Chlorine solubility in evolved alkaline magmas

M R Carroll M R Carroll

doi:10.4401/ag-3223

Abstract

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.

Highlights

Chlorine generally occurs as a minor component in most magmas and fumarolic gases
Experimental data concerning chlorine solubility in phonolitic to trachytic melts and vapor/melt and hydrosaline brine/melt partitioning of Cl under magmatic conditions provide insights concerning the geochemical behavior of Cl in evolved alkaline melts
The experimental data on Cl solubility as a function of magma composition and pressure will provide better constraints on the volcanic degassing of Cl and help in the identification of eruptive products preserved in the geologic record which may have been associated with large Cl emissions

Summary

Introduction

Chlorine generally occurs as a minor component in most magmas and fumarolic gases. it has been observed that the release of Cl from volcanoes to the atmosphere during eruptions and quiescent degassing may contribute to thinning of the ozone layer and the production of acid rain A constant Cl content in the glass (flat distribution or plateau) for further addition of Cl to the system is the typical expression of the melt being saturated with both a hydrosaline liquid and an H2O-rich aqueous fluid and it is assumed to define the solubility limit for Cl in the hydrous silicate melt (Malinin et al, 1989; Shinohara et al, 1989; Métrich and Rutherford, 1992; Webster, 1992a,b; Signorelli and Carroll, 2000, 2001, 2002). Experiments carried out in this region (coexisting silicate melt + aqueous fluid + brine) for hydrous rhyolitic melts have documented that: 1) chlorine does not partition strongly into the water-rich fluid phase and the aqueous fluid/melt partition coefficient is ∼5 at 100 MPa (Webster and Holloway, 1988; Shinohara et al, 1989; Métrich and Rutherford, 1992; Candela and Piccoli, 1995; Williams, 1995); 2) there is a. Given the lack of knowledge concerning Cl speciation in the fluid phase, a simple Nernst-style partition coefficient is often used in discussion of melt-fluid partitioning of Cl

Pressure and Cl solubility

Melt composition and Cl solubility

Volcanological implications

Conclusions