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

Several lines of evidence point to the existence of a pre-eruptive flu id phase in the magmas erupted in 1912 at the Valley of Ten Thousand S mokes, Alaska. The discovery of rare, Cu-rich bubbles in some melt inc lusions is best explained by the random entrapment of a low-density, f luid phase within growing phenocrysts. The H2O, S and Cl concentration s of the high-silica rhyolite are also consistent with fluid saturatio n in the rhyolite. Water contents, as determined by infrared spectrosc opy on doubly polished melt inclusions, cluster between 3.5 and 4.5 wt % H2O, without any apparent differences between magmas vented explosiv ely and effusively. These concentrations would be sufficient for H2O s aturation at a pressure of approximately 100 MPa, though the presence of other volatiles such as CO2 and SO2 could allow saturation with res pect to a fluid phase at higher pressures. The S, Cu and Cl contents o f the phenocryst assemblages, as determined by XRF analyses, are too l ow to account for the decrease in the concentrations of S and Cu in th e melt, and only modest increase in Cl, with differentiation. Therefor e, the behavior of these volatile elements was controlled either by cr yptic fractionation of phenocryst phases or, more likely, by partition ing into a coexisting fluid phase. The presence of this low-density ph ase apparently provided metals such as Cu with a volatile phase into w hich they could partition; the concentration of Cu in this fluid reach ed 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 ma gma was also controlled by sulfides such as intermediate solid solutio n and pyrrhotite, which crystallized directly from the silicate melt.