EVIDENCE FOR A COPPER-BEARING FLUID IN MAGMA ERUPTED AT THE VALLEY OF TEN-THOUSAND-SMOKES, ALASKA

Several lines of evidence point to the existence of a pre-eruptive fluid phase in the magmas erupted in 1912 at the Valley of Ten Thousand Smokes, Alaska. The discovery of rare, Cu-rich bubbles in some melt inclusions is best explained by the random entrapment of a low-density, fluid phase within growing phenocrysts. The H2O, S and Cl concentrations of the high-silica rhyolite are also consistent with fluid saturation in the rhyolite. Water contents, as determined by infrared spectroscopy on doubly polished melt inclusions, cluster between 3.5 and 4.5 wt% H2O, without any apparent differences between magmas vented explosively and effusively. These concentrations would be sufficient for H2O saturation at a pressure of approximately 100 MPa, though the presence of other volatiles such as CO2 and SO2 could allow saturation with respect to a fluid phase at higher pressures. The S, Cu and Cl contents of the phenocryst assemblages, as determined by XRF analyses, are too low to account for the decrease in the concentrations of S and Cu in the melt, and only modest increase in Cl, with differentiation. Therefore, the behavior of these volatile elements was controlled either by cryptic fractionation of phenocryst phases or, more likely, by partitioning into a coexisting fluid phase. The presence of this low-density phase apparently provided metals such as Cu with a volatile phase into which they could partition; the concentration of Cu in this fluid reached tens to hundreds of times that of the coexisting silicate melt and may have been as rich as 0.05 wt% Cu. The distribution of Cu in the magma was also controlled by sulfides such as intermediate solid solution and pyrrhotite, which crystallized directly from the silicate melt.