Spin crossover and iron-rich silicate melt in the Earth's deep mantle
Spin crossover and iron-rich silicate melt in the Earth's deep mantle
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DOI：
10.1038/nature09940
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发表时间：
2011-05-12
期刊：
影响因子：
64.8
通讯作者：
Hiraoka, Nozomu
中科院分区：
文献类型：
--
作者：
Nomura, Ryuichi;Ozawa, Haruka;Hiraoka, Nozomu
熔体的体积大于相同组成的硅酸盐固体。但是这种差异在高压下会减小，并且在地球内部（和其他陆地物体）的压力下，足够富含重元素铁的熔体可能会变得比固体更致密的可能性长期以来一直是相当多的猜测的来源（1，2）。在地球最低地幔中出现如此致密的硅酸盐熔体将对其物理和化学演化产生重要影响，并可以为解释核幔边界区域的各种观测特征提供统一的模型。最近的理论计算（4）结合在较浅地幔条件下（5-7）铁在（Mg，Fe）SiO 3钙钛矿和熔体之间分配的估计表明，在地球最深地幔的压力下，熔体比固体更致密，与冲击波实验的分析一致（8）。在这里，我们延长测量的铁分配在整个地幔压力范围内，并发现一个陡峭的变化，压力大于类似于76 GPa，导致在强大的铁富集熔体。额外的X射线发射光谱测量（Mg0.95Fe0.05）SiO 3玻璃表明自旋崩溃约70 GPa，这表明所观察到的变化，铁分区可以解释由自旋交叉的铁（从高自旋到低自旋）在硅酸盐熔体。这些结果表明，在下地幔1, 800 km深度附近，（Mg，Fe）SiO 3液态比固态密度大。地球形成后不久，吸积和内部分异所消散的热量可能在固体地幔下方产生了厚度接近1，000公里的致密熔融层。我们还推断，（Mg，Fe）SiO 3钙钛矿是在深地幔条件下的液相线上，并预测，致密岩浆的分离结晶将朝着富铁和贫硅的组合物，在核幔边界区域的结构与地震推断一致。
A melt has greater volume than a silicate solid of the same composition. But this difference diminishes at high pressure, and the possibility that a melt sufficiently enriched in the heavy element iron might then become more dense than solids at the pressures in the interior of the Earth (and other terrestrial bodies) has long been a source of considerable speculation(1,2). The occurrence of such dense silicate melts in the Earth's lowermost mantle would carry important consequences for its physical and chemical evolution and could provide a unifying model for explaining a variety of observed features in the core-mantle boundary region(3). Recent theoretical calculations(4) combined with estimates of iron partitioning between (Mg,Fe)SiO3 perovskite and melt at shallower mantle conditions(5-7) suggest that melt is more dense than solids at pressures in the Earth's deepest mantle, consistent with analysis of shockwave experiments(8). Here we extend measurements of iron partitioning over the entire mantle pressure range, and find a precipitous change at pressures greater than similar to 76 GPa, resulting in strong iron enrichment in melts. Additional X-ray emission spectroscopy measurements on (Mg0.95Fe0.05)SiO3 glass indicate a spin collapse around 70 GPa, suggesting that the observed change in iron partitioning could be explained by a spin crossover of iron (from high-spin to low-spin) in silicate melt. These results imply that (Mg,Fe)SiO3 liquid becomes more dense than coexisting solid at similar to 1,800 km depth in the lower mantle. Soon after the Earth's formation, the heat dissipated by accretion and internal differentiation could have produced a dense melt layer up to similar to 1,000 km in thickness underneath the solid mantle. We also infer that (Mg,Fe)SiO3 perovskite is on the liquidus at deep mantle conditions, and predict that fractional crystallization of dense magma would have evolved towards an iron-rich and silicon-poor composition, consistent with seismic inferences of structures in the core-mantle boundary region.