Abstract
The strong influence of physical conditions during magma formation on Fe equilibria offers a large variety of possibilities to deduce these conditions from Fe-bearing phases and phase assemblages found in magmatic rocks. Conditions of magma genesis and their evolution are of major interest for the understanding of volcanic eruptions. A brief overview on the most common methods used is given together with potential problems and limitations. Fe equilibria are not only sensitive to changes in intensive parameters (especially T and fO2) and extensive parameters like composition also have major effects, so that direct application of experimentally calibrated equilibria to natural systems is not always possible. Best estimates for pre-eruptive conditions are certainly achieved by studies that relate field observations directly to experimental observations for the composition of interest using as many constraints as possible (phase stability relations, Fe-Ti oxides, Fe partitioning between phases, Fe oxidation state in glass etc.). Local structural environment of Fe in silicate melts is an important parameter that is needed to understand the relationship between melt transport properties and melt structure. Assignment of Fe co-ordination and its relationship to the oxidation state seems not to be straightforward. In addition, there is considerable evidence that the co-ordination of Fe in glass differs from that in the melt, which has to be taken into account when linking melt structure to physical properties of silicate melts at T and P.
Highlights
Fe is the most abundant 3d-transition element in geological systems and its heterovalent nature is the source of a highly variable geochemical behavior depending mostly on the redox regime during geological processes
Fe-oxidation state in volcanic glass. Another possibility to estimate redox conditions is the direct use of the Fe oxidation state found in volcanic glass (e.g., Helgason et al, 1992a,b)
Despite the uncertainty inherent in this approach, this equation is used in rather complex thermodynamic models predicting magmatic phase relations, such as the program MELTS (Ghiorso and Sack, 1995) or the model used by Sugawara (2000, 2001) for Fe-partitioning between melt and plagioclase. These models can certainly be considered major improvements in the modelling of magmatic systems the use of such an empirical equation in such models obviously puts some restrictions on the applicability to systems that strongly differ in either composition or pressure-temperature conditions. Another approach to model the relationship of Fe oxidation state to intensive and extensive variables only recently introduced to geologically relevant melts is suggested by Ottonello and Moretti (Ottonello et al, 2001; Moretti and Ottonello, 2003; Moretti, 2005)
Summary
Fe is the most abundant 3d-transition element in geological systems and its heterovalent nature is the source of a highly variable geochemical behavior depending mostly on the redox regime during geological processes. In the case of magmatic systems, the valence-state of Fe has strong effects on the stability of Fe-bearing phases as well as on the physical properties of the melt phase itself. The stability of Febearing phases has direct consequences on the overall chemical variation trend of a given magmatic fractionation series as is well documented by the difference of tholeiitic and calc-. Max Wilke sources of error for such redox estimates. The structural role of Fe in silicate glass and melt at high temperature is discussed
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