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

Abstract

Sulfur abundances have been determined in silicic and basaltic melt inclusions in olivines from harzburgitic xenoliths and a basaltic lava sample, all from Batan Island. In mantle xenoliths, olivines (Fo 80–91) are present as neoblasts or in finely recrystallized patches. The most magnesian olivines (Fo 89.7–91, CaO < 0.1 wt%) contain primary silicic melt inclusions (SiO 2 = 63.5–60 wt%) with high sulfur concentrations (1440–2540 ppm S), associated with MSS (Monosulfide Solide 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–H 2O-rich vapor phase, a sulfide globule and casual anhydrite strongly suggests high fS 2. The melt and the composite inclusions are interpreted as resulting from the immiscibility between hydrous S-rich silicic melt, sulfide melt, and H 2O–S rich vapor. The S-rich fluid inclusions also indicate the possible existence of an exsolved fluid phase containing sulfur at depth. The moderately silicic melt inclusions (SiO 2 = 62–56.6 wt%) in the Fe-rich olivine (Fo 88.6–80) are depleted with respect to sulfur (S = 430 to 790 ppm), the concentration of which is controlled by the vapor phase. Because the silicic melt inclusions have been previously interpreted as produced by variable extent of melting of the subducted oceanic crust, sulfur in the silicic melts, in the sulfide globules and in the H 2O-rich phase, should therefore be inherited and recycled from the subducted slab through S–H 2O-silicate rich fluids or supercritical fluids (or melts). In the calc-alkaline basalt sample, the olivine phenocrysts (Fo > 85, CaO = 0.25 wt%) and their melt inclusions (CaO/Al 2O 3 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 σ=390) and S/Cl ratio at nearly 1. This is in agreement with the idea that arc basaltic melts may contain high concentrations of sulfur (S > 2000 ppm), at 1200°C. However, the heterogeneous distribution of S and its partitioning between silicate melts, H 2O-rich vapor and S-bearing solid phases as illustrated by the Batan mantle xenoliths would result in highly variable sulfur concentrations in island arc basaltic magmas, mostly controlled by fO 2 and fS 2.