Chlorine solubility in evolved alkaline magmas

• Main Author: M. R. Carroll
• Format: Article
• Language: English
• Published: Istituto Nazionale di Geofisica e Vulcanologia (INGV) 2005-06-01
• Series: Annals of Geophysics
• Subjects: alkaline magmas; solubility; chlorine; supercritical fluid
• Online Access: http://www.annalsofgeophysics.eu/index.php/annals/article/view/3223

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.