The Oxidation States of Iron in Dry and Wet Olivine: A Thermodynamic Model

Abstract

The oxidation state of iron in olivine is an important control on many of its properties but is poorly constrained in mantle conditions. In this work Density Functional Theory is used to build a thermodynamic model of Fe oxidation states and incorporation mechanisms as a function of silica activity (αSiO2), pressure (P), temperature (T), oxygen fugacity (fO2), and the concentrations of Fe, H2O, Ti, and Al. Total Fe3+/∑Fe, Fe oxidation pathways and by‐products of Fe oxidation (such as Mg and Si vacancies) have a complex dependence upon αSiO2, P, T, and fO2, which are difficult to capture using simple Arrhenius relations and experiments over limited ranges. Our model predicts that in the conditions of the upper mantle, depth is the strongest control on Fe3+/∑Fe and that this relationship is strongly nonmonotonic and varies over several orders of magnitude. Fe3+/∑Fe is predicted to always be low reaching a maximum of 0.003 at around 100 km depth under normal mantle conditions though the presence of large amounts of water could increase this value further (0.03 with 500 ppmw water). While the Ferric iron concentration is predicted to be low, the concentration of Mg and Si vacancies—and thus vacancy dependent properties such as diffusion and likely strength—are predicted to be primarily a function of iron oxidation. These properties are predicted to vary by multiple orders of magnitude across the depth range of the upper mantle and thus olivine is predicted to have highly dynamic rather than static properties.