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Published December 22, 2022 | public
Journal Article

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.

Additional Information

ECH was funded by a Caltech Geology Option Post-Doctoral Fellowship and a Caltech Centre for Comparative Planetary Evolution (3CPE) research grant, and is supported by the Hazards and Risk Management Programme, which is part of New Zealand Strategic Science Investment Funding (SSIF) from the New Zealand Ministry of Business, Innovation & Employment (MBIE). PL is funded by an Embiricos Trust scholarship from Jesus College, University of Cambridge.

Additional details

August 22, 2023
October 24, 2023