Chloride and water solubility in basalt and andesite melts and implications for magmatic degassing

The solubilities of chloride (Cl-) and H2O in aluminosilicate melts of basalt, andesite, and latite compositions saturated in aqueous vapor and/or hydrosaline liquid were determined at 2000 bars and approximate to 1 bar by melting mixtures of NaCl, KCl, H2O, and natural and synthetic rock powders and by measuring Cl- and H2O in the run product glasses. The abundances of Cl- in several of the aqueous run product liquids were also measured, and the partitioning of Cl- between aqueous vapor and silicate melt was determined for these experiments. Chloride is highly soluble in H2O-poor melts. Maximum Cl- solubilities range from 2.9 wt.% in molten basalt to 1 wt.% in molten latite at relatively high oxygen fugacities, 1040 degrees C to 1210 degrees C, and 2000 bars. The solubility of Cl- varies directly with pressure and the molar ((Al+Na+Ca+Mg)/Si) ratio of aluminosilicate melts. Chloride solubility in basalt melt is an order of magnitude greater than that in silicic melts, so the role of Cl- in driving the exsolution of vapor and/or liquid from magma will increase dramatically as mafic, H2O and CO2-undersaturated magmas fractionate and evolve to more silicic compositions. The solubility of H2O in silicate melts saturated in aqueous vapor and/or hydrosaline liquid varies inversely with Cl- content. Chloride has little effect on H2O solubility with up to about 1.9 wt.% Cl- in melt because the coexisting vapor phase contains little Cl-. Hydrosaline liquid is stable with higher Cl- contents in melt, and H2O solubility is highly sensitive to Cl- content at these conditions. This relationship is a result of highly nonideal mixing of H2O and Cl- at magmatic temperatures; in several Cl--enriched andesite experiments, immiscible vapor and hydrosaline liquid are apparently stable instead of a single Cl--bearing volatile phase. At 2000 bars, Cl--bearing aqueous vapor exsolves with <1 wt.% Cl- in the andesite melt, vapor and hydrosaline liquid exsolve with 1 to 2 wt.% Cl- and <4 wt.% H2O in melt, and only hydrosaline liquid exsolves if the andesite melt contains deletion greater than or equal to 2 wt.% Cl- and <4 wt.% H2O. At 2000 bars and temperatures near 1100 degrees C, the distribution coefficients [D-CI = (wt.% Cl- in aqueous vapor/wt.% Cl- in silicate melt)] for basalt and andesite range from 0.9 to 6 for coexisting aqueous vapors containing 1 to 11 wt.% Cl-, respectively. Silicate melt inclusions in phenocrysts from most basalts and andesites contain <1 wt.% Cl- implying that, at these conditions, only Cl-bearing vapor (not vapor and hydrosaline liquid) will exsolve from most basalt and andesite magmas and that the Cl- contents of the aqueous vapors will be <4 wt.% at pressures greater than or equal to 2000 bars. Copyright (C) 1999 Elsevier Science Ltd.