Silica Enhances BaSO 4 Solubility in High‐Pressure Fluids: Implications for Ba Enrichment in Arc Magmas

The coexistence of Ba 2+ and SO 4 2‒ in subduction zone fluids without barite (BaSO 4 ) precipitation presents a geochemical paradox, given the traditionally recognized ultra‐low solubility of barite in aqueous solutions. This study investigates barite solubility at temperatures up to 558°C and pressure up to 1.8 GPa in both silica‐saturated (BaSO 4 ‐SiO 2 ‐H 2 O and BaSO 4 ‐Na 2 SO 4 ‐SiO 2 ‐H 2 O) and silica silica‐free systems by coupling a hydrothermal diamond anvil cell (HDAC) with confocal laser Raman microscopy (CLRM). It is confirmed that the existence of dissolved silica plays a primary role in maintaining elevated solubility of barite. In silica‐saturated solutions, the solubility of barite reaches ∼1200 ppm at 452°C and 1.1 GPa in the BaSO 4 ‐SiO 2 ‐H 2 O fluid, and ∼1700 ppm at 558°C and 1.3 GPa in solutions containing 1.5 m (mol/kg H 2 O) dissolved Na 2 SO 4 . These values were approximately six orders of magnitude higher than those in silica‐free solutions at the same temperatures. The experiment results may account for the presence of both aqueous Ba and sulphate in slab‐derived fluids and therefore enrichment of Ba in arc magmas, since these fluids are in equilibrium with coesite and/or silicate minerals.