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

Abstract

Mafic melt inclusions hosted in olivine phenocrysts (Fo89–78) 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 ∼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 34S enrichments that commonly characterize arc magmas and gases. Although modeling suggests that elevated copper concentrations relative to MORB can be the consequence of the higher solubility of sulfur in Galunggung melts, the results do not rule out that the subarc mantle was enriched by slab-derived Cu. Lead must have been added, which is readily explained by slab-derived fluids. Other chalcophile elements (Co, Ni) are largely controlled by the nature of the presubduction mantle.