We examined the solubility and behavior of chlorine in hydrous silicate melts, ranging from mid-ocean ridge basalt (MORB) to high-silica rhyolite, under high-pressure (0.5-1.5 GPa) and high-temperature (1200-1300°C) conditions using the chlorine fugacity control method ([1]). By increasing the water content from 0 to 4 weight %, while keeping chlorine and oxygen fugacities constant, we observed that the chlorine concentration increased with the addition of H₂O across all conditions.

To develop a general equation for chlorine solubility, we combined our results from hydrous basalt, andesite, dacite, and rhyolite (40 experiments) with 60 data points from anhydrous compositions ([1], [2]). Chloride capacity (CCl) for each experiment was defined as:

CCl = Cl(wt.%)/(fCl2^0.5)*(fO2^0.25)

We applied stepwise linear regression, including terms for pressure, temperature, and composition. Excluding non-significant terms (α = 0.05), we obtained the following equation:

logCCl=1.492+(4331XCa−3508XSi+2440XFe−3921XK−741P)/T

The fit had a standard error of 0.083 and R² = 0.963. Surprisingly, the HO₀.₅ term was not significant, suggesting water behaves as an ideal diluent for chlorine.

Using this chloride capacity equation, we calculated the activity of NaCl in hydrous melts and derived an expression for HCl fugacity in equilibrium with experimental melts. These equations were used to calculate chlorine partitioning between NaCl and HCl in both our experiments and those from the literature. Finally, we applied these expressions to determine fluid compositions in equilibrium with melt inclusions from various volcanic settings.

[1] Thomas, R. W., & Wood, B. J. (2021). Geochimica et Cosmochimica Acta, 294, 28-42.
[2] Thomas, R. W., & Wood, B. J. (2023). American Mineralogist, 108(5), 814-825.