The ferric/ferrous ratio in silicic melts: experimental investigation and modelling

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Geochemical processes occurring in magmatic systems depend strongly on oxygen fugacity and on the Fe3+/Fe2+ ratio in silicate melts. This ratio can now be predicted adequately for mafic melts using the refined model proposed recently by Borisov et al. (2018, Contributions to Mineralogy and Petrology), but all models proposed so far are not suitable or satisfying for silica-rich compositions (SiO2 > 65-68 wt%). In this proposal, it is planned to improve the models predicting Fe3+/Fe2+ ratios by testing the role of network-forming and network-modifying cations in various rhyolitic compositions containing up to 10 wt% Fe2O3tot. The experimental work, conducted at atmospheric pressure, will consist in equilibrating aluminosilicate melts at various temperatures and oxygen fugacities. The Fe3+/Fe2+ ratio will be determined by wet chemistry. The results will be combined with previous datasets to provide a consistent thermodynamic model predicting accurately the effect of temperature, fO2 and melt composition on the Fe3+/Fe2+ ratio over a wide compositional range (mafic to silicic melts), with particular attention to silica-rich melts.