The role of dissolved sulfide in controlling copper speciation in basaltic melts

Abstract

This study evaluates changes in copper (Cu) speciation that occur in sulfate-dominated basaltic and andesitic magmas equilibrated at oxygen fugacities (fO2′s) above the nickel-nickel oxide (NNO) buffer. Cu K-edge microfocused X-ray absorption fine structure spectroscopy (XAFS) data are presented from both natural and synthetic silicate glasses. Natural samples analyzed include olivine-hosted melt inclusions from tephra of mafic cinder cones in the Lassen segment of the Cascade arc (USA) and from the Michoacán-Guanajuato volcanic field (Mexico) as representative samples from melts equilibrated at fO2 > NNO. A comparison with melts equilibrated at fO2 < NNO is provided by analysis of olivine-hosted melt inclusions from Kīlauea Volcano. Data are also presented from copper- and sulfur-bearing synthetic hydrous andesitic glasses synthesized over a range of fO2, from roughly NNO-2 to NNO+2. The Cu spectroscopy data from the natural and synthetic glasses show two dominant Cu species, Cu1+ oxides (referred to here as Cu–O) and Cu1+ sulfides (referred to here broadly as Cu-S, but not precluding Cu-Fe-S species). The relative proportion of each species present correlates with the relative concentration of dissolved sulfide in the melt. Synthetic sulfur-bearing glasses equilibrated at NNO-1.2 were found to contain exclusively Cu-S species. Sulfur-bearing experimental glasses equilibrated at NNO-0.5 give calculated Cu–O/(Cu–O + Cu-S), defined here as the “Cu–O fraction”, of < 0.10, whereas sulfur-bearing glasses synthesized at NNO+0.6 and NNO+1.8 give calculated Cu–O fraction > 0.96. Natural melt inclusions from Lassen and Kīlauea show a bimodal distribution in Cu–O fraction, with overlapping ranges, of 0.14–0.77 for Lassen and 0.18––0.78 for Kīlauea. Michoacán-Guanajuato inclusions yield Cu–O fractions of 0.68–0.91. The difference in the calculated proportions of Cu–O to Cu-S species appear correlated with available sulfide in the melt. As relative S2- concentrations decrease, the dissolved Cu species in the melt evolves from dominantly Cu-S to Cu–O. This includes melts equilibrated at fO2’s where S6+ is the dominant S species. At intermediate sulfide abundances both species appear to coexist. Thermodynamic modeling of the Cu speciation in these silicate glasses suggests that speciation of Cu as a CuFeS2 melt species (akin to chalcopyrite or intermediate solid solution) most accurately predicts the measured Cu species. The modeling suggests that aFeO in the silicate melt, fO2 and melt S2- (expressed as fS2) are the most important parameters controlling the proportions of Cu–O vs. Cu-S species. Our results provide a new perspective for understanding Cu solubility, transport, and partitioning in magmatic systems.