Chlorine partitioning between melt and aqueous chloride fluid during granite magma. Degassing I. Decompression-induced melt degassing

Abstract

Based on available experimental data, a computer model was developed for the description of chlorine distribution between a magmatic melt of metaluminous granite composition and an aqueous chloride fluid phase formed during the decompression degassing of magma at pressures ranging from 5 to 0.5–0.3 kbar and temperatures of 800 ± 25°C. The model accounts for the dependence of fluid/melt Cl partition coefficient on pressure and Cl content of the melt. It allows the calculation of Cl and H2O redistribution between melt and fluid during the decompression degassing of magmas under both closed-system conditions, when fluid remains in the system, and open-system conditions, when the released fluid phase is completely of partly removed from the system. The results of numerical modeling revealed the influence of the initial Cl and H2O contents in melt and the degree of system openness on variations in the concentrations of these volatiles in aqueous fluid and melt during magma ascent toward the surface under near-isothermal conditions. The highest Cl concentrations of the fluid (CClfl) are attained at the early stages of decompression degassing at high pressures. With decreasing pressure, CClfl decreases at a rate that increases at increasing degree of system openness. During open-system degassing, the Cl content of melt (CClm) also decreases. In contrast, under closed-system conditions, CClm decreases initially with decreasing pressure, reaches a minimum, and then increases. If decompression degassing begins at P ≤ 1.0-1.25 kbar, CClm changes only slightly irrespective of the initial Cl content of the melt and the degree of system openness.