CSEDI Collaborative Research: Valence state of iron in the lower mantle

氧化还原状态控制着自然系统中的许多重要化学反应，包括地球深处的反应。铁的氧化态已被广泛用于推断氧化还原状态。众所周知，上地幔有还原的氧化还原条件，热力学研究预测下地幔有更多的还原条件。然而，最近与最顶端下地幔有关的压力-温度条件下的研究表明，在下地幔的主要矿物--镁硅酸盐钙钛矿中，大约60%的铁是铁。这非常令人惊讶，因为在上地幔矿物中，只有2%的铁是铁。如果将这一结果外推到深部下地幔，将意味着下地幔与上地幔在化学上是不同的，这可能与最近地震层析成像上、下地幔混合的证据相冲突。我们的初步工作表明，在高压下，地幔硅酸盐钙钛矿中铁的氧化态与铁的电子组态有很强的偶合关系。由于最近的研究表明，铁在地幔矿物中的电子组态随着深度的变化而变化，我们的初步工作表明，铁的氧化态也可能随着深度的变化而变化。此外，硅酸盐钙钛矿中的铝会影响铁的氧化状态。在这个项目中，我们将结合实验(Shim)和理论(Morgan)来测量下地幔硅酸盐钙钛矿中铁的氧化状态如何随着深度和成分的变化而变化。在实验工作中，我们将通过控制样品腔的氧化还原状态来测量铁的氧化状态，以达到高温高压下地幔的氧化还原状态。我们使用基于从头算的技术，即求解材料的基本量子力学方程，来预测作为组成、压力和温度的函数的反应能量和体积。这一联合努力将为研究生和博士后提供新的机会，将实验方法和理论方法结合起来。

The redox state controls many important chemical reactions in natural systems, including the reactions in the deep interior of the Earth. The oxidation state of iron has been extensively used to infer the redox state. It has been well understood that the upper mantle has reducing redox conditions and thermodynamic studies have predicted even more reducing conditions in the lower mantle. However, recent studies at pressure-temperature conditions related to the topmost lower mantle have shown that about 60% of iron in the dominant lower-mantle mineral, magnesium silicate perovskite, is ferric. This is very surprising because only 2% of iron is ferric in upper-mantle minerals. If this result is extrapolated to the deep lower mantle, it would imply that the lower mantle is chemically distinct from the upper mantle, which may conflict with recent seismic tomography evidence of mixing between the upper and lower mantle. Our preliminary work showed that the oxidation state of iron in mantle silicate perovskite is strongly coupled with the electronic configuration of iron at high pressure. As recent studies have shown that the electronic configuration of iron in mantle minerals changes with depth, our preliminary work implies that the oxidation state of iron may change with depth as well. In addition, aluminum in silicate perovskite can influence the oxidation state of iron. In this project, we will combine experiment (Shim) and theory (Morgan) to measure how the oxidation state of iron in lower-mantle silicate perovskite changes with depth and composition. In the experimental effort, we will measure the oxidation state of iron by controlling the redox state of the sample chamber to that of the lower mantle at high pressure-temperature. We use ab-initio based techniques, which solve the fundamental quantum mechanical equations for the material, to predict reaction energies and volumes as a function of composition, pressure, and temperature. This joint effort will provide graduate students and postdocs with new opportunities to combine experimental and theoretical approaches.