The Journal of Petrology, Volume 38, Issue 12: December 1997.

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

JD Webster

Department of Earth and Planetary Sciences, American Museum of Natural History, Central park West at 79th Street, New York, NY 10024-5192, USA. E-mail: jdw@amnh.org

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 H₂O-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 phase-saturated felsic liquids, suggesting that some portions of each of these magmas might have been saturated in volatile phases before melt inclusion entrapment and eruption.

Key words: chloride; volcanic degassing; experimental petrology; melt inclusions

Pages 1793-1807