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

岩浆系统中发生的地球化学过程强烈地依赖于氧逸度和硅酸盐熔体中的Fe3+/Fe2+比值。现在，使用Borisv等人最近提出的改进模型，可以对镁铁质熔体的这一比率进行充分的预测。(2018年，对矿物学和岩石学的贡献)，但到目前为止提出的所有模型都不适合或不能满足富硅成分(SiO_2&GT；65-68wt%)。在这项建议中，计划通过测试网络形成和网络修饰阳离子在各种含铁量高达10wt%的流纹岩成分中的作用来改进预测Fe3+/Fe2+比值的模型。这项在大气压下进行的实验工作将包括在不同温度和氧逸度下平衡铝硅酸盐熔体。Fe~(3+)/Fe~(2+)的比值将用湿化学法测定。这些结果将与以前的数据集相结合，以提供一个一致的热力学模型，准确地预测温度、fO2和熔体组成对广泛成分范围(镁铁质到硅质熔体)Fe3+/Fe2+比率的影响，特别是对富硅熔体的影响。

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.