Ionic syntax and equilibrium approach to redox exchanges in melts: Basic concepts and the case of iron and sulfur in degassing magmas

The properties of melts and glasses are heavily affected by redox equilibria and these are sensitive to changes in temperature, oxygen fugacity and composition. The compositional control includes the amphoteric behaviour of some components such as iron oxides and demands the understanding of the link between redox, acid-base properties and structural coordination. This goes through a precise and coherent chemical "syntax" for redox and more in general chemical exchanges in melts and magmas. This syntax applies also to the understanding of how bulk composition drives the redox evolution via melt and gas speciation from source depths up to surface. Useful shortcuts and rules of thumbs might be adopted, but we must keep in mind that redox is not a Maxwell's demon or an adjustable parameter. Rather, it is the twin-face of acid-base exchanges that occur in chemically complex mixtures and solutions. In this study we show that recognizing chemically reactive entities allows decrypting the effect of composition on redox patterns of iron and sulfur. Besides, this allows proposing a stoichiometry-based methodology to describe in detail the chemical mechanisms that drive the melt system to get rid of sulfur via decompression, degassing, and precipitation of S-bearing phases. These stoichiometric constraints allow exploring the different features of magma degassing in basaltic and andesitic systems, independently of any model of redox state proposed in the literature.