SolEx: A model for mixed COHSCl-volatile solubilities and exsolved gas compositions in basalt

We present a software application, SolEx, to calculate basaltic melt and coexisting vapour compositions in the system C-O-H-S-Cl. Such a model has great utility in interpreting emitted gas and melt inclusion compositions, especially through the incorporation of sulphur and chlorine, the most commonly measured volcanic gas species. We assume that the behaviour of the fluid phase is controlled by the volumetrically dominant volatile species, H2O and CO2, whereas sulphur and chlorine partition between the melt and fluid phases. Melt-fluid partition coefficients for S and Cl were parameterised from measurements by Lesne et al. (2011a, p. 1737). The model of Churakov and Gottschalk (2003a, p. 2415) was applied to calculate fugacity coefficients and the equilibrium constants for the reaction i(melt) -> i(fluid) were thereby deduced. SO2 dominates at oxidation states of Delta NNO > 0.5 (Jugo et al., 2010, p. 5926), where this model is applicable.

In the forward model, total volatile inventories and melt composition are specified by the user. The parameterisation of Dixon (1997, p. 368) is used to predict the partitioning of CO2 and H2O between vapour and melt phases. An iterative procedure is employed to predict the partitioning of S and Cl components between fluid and melt phases. Melt and gas compositions and gas volume fraction are thereby modelled over pressures in the range 5-4000 bar. This approach satisfactorily reproduces independent literature data on S and Cl behaviour in basalt. SolEx is a user-friendly software package available for OS X and Windows, facilitating modelling of closed- and open-system C-O-H-S-Cl degassing in basalts. (C) 2011 Elsevier Ltd. All rights reserved.