Chlorine solubility in evolved alkaline magmas

Experimental studies of Cl solubility in trachytic to phonolitic melts provide insights into the capacity of alkaline
magmas to transport Cl from depth to the earth’s surface and atmosphere, and information on Cl solubility
variations with pressure, temperature and melt or fluid composition is crucial for understanding the reasons for
variations in Cl emissions at active volcanoes. This paper provides a brief review of Cl solubility experiments
conducted on a range of trachytic to phonolitic melt compositions. Depending on the experimental conditions
the melts studied 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 fluids of similar Cl molality, higher Cl
concentrations are observed in peralkaline phonolitic melts compared with peraluminous phonolitic melts; overall
the Cl concentrations observed in phonolitic and trachytic melts are approximately twice those found in calcalkaline
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 (assuming melt-vapor equilibrium is maintained). The high Cl contents (approaching
1 wt% Cl) observed in some melts/glasses from the Vesuvius and Campi Flegrei areas suggest saturation
with a Cl-rich brine prior to eruption.