Clorine Solubility in Silicate Melts: New Experiments and Thermodynamic Mixing Model

Abstract

We present new experimental data on Cl solubility in model basalt melts of eutectic compositions diopside (Di)–albite (Ab) and Di–anorthite ± quartz (Qtz). The starting glasses were equilibrated with aqueous fluid H₂O-NaCl-CaCl₂ at 4 kbar in the temperature range 900–1200°C. The experiments show that the Cl solubility decreases with increasing NaCl in the fluid. Ca-Na partitioning between melts and fluid is weekly temperature dependent and resembles that of the plagioclase-fluid system. The new experimental data, along with the previously published results on the model granite melting in the presence of (Na, K)Cl brines (Aranovich et al., 2013) are used to calibrate an empirical thermodynamic model for the salt species (NaCl, KCl, CaCl₂) in silicate melt. Calculations show that Cl solubility in the haplogranite melt decreases with increasing K/Na ratio in the fluid (and, correspondingly, melt). At high pressure (10 kbar) Cl solubility in the granite increases with increasing H₂O content. Calculated phase diagram for a simple pseudo-ternary system Ab–H₂O–NaCl demonstrates complex phase relations and, correspondingly, evolution of the H₂O and NaCl concentrations in the melt. Literature data on the variations of H₂O and NaCl in the melt and fluid inclusions in Qtz from the granite of Badzhal tin deposit is used to illustrate complex evolution of a fluid-magmatic system.