The effect of network-modifiers (Mg, Ca, Na and K) on ferric/ferrous ratio in silicate melts: experimental study and improvement of existing models.

岩浆系统中发生的地球化学过程强烈依赖于氧逸度和硅酸盐熔体中Fe3+/Fe2+的比值。这一比率可以用现有的热力学模型在理论上进行预测。这些模型考虑了玄武岩体系的成分参数，但尚未针对较宽的成分范围进行校准。前人对硅酸盐熔体中Fe3+/Fe2+比率的作用网络形成者（Si, Al, Ti， P）的研究也表明，热力学模型中采用的形式需要改进，需要在方程中添加一些关键元素的相互作用项（简单的形式加上每种氧化物的单独影响ΣdiXi是不够的）。在本项目中，我们实验探索了网络改性剂（Mg, Ca, Na， K）对硅酸盐熔体中Fe3+/Fe2+比率的影响。到目前为止获得的实验结果被用于建立一个预测多组分铝硅酸盐熔体中Fe3+/Fe2+比率的方程（发表于2017年）。然而，这个方程只在氧化条件下校准。在这个项目的扩展中，我们计划纳入以前在减少条件下研究中获得的各种数据集。还计划进行一些额外的实验，以测试迄今为止在自然多组分系统（氧化和还原条件）下开发的配方。利用该扩展数据集，将提出一个一致的热力学模型，准确预测氧逸度和熔体成分对天然基硅熔体Fe3+/Fe2+比的影响。

Geochemical processes occurring in magmatic systems depend strongly on oxygen fugacity and on the Fe3+/Fe2+ ratio in silicate melts. This ratio can be theoretically predicted using available thermodynamic models. These models take compositional parameters for basaltic systems into account, but have not been calibrated for a wide compositional range. Previous studies of the proponents on the role network formers (Si, Al, Ti, P) on the Fe3+/Fe2+ ratio in silicate melts also show that the formalism adopted in the thermodynamic models needs to be improved by adding interaction terms for some key elements in the equations (a simple formalism adding the individual effect of each oxide, ΣdiXi, is not sufficient). In this project, we explore experimentally the effect of network-modifiers (Mg, Ca, Na, K) on the Fe3+/Fe2+ ratio in silicate melts. The experimental results obtained so far were used to develop an equation predicting the Fe3+/Fe2+ ratio in multicomponent aluminosilicate melts (published in 2017). However, this equation is only calibrated at oxidizing conditions. In an extension of this project, we plan to incorporate various datasets obtained in previous studies at reducing conditions. It is also planned to make some additional experiments to test the formulation developed so far with natural multicomponent systems (oxidizing and reducing conditions). Using this extended dataset, a consistent thermodynamic model predicting accurately the effect of oxygen fugacity and melt composition on the Fe3+/Fe2+ ratio of natural mafic to silicic melts will be proposed.