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

This study evaluates changes in copper (Cu) speciation that occur in sulfate-dominated basaltic and andesitic magmas equilibrated at oxygen fugacities ( f O 2 ′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 Michoacan-Guanajuato volcanic field (Mexico) as representative samples from melts equilibrated at f O 2 > NNO. A comparison with melts equilibrated at f O 2 < 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 f O 2 , from roughly NNO-2 to NNO + 2. The Cu spectroscopy data from the natural and synthetic glasses show two dominant Cu species, Cu 1+ oxides (referred to here as Cu–O) and Cu 1+ 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. Michoacan-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 S 2- concentrations decrease, the dissolved Cu species in the melt evolves from dominantly Cu-S to Cu–O. This includes melts equilibrated at f O 2 ’s where S 6+ is the dominant S species. At intermediate sulfide abundances both species appear to coexist. Thermodynamic modeling of the Cu speciation in these silicate melts suggests that speciation of Cu as a CuFeS 2 melt species (akin to chalcopyrite or intermediate solid solution) most accurately predicts the measured Cu species. The modeling suggests that a FeO in the silicate melt, f O 2 and melt S 2- (expressed as f S 2 ) 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.