Transfer of sulfur in subduction settings: an example from Batan Island (Luzon volcanic arc, Philippines)

Sulfur abundances have been determined in silicic and basaltic melt inclusi ons in olivines from harzburgitic xenoliths and a basaltic lava sample, all from Batan Island. In mantle xenoliths, olivines (Fo(80-91)) are present a s neoblasts or in finely recrystallized patches. The most magnesian olivine s (Fo(89.7-91) CaQ < 0.1 wt%) contain primary silicic melt inclusions (SiO2 = 63.5-60 wt%) with high sulfur concentrations (1440-2540 ppm S), associat ed with MSS (Monosulfide Solids Solution) globules. The occurrence of both S-rich silicic melts saturated with respect to sulfur, at fO(2) close to Ni -NiO buffer, and multiphase fluid inclusions composed of an S-H2O-rich vapo r phase, a sulfide globule and casual anhydrite strongly suggests high fS(2 ) The melt and the composite inclusions are interpreted as resulting from t he immiscibility between hydrous S-rich silicic melt, sulfide melt, and H2O -S rich vapor. The S-rich fluid inclusions also indicate the possible exist ence of an exsolved fluid phase containing sulfur at depth. The moderately silicic melt inclusions (SiO2=62-56.6 wt%) in the Fe-rich olivine (Fo(88.6- 80)) an depleted with respect to sulfur (S = 430 to 790 ppm), the concentra tion of which is controlled by the vapor phase, Because the silicic melt in clusions have been previously interpreted as produced by variable extent of melting of the subducted oceanic crust, sulfur in the silicic melts, in th e sulfide globules and in the H2O-rich phase, should therefore be inherited and recycled from the subducted slab through S-H2O-silicate rich fluids or supercritical fluids (or melts). In the calc-alkaline basalt sample, the o livine phenocrysts (Fo > 85, CaO = 0.25 wt%) and their melt inclusions (CaO /Al2O3 from 0.8 to 1.15) have recorded early stages of crystallization. The sulfur concentrations for these calc-alkaline basaltic melts are estimated between 1720 and 3200 ppm, with a mean value at 2550 ppm (1 sigma = 390) a nd S/Cl ratio at nearly 1. This is in agreement with the idea that are basa ltic melts may contain high concentrations of sulfur (S > 2000 ppm), at 120 0 degrees C. However, the heterogeneous distribution of S and its partition ing between silicate melts, H2O-rich vapor and S-bearing solid phases as il lustrated by the Batan mantle xenoliths would result in highly variable sul fur concentrations in island are basaltic magmas, mostly controlled by fO(2 ) and fS(2) (C) 1999 Elsevier Science B.V. All rights reserved.