Sulfur Speciation, Oxidation-State, and Sulfur Concentration in Backarc Magmas

We have determined the Fe3+/Fe2+ ratio, S content, and the SO42-/S2- ratio (by SK(alpha) X-ray peak shift) for a suite of twelve mid-ocean ridge and backarc basin glasses. These samples range in oxidation state from two orders of magnitude below to 1.5 orders of magnitude above the Ni-NiO oxygen buffer. Sulfur speciation in the glasses is strongly dependent on magmatic oxidation state. We propose that the solubility mechanism for S in basaltic melts changes over the f(O2) range which characterizes convergent margin magmas. In low f(O2), sulfide saturated basaltic melts, such as most MORBs, S is present largely as S2-, and its concentration is controlled by sulfide melt-silicate melt equilibrium and varies positively with f(O2). At higher f(O2)'s, where sulfide melt becomes unstable, S concentration is controlled by melt-vapor equilibrium and decreases with increasing f(O2). For the range of f(O2)'s in the back arc basin basalts in this study (NNO-1 to NNO+1.5), S is dissolved in the silicate in both oxidized and reduced forms. For the lower f(O2) end of this range, S2- dominates and equilibria involving reduced S-species control S melt concentration. At higher f(O2)'s, the proportion of oxidized S increases and melt-vapor equilibria involving these species become more important. Because S partitions strongly into the vapor phase over the f(O2) range investigated, magmatic degassing may be effective at stripping S from the melt. The low S concentrations in island arc and backarc lavas do not necessarily reflect a low S source but rather result from higher f(O2) in the convergent margin environment.