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(89-79)) in high-Mg basalts of Galunggung volcano (Java, Indonesia) were analyzed in situ by la ser ablation ICP-MS to determine concentrations of chalcophile and associat ed trace elements. Our results indicate that sulfur in the mantle beneath G alunggung is significantly enriched relative to MORB source mantle, suggest ing large-scale fluxing of sulfur into the mantle wed-e during slab dehydra tion. Melt-inclusion compositions range from strongly undersaturated to transitio nal basaltic and are characterized by a wide range of sulfur contents (350- 2900 ppm). Chalcophile element concentrations are not affected by exsolutio n of immiscible sulfide liquids and generally fall within the range of whol e-rock samples from other arcs. We infer that primary Galunggung melts cont ain approximately 290 ppm. Ni, 60 ppm Co, 190 ppm. 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 mo del calculations to investigate whether slab-derived contributions to a MOR B-source mantle are required to balance the budgets of sulfur and associate d chalcophile elements, assuming that all Cu and S originally resided in su lfides and adopting a Cu concentration of 28 ppm in the presubduction mantl e, similar to that in MORB-type mantle. Modeling results predict at least 256 to 465 ppm S in the magma source of G alunggung, which is up to twice the amount commonly assumed for MORB source s. A slab-derived origin of the excess sulfur is consistent with S-34 enric hments that commonly characterize arc magmas and gases. Although modeling s uggests that elevated copper concentrations relative to MORB can be the con sequence of the higher solubility of sulfur in Galunggung melts, the result s do not rule out that the subarc mantle was enriched by slab-derived Cu. L ead 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. Copyright (C) 2001 Elsevier Science Ltd.