Chloride solubility in felsic melts and the role of chloride in magmatic degassing

Abstract

The degassing of Cl- and H2O-bearing magmas has been investigated experimentally at ∼1 bar and 2000 bars and 800-1075°C by determining the solubility of H2O and Cl- in felsic to intermediate liquids variably enriched in F, P, B, C, and excess alkalis or aluminum. Chloride solubility in H20-undersaturated silicate liquids is low and increases with increasing values of the molar [(Al+Na+Ca+Mg)/Si] of silicate liquids, F content, the abundance of network-modifying Al, and pressure. The presence of B or P in peraluminous felsic liquids has no discernible influence on Cl- solubility. The experimental solubility data are compared with H2O and Cl- in silicate melt inclusions from seven high-silica rhyolites and tin and topaz rhyolites of western North America. Small ranges in Cl- concentration, minimum Cl- contents 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 inclusions are similar to abundances and trends of H2O vs Cl- for volatile phasesaturated felsic liquids, suggesting that some portions of each of these magmas might have been saturated in volatile phases before melt inclusion entrapment and eruption.