Application of the electron microprobe to quantitative analysis of ferric-ferrous ratio in silicate glasses, new experimental calibration

天然硅酸盐熔体中的铁/铁比被广泛用于估计氧化还原条件，氧化还原条件与其他热力学变量（如温度、压力和水活度）一起控制岩浆熔体的物理和化学性质。尽管分析技术的发展取得了很大的进步，但在熔体夹杂物或自然淬火玻璃等非均质自然物体以及部分结晶或熔融的实验产品中，仍然迫切需要直接测定铁-铁比的方法。在局部尺度上，实验和自然样品中的Fe3+/Fe2+可以通过多种方法进行测量。在这些复杂的技术中，由于其成本低，使用电子探针进行铁-铁比测定将是理想的。利用电子探针测量铁铁比是基于过渡金属的性质，即随着氧化态的变化，在x射线发射光谱中l - α和l - β峰以及l - α和l - β强度比发生位移。对含铁硅酸盐矿物进行了峰移法和侧边法等几种方法的开发和验证。在应用于硅酸盐玻璃的文献中，峰移法只有一次校准（Fialin et al., 2011）。然而，将此校准应用于强氧化和还原玻璃表明预测模型存在显著偏差。该项目的重点是Fialin等人（2001）模型的下一步发展，并基于使用一套新的硅酸盐玻璃对Fe- l - α峰的位置作为Fe含量的函数进行新的校准。该数据集包括在广泛的氧化还原条件下产生的更大范围的玻璃成分。与之前使用两组含铁和亚铁的矿物来校准纯含Fe2+和纯含Fe3+曲线的方法相反，将使用在还原和氧化条件下生产的实验玻璃。

The ferric/ferrous ratio in natural silicate melts is widely used for estimations of the redox conditions, which together with other thermodynamic variables such as temperature, pressure and water activity control the physical and chemical properties of the magmatic melts. Despite the progress in developments of analytical techniques, there is still crucial need of the method for direct determinations of ferric-ferrous ratio in heterogeneous natural objects such as melt inclusions or naturally quenched glasses as well as in the experimental products of partial crystallization or melting. At local scale, Fe3+/Fe2+ measurements in experimental and natural samples can be performed by a number of methods. Amongst this sophisticated techniques, the use of the electron microprobe for ferric-ferrous ratio determinations would be the ideal due to its low costs. The measurement of ferric-ferrous ratio using electron microprobe is based on the property of transition metals to exhibit a shift of the L-alpha and L-beta peaks and L-alpha and L-beta intensity ratios in X-ray emission spectra with a change in the oxidation states. Several methods (peak shift and flanks methods) has been developed and verified for the iron-bearing silicate minerals. Only one calibration of the peak shift method (Fialin et al., 2011) exists in the literature in application to silicate glasses. However application of this calibration to strongly oxidized and reduced glasses demonstrate significant deviation of the predictive model. This project is focused on a next development of the model of Fialin et al. (2001) and is based on new calibration of the location of the Fe-L-alpha peak as a function of Fe content, using a new set of silicate glasses. This dataset includes significantly larger range of glass compositions, which was produced in a wide range of redox conditions. In contrast to the previous approach where two sets of minerals with ferric and ferrous iron were used to calibrate pure Fe2+-bearing and pure Fe3+-bearing curves, experimental glasses produced at reduced and oxidized conditions will be utilized.