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

Abstract

The solubilities of chloride (Cl −) and H 2O in aluminosilicate melts of basalt, andesite, and latite compositions saturated in aqueous vapor and/or hydrosaline liquid were determined at 2000 bars and ≈1 bar by melting mixtures of NaCl, KCl, H 2O, and natural and synthetic rock powders and by measuring Cl − and H 2O 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 H 2O-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°C to 1210°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, H 2O- and CO 2-undersaturated magmas fractionate and evolve to more silicic compositions. The solubility of H 2O in silicate melts saturated in aqueous vapor and/or hydrosaline liquid varies inversely with Cl − content. Chloride has little effect on H 2O 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 H 2O solubility is highly sensitive to Cl − content at these conditions. This relationship is a result of highly nonideal mixing of H 2O 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.% H 2O in melt, and only hydrosaline liquid exsolves if the andesite melt contains deletion ≥2 wt.% Cl − and <4 wt.% H 2O. At 2000 bars and temperatures near 1100°C, the distribution coefficients [D Cl = (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 ≥2000 bars.