Sulfur solubility and magmatic sulfides in submarine basalt glass

Abstract

A suite of 35 fresh basalt glasses collected in >1000-m water depth from 16 localities on or near the Juan de Fuca Ridge and 17 localities elsewhere in the Atlantic and Pacific basins were examined petrographically and with the microprobe. Most are ridge-type tholeiites, but they span the entire range from quartz to nepheline normative varieties. Every sample contains magmatic sulfide. These are globules which occur in clear glass and are composed of finely intergrown pyrrhotite, Cu-Fe sulfide, and frequently pentlandite. Magnetite can be observed in some of the more coarsely intergrown globules. Bulk sulfide compositions generally include >10% Cu + Ni. Petrographie and chemical data suggest that some basalts erupted in a sulfide-saturated state, and the remainder were nearly saturated prior to their being quenched. Glass inclusions in phenocrysts either have compositions similar to matrix glass or are relatively depleted in elements entering host phenocrysts. In either case, their S concentrations are generally similar to or greater than S concentrations of matrix glasses. This and the usually pristine character of fresh basalts with respect to other volatile and trace elements suggest that S in glasses is predominantly juvenile. Concentrations of S dissolved in glass range from 1000 to 1800 ppm and exhibit a nearly perfect linear covariance with Fe concentrations. Because all the glasses contain sulfides this relationship implies that the Fe concentration in the melt controls gas composition with respect to oxygen and sulfur species. However, the moderate range in Fe concentration of the submarine basalts of this study corresponds to a range in ƒs2 and ƒo2 of only about half an order of magnitude each. That is, they all existed under essentially similar ƒs2 and ƒo2 at equivalent temperatures. Fractional crystallization of olivine should cause sulfide liquids to exsolve from the magma, whereas plagioclase fractionation should drive the liquid further from sulfide saturation. However, in more typical fractionation models involving plagioclase and olivine separation in a ratio of ∼10:3, liquid compositions are driven approximately parallel to a sulfide saturation plane if constant temperature is assumed. Thus fractional crystallization need not involve separation of large quantities of sulfide. Many of the most juvenile unfractionated submarine basaltic magmas are probably sulfide saturated and contain ∼900 ppm S.