (⁠w^(m)_(S_T)⁠) is held constant with increasing f_O₂⁠, the SSᵐⁱⁿ is expressed as a maximum in P. The SSᵐⁱⁿ occurs when the fraction of S⁶⁺ in the melt ([S⁶⁺/S_(T)]ᵐ) is 0.25 for constant f_S₂ and [S⁶⁺/S_(T)]ᵐ ≅ 0.75 for constant (⁠w^(m)_(S_T)⁠) and P. A minimum in (⁠w^(m)_(S_T)⁠) is not encountered during closed- or open-system depressurisation in the simple system we modelled. However, the SSᵐⁱⁿ marks a change from reduction to oxidation during degassing. Various SSᵐᵃˣ occur when the silicate melt is multiply-saturated with at least two phases: vapour, sulfide melt, and/or anhydrite. The SSᵐⁱⁿ and SSᵐᵃˣ are important features of magmatic process involving S, such as mantle melting, magma mixing, and degassing. These concepts influence calculations of the pressures of vapour-saturation, f_O₂, and SO₂ emissions using melt inclusions.