Chlorine (Cl) and hydrogen chloride (HCl) solubility in hydrous silicate melts: implications for volcanic gas composition

Abstract Chlorine is a key volatile in magmatic and volcanic systems, being a major consumer of atmospheric ozone and an important ligand for metal transport. Despite chlorine’s importance however, the combined effects of melt composition and H₂O on Cl solubility and HCl degassing remain poorly constrained, particularly at magmatic pressures. We present high-pressure (0.5–1.63 GPa), 1200 °C experiments on hydrous andesite, dacite and rhyolite (up to ~ 8–9 wt% H₂O) equilibrated with Ag–AgI–AgCl buffers to fix fCl₂, and with graphite-saturated C–O–H fluids to buffer fO₂ near CCO. Chlorine concentrations in the glasses define chloride capacities (C Cl ) that we combine with anhydrous data from earlier work. Using LASSO regression, we obtain a revised chloride-capacity model that improves predictions for evolved, silica- and alkali-rich melts. The explicit HO 0.5 term remains statistically insignificant up to ~ 8–9 wt% H₂O, confirming that H₂O acts primarily as an ideal diluent of network-forming and network-modifying cations rather than exerting a strong, specific control on Cl dissolution. We couple the new C Cl model with H₂–H₂O–HCl equilibria and a C–H–O–S–Cl fluid speciation code to derive a simple expression for HCl fugacity in terms of melt composition, temperature, pressure and fH₂O, without requiring explicit values of fCl₂ or fO₂. Applied to literature experiments and melt inclusions, the model predicts (i) low fCl₂ (~ 10 − 10 –10 − 6 bar) and much higher fHCl (~ 10⁻ 1 –10 2 bar) at near-surface pressures, consistent with HCl-dominated volcanic emissions; (ii) SO₂/HCl ratios for the 2002–2003 Mt. Etna eruption that match plume measurements; and (iii) strong variations in SO₂/HCl and S/Cl during decompression due to contrasting pressure dependences of sulphur and chlorine capacities. Our framework links petrological archives (melt inclusions, matrix glasses) to HCl contents of magmatic gases, providing a tool to interpret excess degassing, magma plumbing and volcanic halogen fluxes to the atmosphere. We also used HSC Chemistry software to calculate the production of reactive chlorine species (ClO) from HCl in volcanic gases upon mixing with air at the vent.