Abstract 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 its β-factor, which is the most significant effect compared to Fe site occupancy and valence. Fe2+-bearing Bdg varieties have smaller β-factor 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‰.

中文翻译：

---

高压下布里奇曼石的铁力常数：对深部地幔中铁同位素分馏的影响

摘要 主要地幔矿物中铁的同位素组成可能记录了由于早期分化和正在进行的板块构造过程导致的深地储层之间的化学交换。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，还可以掺入具有不同氧化态和自旋态的 Fe，进而影响 Bdg 和亚铁镁石 (Fp) 之间以及之间的 Fe 同位素分布。下地幔和地核。在这项研究中，我们将第一性原理计算与高压核共振非弹性 X 射线散射测量相结合，以评估下地幔条件下 Fe 位点占据、价态和自旋态对降低的 Fe 分配函数比（β -因素）的 Bdg。我们的结果表明，在中下地幔条件下，Bdg 中八面体位点（B 位点）Fe3+ 的自旋跃迁使其 β 因子增加了 +0.09‰，与 Fe 位点占有率相比，这是最显着的影响。价。与含 Fe3+ 的品种相比，含 Fe2+ 的 Bdg 品种具有较小的 β 因子，尤其是那些含有 B 位 Fe3+ 的品种。我们的模型表明，由于 Fp 中低自旋 Fe2+ 的发生，Bdg 和 Fp 之间的 Fe 同位素分馏仅在最下地幔中显着。假设铁核从深岩浆海洋早期分离，我们发现核的形成和岩浆海洋结晶都不会导致可解析的 Fe 同位素分馏。相比之下，