Iron force constants of bridgmanite at high pressure: Implications for iron isotope fractionation in the deep mantle
Iron force constants of bridgmanite at high pressure: Implications for iron isotope fractionation in the deep mantle
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DOI：
10.1016/j.gca.2020.11.025
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发表时间：
2020-12
影响因子：
5
通讯作者：
Wenzhong Wang;Jiachao Liu;Hong Yang;Susannah M Dorfman;M. Lv;Jie Li;F. Zhu;Jiyong Zhao;
中科院分区：
文献类型：
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作者：
Wenzhong Wang;Jiachao Liu;Hong Yang;Susannah M Dorfman;M. Lv;Jie Li;F. Zhu;Jiyong Zhao;
主要地幔矿物中铁的同位素组成可能记录了早期分异作用和板块构造过程中深部储层间的化学交换。硼镁石（Bridgmanite，Bdg）是地球下地幔中含量最丰富的矿物，它不仅可以掺入Al，还可以掺入不同氧化态和自旋态的Fe，从而影响Bdg与铁方镁石（ferropericlase，Fp）之间以及下地幔与地核之间Fe同位素的分布。在这项研究中，我们结合第一性原理计算与高压核共振非弹性X射线散射测量，以评估Fe占位，价态和自旋状态在下地幔条件下对Bdg的约化Fe配分函数比（β因子）的影响。结果表明，Bdg中八面体位（B位）Fe ~（3+）在中下地幔条件下的自旋跃迁使其β因子增加了+0.09‰，这是与Fe位占位和价态相比最显著的效应。含Fe 2+的Bdg品种相对于含Fe 3+的品种具有较小的β因子，尤其是含B位Fe 3+的品种。我们的模型表明，Fe同位素分馏之间的Bdg和Fp是唯一显着的最低地幔由于低自旋Fe 2+的出现在Fp。假设早期分离的铁核心从深岩浆海洋，我们发现，无论是核心的形成，也不是岩浆海洋结晶将导致可溶性铁同位素分馏。与此相反，在核幔边界低自旋含Fe ~（3+）的Bdg/含Fe ~（2+）的Fp与金属铁之间的Fe同位素分馏作用可能使地幔最下部的重Fe同位素富集了+0.20‰。
The isotopic compositions of iron in major mantle minerals may record chemical exchange between deep-Earth reservoirs as a result of early differentiation and ongoing plate tectonics processes. Bridgmanite (Bdg), the most abundant mineral in the Earth’s lower mantle, can incorporate not only Al but also Fe with different oxidation states and spin states, which in turn can influence the distribution of Fe isotopes between Bdg and ferropericlase (Fp) and between the lower mantle and the core. In this study, we combined first-principles calculations with high-pressure nuclear resonant inelastic X-ray scattering measurements to evaluate the effects of Fe site occupancy, valence, and spin states at lower-mantle conditions on the reduced Fe partition function ratio (β-factor) of Bdg. Our results show that the spin transition of octahedral-site (B-site) Fe3+in Bdg under mid-lower-mantle conditions generates a +0.09‰ increase in itsβ-factor, which is the most significant effect compared to Fe site occupancy and valence. Fe2+-bearing Bdg varieties have smallerβ-factors relative to Fe3+-bearing varieties, especially those containing B-site Fe3+. Our models suggest that Fe isotopic fractionation between Bdg and Fp is only significant in the lowermost mantle due to the occurrence of low-spin Fe2+in Fp. Assuming early segregation of an iron core from a deep magma ocean, we find that neither core formation nor magma ocean crystallization would have resulted in resolvable Fe isotope fractionation. In contrast, Fe isotopic fractionation between low-spin Fe3+-bearing Bdg/Fe2+-bearing Fp and metallic iron at the core-mantle boundary may have enriched the lowermost mantle in heavy Fe isotopes by up to +0.20‰.