In order to elucidate the processes involved in iron and silicon isotopes partitioning during magmatic differentiation, it is essential to know the precise value of equilibrium fractionation factors between the main minerals present in the evolving silicic melts. In this study, we performed first-principles calculations based on the density functional theory to determine the equilibrium iron and silicon isotopes fractionation factors between eleven relevant silicate or oxide minerals in the context of magmatic differentiation, namely: aegirine, hedenbergite, augite, diopside, enstatite, fayalite, hortonolite, Fe-rich and Fe-free forsterites, magnetite and ulvospinel. Results show that Fe²⁺-bearing silicate minerals display significant differences in iron isotope fractionation factors that cannot be neglected, even at high temperature (1000 °C). Various physical and chemical parameters control the iron isotopic fractionation of silicate minerals. However, the main parameter, after temperature and the iron oxidation state, is the nature and number of iron second neighbors (i.e. the local chemical composition around Fe atoms). This conclusion is also valid for silicon isotopes. In the investigated nesosilicates and inosilicates, silicon isotope reduced partition function ratios (also called β-factors) show no correlation with the average Si-O bond length, which remains almost constant, but Si β-factors are correlated with the local chemical composition of the minerals. Fractional crystallization is one of the mechanisms, which could explain the evolution of iron isotopic compositions during magmatic differentiation. Using the present theoretical set of equilibrium fractionation factors allows us to assess the impact of inter-mineral isotopic fractionations, and shows that pyroxene appears to be the main mineral phase driving the isotopic evolution to a heavier signature in the most evolved lavas.

为了阐明岩浆分异过程中铁和硅同位素分配所涉及的过程，必须知道正在演化的硅质熔体中存在的主要矿物之间的平衡分馏因子的精确值。在这项研究中，我们基于密度泛函理论进行了第一性原理计算，以确定岩浆分化背景下的11种相关硅酸盐或氧化物矿物之间的平衡铁和硅同位素分馏因子，其中包括：aegirine，hedenbergite，augite，透辉石，顽辉石，铁橄榄石，菱沸石，富铁和无铁镁橄榄石，磁铁矿和ulspinepinel。结果表明Fe ²⁺含硅酸盐矿物即使在高温（1000°C）下，铁同位素分馏因子也存在不可忽视的显着差异。各种物理和化学参数控制着硅酸盐矿物的铁同位素分馏。然而，在温度和铁的氧化态之后，主要参数是第二铁邻位的性质和数量（即铁原子周围的局部化学组成）。这个结论对于硅同位素也是有效的。在所研究的nesosilicates和链状硅酸盐，硅同位素降低分区函数比（也称为β -因子）示出与所述平均Si-O键长度，这几乎保持不变无相关性，但硅β因子与矿物的局部化学成分相关。分数结晶是其中的一种机制，可以解释岩浆分化过程中铁同位素组成的演化。利用目前理论上的平衡分级分离因子集，我们可以评估矿物间同位素分级分离的影响，并表明辉石似乎是驱动同位素演化的主要矿物相，在大多数演化的熔岩中都具有较重的特征。