On the behavior of redox pairs in anhydrous and hydrous silicate melts: from the oxygen electrode to the mutual interactions of Fe and S (INVITED)

The need of techniques for determining the oxidation state of magmas from iron and/or sulfur redox ratios has pushed scientists to yield composition-based semi-empirical equations, without much interest for the understanding of how electron transfer takes place, thus disregarding true, or at least most plausible, redox exchanges occurring in melts. Not secondary, it has generated notations (i.e., chemical equilibria) in which standard states, species and components are mixed. Let us then go back to basics, by taking the most geologically important element having multiple oxidation states: iron. In order to model redox exchanges we need i) a formalism for acid-base reactions in silicate melts, ii) a reference electrode, iii) a model for computing proportions and activities of species intervening in acid-base exchanges and redox electrodes, including that of reference. Briefly, the above requirements converge in the adoption of the normal oxygen electrode plus the Toop-Samis polymeric approach, which is based on the ionic Temkin notation (Ottonello et al., 2001). Therefore, in silicate melts redox processes and polymerization are intimately related. Under certain conditions, some unexpected features can be explored, such as the oxidation of iron in closed system with decreasing temperature. Let us now complicate the things by introducing the most geologically important volatile: water. Processing of data on the iron redox ratio in hydrous glasses allows one to model the role of composition, temperature, pressure and oxygen fugacity i) by assessing the acid-base properties of the water component in a notation consistent with the above and ii) by introducing volume terms of interest. The central role of water speciation can be then discussed in terms of its amphoteric behavior, in line with the earlier prediction of Fraser (1977) and the recent NMR findings of Xue and Kanzaki (2004). Finally, let us add another multiple valence state element such as S and investigate how mutual interactions between iron and sulfur take place under various conditions. The adoption of an internal consistent model for sulfur solubility and speciation (Moretti and Ottonello, 2005), based on the same premises adopted so far, permits a full parameterization of how Fe and S interacts. A volcanological application is also given. References: Fraser (1997) Thermodynamics in Geology, D. Reidel Pub. Co.; Moretti and Ottonello (2005) Geochim. Cosmochim. Acta 69, 801-823; Ottonello et al. (2001) Chem. Geol. 174, 157-179; Xue and Kanzaki (2004) Geochim. Cosmochim. Acta 68, 5027-5057