MODELLING THE SULPHIDE CAPACITY OF SILICATE MELTS: IMPLICATIONS FOR VOLCANIC DEGASSING

The assessment of sulphide capacity of silicate melts over the wide range of natural compositions represents an important task for geochemists and petrologists dealing with the study of volcanic degassing in order to allow a better comprehension of the deep system originating sulphur emission. It has been yet recognised that sulphur dissolves in silicate melts both as sulphide and sulphate depending on oxygen fugacity, giving rise to a V-shaped function on a log[S wt%] Vs. logfO2 plot. The solubility of sulphide in the low oxygen fugacity side –the most important to the purposes of gas-magma interactions, i.e. logfO2 less than FMQ - is practically given by the Sulphide Capacity. Here we present our results on about 300 data of literature, including both synthetic systems of compositions relevant to geosciences and “simple” metallurgical slags. Given the equilibrium O2- + ½S2 ó S2- + ½O2 (1), sulphide capacity is normally defined as Cs = (S wt%)·(fO2/fS2)1/2. We can estimate the molar sulphide capacity of silicate melts through a modified Flood – Grjotheim approach [1] which considers the quantity log K’1 = SNilogK’i - where Ni is the fraction of the ith ion over the appropriate matrix and logK’i is related to partial equilibria of the type MeO + ½S2 ó MeS + ½O2 and is defined as logKi·nO 2- = log [nS2-·(fO2/fS2)1/2]. It is important to remark that each K’i term embodies equilibrium constants related to dissociation equilibria of the corresponding Me-oxide and Mesulphide in the melt phase. The addition of Margules-type interaction terms over the cationic and anionic matrix is required and shows a dependence of sulphide solubility on melt composition more complex than that currently assumed. Further improvements may be achieved taking into account for iron speciation and combining the equations developed with a polymeric approach of silicate melts in order to give us oxide ion O2- activity in silicate melts. Applications and case studies concerning sulphur degassing of different magmas are discussed as well as previous results given in literature. BIBLIOGRAPHY: [1]Flood and Grjotheim (1952) J. Iron Steel Inst., 171, 64-70.