A missing link between volcanic degassing and experimental studies on chloride partitioning

Abstract

Chlorine behavior in a silicate melt and aqueous fluid system is affected by various pressure dependencies: HCl solubility in a melt has a positive pressure dependence, NaCl partitioning into a melt shows a large and negative pressure dependence, and the HCl–NaCl exchange reaction between a silicate melt and an aqueous fluid favors HCl in aqueous fluids at lower pressures. These pressure dependencies cause chlorine to appear in contrasting forms in magmatic aqueous fluids: HCl-rich volcanic gases at low pressure (~ 0.1 MPa) and NaCl-rich fluids at high pressure (> 50 MPa). This paper reviews the pressure dependencies of these reactions obtained by experimental studies through a theoretical evaluation, and applies the findings to modeling chloride degassing from ascending magma. The model calculation results are qualitatively consistent with the distribution of Cl in natural samples, such as volcanic glasses and volcanic gases. However, experimental conditions do not cover those of real magmatic systems and the model calculation relies on extrapolating experimental data, resulting in limited reliability. In particular, there is a large gap in pressure conditions between HCl solubility experiments at 0.1 MPa and chloride partition experiments at high pressure (> 50 MPa), and further experiments with theoretical evaluation are necessary to fill the gap for quantitatively reliable modeling.