A new method to estimate the oxidation state of basaltic series from microprobe analyses

The oxygen fugacity and therefore the iron redox state of a melt is known to have a strong influence on the liquid line of descent of magmas and thus on the composition of the coexisting melts and crystals. We present a new method to estimate this critical parameter from electron probe microanalyses of two of the most common minerals of basaltic series, plagioclase and clinopyroxene. This method is not based on stoichiometric calculations, but on the different partitioning behaviour of Fe(3+) and Fe(2+) between both minerals and a melt phase: plagioclase can incorporate more Fe(3+) than Fe(2+), while clinopyroxene can incorporate more Fe(2+) than Fe(3+). For example, the effect of oxidizing a partly molten basaltic system (Fe(3+) is stabilized with respect to Fe(2+)) results in an increase of FeO(total) in plagioclase, but a decrease in the associated clinopyroxene. We propose an equation, based on published partition coefficients, that allows estimating the redox state of a melt from these considerations. An application to a set of experimental and natural data attests the validity of the proposed model. The associated error can be calculated and is on average <1 log unit of the prevailing oxygen fugacity. In order to reduce the number of different variables influencing the Fe(2+)/Fe(3+) mineral/melt equilibrium, our model is restricted to basaltic series with SiO(2)<60% that have crystallized at intermediate to low pressure (<0.5 GPa) and under relatively oxidizing conditions (Delta FMQ>0; where FMQ is the fayalite-magnetite-quartz oxygen buffer equilibrium), but it may be parameterized for other conditions. A spreadsheet is provided to assist the use of equations, and to perform the error propagation analysis. (C) 2009 Elsevier B.V. All rights reserved.