Sulfur mobility during magma mixing

Sulfur (S) is one of the most enigmatic elements in the magmatic cycle – highly mobile in the magma as it dissolves in the silicic melt, forms a sulfide or sulfate mineral phase (e.g., pyrrhotite, anhydrite), separates into an immiscible sulfide melt or partitions into a volatile phase. It is known that the specific behavior of sulfur is strongly dependent on oxygen fugacity (fO2, in the following referenced by comparing it to the Ni-NiO buffer, NNO) and melt composition as these parameters control sulfur speciation and the sulfur concentration at sulfide/sulfate saturation in the silicic melt. The separation of sulfur from the magma is important for the formation of magmatic and magmatic-hydrothermal ore deposits and is expressed in fumarolic activity in volcanic areas as well as the catastrophic release during volcanic eruptions. How sulfur is partitioned during the mixing of two magmas of distinct composition as the two magmas equilibrate compositionally and with respect to oxidation state, in the case where the magmas may initially be at different fO2, is therefore paramount for our understanding of magma degassing and the mobility of the highly S-affine ore metals such as Cu and Au. We use diffusion-couple experiments of S-bearing andesite and dacite magmas to elucidate transient phenomena.

collaborator: Adam Simon (UMichigan) & Adrian Fiege (AMNH)