EXPERIMENTAL INVESTIGATION OF H2O AND CL- SOLUBILITIES IN F-ENRICHED SILICATE LIQUIDS - IMPLICATIONS FOR VOLATILE SATURATION OF TOPAZ RHYOLITE MAGMAS

To interpret the degassing of F-bearing felsic magmas, the solubilitie s of H2O, NaCl, and KCl in topaz rhyolite liquids have been investigat ed experimentally at 2000, 500, and approximate to 1 bar and 700 degre es to 975 degrees C. Chloride solubility in these liquids increases wi th decreasing H2O activity, increasing pressure, increasing F content of the liquid from 0.2 to 1.2 wt% F, and increasing the molar ratio of ((Al + Na + Ca + Mg)/ Si). Small quantities of Cl- exert a strong inf luence on the exsolution of magmatic volatile phases (MVPs) from F-bea ring topaz rhyolite melts at shallow crustal pressures. Water- and chl oride-bearing volatile phases, such as vapor, brine, or fluid, exsolve from F-enriched silicate liquids containing as little as 1 wt% H2O an d 0.2 to 0.6 wt% Cl at 2000 bar compared with 5 to 6 wt% H2O required for volatile phase exsolution in chloride-free liquids. The maximum so lubility of Cl- in H2O-poor silicate liquids at 500 and 2000 bar is no t related to the maximum solubility of H2O in chloride-goer liquids by simple linear and negative relationships; there are strong positive d eviations from ideality in the activities of each volatile in both the silicate liquid and the MVP(s). Plots of H2O versus Cl- in rhyolite l iquids, for experiments conducted at 500 bar and 910 degrees-930 degre es C, show a distinct 90 degrees break-in-slope pattern that is indica tive of coexisting vapor and brine under closed-system conditions. The presence of two MVPs buffers the H2O and Cl- concentrations of the si licate liquids. Comparison of these experimentally-determined volatile solubilities with the pre-eruptive H2O and Cl- concentrations of five North American topaz and tin rhyolite melts, determined from melt inc lusion compositions, provides evidence for the exsolution of MVPs from felsic magmas. One of these, the Cerro el Lobo magma, appears to have exsolved alkali chloride-bearing vapor plus brine or a single supercr itical fluid phase prior to entrapment of the melt inclusions and prie r to eruption.