Solubility and fluid-melt partitioning of Cl in hydrous phonolitic melts

Abstract

Chlorine solubilities and partition coefficients between aqueous chloride solutions and phonolitic melts were investigated at 860–890°C and pressures of 25 to 250 MPa using both peraluminous (Vesuvius) and peralkaline (Montaña Blanca, Tenerife) phonolitic melt compositions. Depending on the experimental conditions the phonolites were in equilibrium with either a Cl-bearing aqueous fluid or a subcritical assemblage of low-Cl aqueous fluid + Cl-rich brine. The nature of the fluid phase(s) was identified by examination of fluid inclusions present in run product glasses and the fluid bulk composition was calculated by mass balance. Chlorine concentrations in the glass increase with increasing Cl molality in the fluid phase until a plateau in Cl concentration is reached when melt coexists with aqueous fluid + brine. With constant Cl molality in the fluid phase, the concentration of Cl in phonolitic melt increases as the pressure decreases. With fluids of similar Cl concentration, higher Cl concentrations are observed in peralkaline phonolitic melts compared with the peraluminous phonolitic melts; overall the Cl concentrations observed in phonolitic melts are approximately twice those found in rhyolitic melts under similar conditions. The observed negative pressure dependence of Cl solubility implies that Cl contents of melts may actually increase during magma decompression if the magma coexists with aqueous fluid and Cl-rich brine.