Hydrous Fe³⁺-bearing bridgmanite (Bdg) is potentially a critical water host in the lowermost mantle. The spin transition behaviors of such materials are pivotal for understanding geophysical heterogeneity in the deep Earth but are poorly understood. Here, we investigated the spin transition and related geophysical properties of Fe³⁺ with associated H defects [Fe³⁺-H] at high P-T conditions using first-principles simulations. Our calculations predict that the presence of hydrogen reduces the onset pressure of the spin transition of Fe³⁺ in Bdg, leading to higher fractions of low spin Fe³⁺. Along standard geotherms, spin transition is predicted to remain incomplete even at the core-mantle boundary (CMB), and lateral temperature variations would significantly affect the proportions of high and low spin Fe and related properties like elasticity. The thermoelastic property of hydrous Fe³⁺-bearing bridgmanite exhibit stronger softening anomalies at the lower mantle conditions compared to dry system, which potentially enhancing the seismic detectability of the hydrous Bdg in the deep earth. Density profile of hydrous Fe³⁺-bearing bridgmanite indicates that the [Fe³⁺-H] defect modestly increases the system's density, but much less than that caused by incorporating an equivalent amount of iron alone. This is crucial for understanding regions like Large Low Shear Velocity Provinces (LLSVPs), which exhibits large velocity drops but only minor density changes. The co-adsorption of Fe and H allows for the introduction of Fe to induce velocity drops without the concomitant sharp increase in density, as pure iron would, thus enabling Fe-H enrichment as a potential source of LLSVPs.

中文翻译：

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含水 Fe3+ 布里奇曼石中的铁自旋转变及其在下地幔中的地球物理特性


含水 Fe³⁺ 桥铜矿 （Bdg） 可能是最下层地幔中的关键水主体。这种材料的自旋跃迁行为对于理解地球深处的地球物理异质性至关重要，但人们对此知之甚少。在这里，我们使用第一性原理模拟研究了 Fe³⁺ 与相关 H 缺陷 [Fe³⁺-H] 在高 P-T 条件下的自旋跃迁和相关地球物理特性。我们的计算预测，氢的存在降低了 Bdg 中 Fe³⁺ 自旋转变的起始压力，导致低自旋 Fe³⁺ 的分数更高。沿着标准地热，即使在核-地幔边界 （CMB） 处，预计自旋转变也保持不完全，横向温度变化将显着影响高低自旋铁的比例以及弹性等相关特性。与干系相比，含水 Fe³⁺ 桥桥石的热弹性在下地幔条件下表现出更强的软化异常，这可能会增强含水 Bdg 在地球深处的地震可探测性。含水 Fe³⁺ 桥曼石的密度剖面表明，[Fe³⁺-H] 缺陷适度增加了系统的密度，但远小于单独加入等量铁引起的密度。这对于理解大型低剪切速度省 （LLSVP） 等区域至关重要，这些区域表现出较大的速度下降，但密度变化很小。Fe 和 H 的共吸附允许引入 Fe 以诱导速度下降，而不会像纯铁那样随之急剧增加密度，从而使 Fe-H 富集成为 LLSVP 的潜在来源。