Sulfur and chalcophile elements in subduction zones: constraints from a laser ablation ICP-MS study of melt inclusions from Galunggung Volcano, Indonesia

Mafic melt inclusions hosted in olivine phenocrysts (Fo sub(89-78)) in high-Mg basalts of Galunggung Volcano (Java, Indonesia) were analyzed in situ by laser ablation ICP-MS to determine concentrations of chalcophile and associated trace elements. Our results indicate that sulfur in the mantle beneath Galunggung is significantly enriched relative to MORB source mantle, suggesting large-scale fluxing of sulfur into the mantle wedge during slab dehydration. Melt-inclusion compositions range from strongly undersaturated to transitional basaltic and are characterized by a wide range of sulfur contents (350-2900 ppm). Chalcophile element concentrations are not affected by exsolution of immiscible sulfide liquids and generally fall within the range of whole-rock samples from other arcs. We infer that primary Galunggung melts contain approximately 290 ppm Ni, 60 ppm Co, 190 ppm Cu, and 3 ppm Pb. Patterns of refractory trace elements point to similar to 15% melting of a MORB-source mantle below Galunggung, which was enriched in LILE and LREE by slab-derived fluids before melting. Based on this value, we use melting model calculations to investigate whether slab-derived contributions to a MORB-source mantle are required to balance the budgets of sulfur and associated chalcophile elements, assuming that all Cu and S originally resided in sulfides and adopting a Cu concentration of 28 ppm in the presubduction mantle, similar to that in MORB-type mantle. Modeling results predict at least 256 to 465 ppm S in the magma source of Galunggung, which is up to twice the amount commonly assumed for MORB sources. A slab-derived origin of the excess sulfur is consistent with super(34)S enrichments that commonly characterize arc magmas and gases.