中国科学技术大学吴忠庆、上海高压科学与技术研究中心毛河光等人合作，在研究下地幔深部硅酸铁钙钛矿和后钙钛矿中取得新进展。相关论文于2024年4月15日发表在《美国科学院院刊》上。

据研究人员介绍，铁镁硅酸盐是地幔的主要成分，占地球体积的80%以上。为了解释下地幔的低剪切速度异常，研究需要构建一个可靠的铁镁硅酸盐在宽压力-温度(P-T)范围内的转换图。尽管钙钛矿结构中的MgSiO₃由于其在地球上的优势地位已被广泛研究，但铁硅酸盐在下地幔条件下的相变仍未得到解决。

研究人员在下地幔P-T条件下，以合成铁橄榄石(Fe₂SiO₄)为起始物质，获得了的钙钛矿(Pv)结构中的铁硅酸盐相。化学分析显示，与金属铁颗粒共存的Pv相的Fe/Si比高达1.72(3)，表明Pv相中并入了约25mol%的Fe₂O₃，其化学式近似为(Fe²⁺_0.75Fe³⁺_0.25)(Fe³⁺_0.25Si_0.75)O₃。研究人员高于95GPa的后钙钛矿(PPv)结构中进一步获得了铁硅酸盐相。

计算得到硅酸铁Pv和PPv的压缩速度(V_P)和剪切速度(V_S)随压力的变化曲线，与MgSiO₃的曲线基本平行。据研究人员所知，铁硅酸盐Pv和PPv是所有已报道的，在下地幔P-T条件下稳定的硅酸盐中密度最大的相。在高于55GPa的Si原位解释地震观测结果，应考虑到硅酸盐相中高价铁含量和高价铁的自旋交叉。研究结果将为限制下地幔的地球物理和地球化学模型提供关键信息。

附：英文原文

Title: Iron silicate perovskite and postperovskite in the deep lower mantle

Author: Yang, Ziqiang, Song, Zijun, Wu, Zhongqing, Mao, Ho-kwang, Zhang, Li

Issue&Volume: 2024-4-15

Abstract: Ferromagnesian silicates are the dominant constituents of the Earth’s mantle, which comprise more than 80% of our planet by volume. To interpret the low shear-velocity anomalies in the lower mantle, we need to construct a reliable transformation diagram of ferromagnesian silicates over a wide pressure–temperature (P-T) range. While MgSiO₃ in the perovskite structure has been extensively studied due to its dominance on Earth, phase transformations of iron silicates under the lower mantle conditions remain unresolved. In this study, we have obtained an iron silicate phase in the perovskite (Pv) structure using synthetic fayalite (Fe₂SiO₄) as the starting material under P-T conditions of the lower mantle. Chemical analyses revealed an unexpectedly high Fe/Si ratio of 1.72(3) for the Pv phase in coexistence with metallic iron particles, indicating incorporation of about 25 mol% Fe₂O₃ in the Pv phase with an approximate chemical formula (Fe²⁺_0.75Fe³⁺_0.25)(Fe³⁺_0.25Si_0.75)O₃. We further obtained an iron silicate phase in the postperovskite (PPv) structure above 95 GPa. The calculated curves of compressional (V_P) and shear velocity (V_S) of iron silicate Pv and PPv as a function of pressure are nearly parallel to those of MgSiO₃, respectively. To the best of our knowledge, the iron silicate Pv and PPv are the densest phases among all the reported silicates stable at P-T conditions of the lower mantle. The high ferric iron content in the silicate phase and the spin-crossover of ferric iron at the Si-site above ~55 GPa should be taken into account in order to interpret the seismic observations. Our results would provide crucial information for constraining the geophysical and geochemical models of the lower mantle.

DOI: 10.1073/pnas.2401281121

Source: https://www.pnas.org/doi/abs/10.1073/pnas.2401281121