CHLORIDE SOLUBILITY IN FELSIC MELTS AND THE ROLE OF CHLORIDE IN MAGMATIC DEGASSING

The degassing of Cl- and H2O-bearing magmas has been investigated expe rimentally at similar to 1 bar and 2000 bars and 800-1075 degrees C by determining the solubility of H2O and Cl- in felsic to intermediate l iquids variably enriched in F, P, B, C, and excess alkalis or aluminum . Chloride solubility in H2O-undersaturated silicate liquids is low an d increases with increasing values of the molar [(Al + Na + Ca + Mg)/S i] of silicate liquids, F content, and abundance of network-modifying Al, and pressure. The presence of B or P in peraluminous felsic liquid s has no discernible influence on Cl- solubility. The experimental sol ubility data are compared with H2O and Cl- in silicate melt inclusions from seven high-silica rhyolites and tin and topaz rhyolites of weste rn North America. Small ranges in Cl- concentration, minimum Cl- conte nts of 600-800 ppm, large ranges in H2O content, and steeply negative to near-infinite slopes in plots of H2O vs Cl- for many of the melt in clusions are similar to abundances and trends of H2O vs Cl- for volati le phase-saturated felsic liquids, suggesting that some portions of ea ch of these magmas might have been saturated in volatile phases before melt inclusion entrapment and eruption.