Insights into global sulfur cycling from the melt inclusion record

Abstract:

Arc magmas are oxidized relative to MORBs, but the origin of this oxidized signature is debated. It may originate from oxidized slab material present in the mantle during melting. Alternatively, it may be that arc magmas increase in fO₂ during the early stages of differentiation. The solubility of S in magmas is strongly dependent on fO₂. At oxidizing conditions, S solubility is several times greater than at more reduced conditions similar to mid-ocean ridge settings [1, 2]. We take advantage of this relationship by using S contents of olivine-hosted melt inclusions to place constraints on the fO₂ of primitive arc magmas. Concentrations of S were drawn from a compilation of Mg-rich olivine-hosted melt inclusion compositions corrected for olivine crystallization [3]. Primary magma volatile contents were estimated via reverse crystallization corrections using the least-degassed inclusions from each volcano. The data set spans 18 subduction zone segments.

While the S contents of primary arc magmas span a wide range of values, many are higher than that of MORB and require more oxidizing conditions. In arcs with S contents above S solubility limits for reducing conditions, the calculated minimum required fO₂ ranges from QFM to QFM + 1.3. These estimates, calculated using high-Mg inclusions formed during the earliest stages of crystallization, imply that either S content and fO₂ both increase dramatically during early differentiation, or that many arc magmas form in an oxidized mantle. S contents in arc magmas correlate with Cl and Ba, which are commonly released during slab dehydration, suggesting that S is sourced from the subducting slab. High S, and therefore high estimated fO₂, occurs most often in arcs with an intermediate thermal regime. This distribution suggests that the thermal regime of arcs may exert some control on the S content and/or fO₂ of primitive arc magmas.