Controls on the distribution of hydrous defects in forsterite from a thermodynamic model
Controls on the distribution of hydrous defects in forsterite from a thermodynamic model
复制标题
DOI：
10.1007/s00269-022-01182-w
复制
发表时间：
2022-03
影响因子：
1.4
通讯作者：
J. Muir;M. Jollands;Feiwu Zhang;A. Walker
中科院分区：
文献类型：
--
作者：
J. Muir;M. Jollands;Feiwu Zhang;A. Walker
氢在镁橄榄石中不同晶体位置和点缺陷上的分布决定了水对流变性、导电性和扩散性等许多性质的影响。本研究利用晶格动力学和密度泛函理论建立了含Al、Ti镁橄榄石中含氢缺陷的热力学模型。由此，确定了镁橄榄石中氢的分布作为压力（P）、温度（T）、水、Al和Ti浓度的函数。首先，氢在镁橄榄石中的分布是复杂的，并且在不同的P、T和成分制度下变化很大。因此，外推这些不同制度之间的水依赖的属性是不平凡的。这种外推通常是通过确定指数来完成的，这些指数描述了缺陷特定的缺陷浓度或依赖于它们的性质如何随水浓度/逸度而变化。我们在这里表明，这些指数可以从根本上改变常见的实验和地球物理P，T和[H2O]bulkranges作为有利的含氢缺陷的变化。通常，在低压下，氢有利于Mg空位（高温）或与钛的络合物（低温），而在高压下，氢有利于Si空位，而不管所有其他条件。较高的[H2O]体积值也有利于水合Si空位。我们评估这些分布沿着地热，并发现氢的分布，从而其对镁橄榄石性质的影响是高度不同的预期条件的上地幔，因此不能简单地表示。没有这样的分布的氢已建成以前，它是必要的，以考虑这种氢分布时，考虑湿地幔的性质。
The distribution of hydrogen across different crystallographic sites and point defects in forsterite determines how many properties, such as rheology, conductivity and diffusion are affected by water. In this study, we use lattice dynamics and Density Functional Theory (DFT) to build a thermodynamic model of H-bearing defects in Al,Ti bearing forsterite. From this, the distribution of hydrogen in forsterite as a function of pressure (P), temperature (T), water, Al and Ti concentration is determined. Primarily, hydrogen distribution in forsterite is complex and highly varied in different P, T and composition regimes. Therefore, extrapolation of properties that depend upon water between these different regimes is non-trivial. This extrapolation has often been done by determining exponents which describe how defect-specific defect concentrations or properties dependent upon them vary with water concentration/fugacity. We show here that these exponents can vary radically across common experimental and geophysical P, T and [H2O]bulkranges as the favoured hydrogen-bearing defects change. In general, at low pressures hydrogen favours Mg vacancies (high temperatures) or complexes with titanium (low temperatures) whilst at high pressures, hydrogen favours Si vacancies regardless of all other conditions. Higher values of [H2O]bulkalso favours hydrated Si vacancies. We evaluate these distributions along geotherms and find that hydrogen distribution and thus its effect on forsterite properties is highly varied across the expected conditions of the upper mantle and thus cannot be simply represented. No such distribution of hydrogen has been previously constructed and it is essential to consider this hydrogen distribution when considering the properties of a wet mantle.