Fluid-melt partitioning of sulfur in differentiated arc magmas and the sulfur yield of explosive volcanic eruptions

Abstract The fluid-melt partitioning of sulfur (D S fluid/melt ) in differentiated arc magmas has been experimentally investigated under oxidizing conditions (Re–ReO 2 buffer) from 800 to 950 °C at 200 MPa. The starting glasses ranged in composition from trachyte to rhyolite and were synthesized targeting the composition of the residual melt formed after 10–60% crystallization of originally trachy-andesitic, dacitic and rhyodacitic magmas (Masotta and Keppler, 2015). Fluid compositions were determined both by mass balance and by Raman spectroscopy of fluid inclusions. D S fluid/melt increases exponentially with increasing melt differentiation, ranging from 2 to 15 in the trachytic melt, from 20 to 100 in the dacitic and rhyodacitic melts and from 100 to 120 in the rhyolitic melt. The variation of the D S fluid/melt is entirely controlled by the compositional variation of the silicate melt, with temperature having at most a minor effect within the range investigated. Experiments from this study were used together with data from the literature to calibrate the following model that allows predicting D S fluid/melt for oxidized arc magmas: ln D S fluid / melt = 9.2 - 31.4 · nbo t - 1.8 · ASI - 29.5 · Al # + 4.2 · Ca # where nbo t is the non-bridging oxygen atoms per tetrahedron, ASI is the alumina saturation index, Al# and Ca# are two empirical compositional parameters calculated in molar units ( Al # = X Al 2 O 3 X SiO 2 + X TiO 2 + X Al 2 O 3 and Ca # = X CaO X Na 2 O + X K 2 O ). The interplay between fluid-melt partitioning and anhydrite solubility determines the sulfur distribution among anhydrite, melt and fluid. At increasing melt polymerization, the exponential increase of the partition coefficient and the decrease of anhydrite solubility favor the accumulation of sulfur either in the fluid phase or as anhydrite. On the other hand, the higher anhydrite solubility and lower partition coefficient for less polymerized melts favor the retention of sulfur in the melt. At equilibrium conditions, these effects yield a maximum of the sulfur fraction in the fluid phase for slightly depolymerized melts ( nbo t  = 0.05–0.15). Our data allow quantitative predictions of the sulfur yield of explosive volcanic eruptions over a wide range of magma compositions.