1] Centre for Science at Extreme Conditions and School of Physics and Astronomy, University of Edinburgh, Scottish Universities Physics Alliance, Edinburgh EH9 3JZ, UK [2] Université Pierre et Marie Curie, UMR-CNRS 7193, Institut des Sciences de la Terre Paris, F-75005, Paris, France.
Nature. 2013 Nov 7;503(7474):104-7. doi: 10.1038/nature12668.
Silicate liquids play a key part at all stages of deep Earth evolution, ranging from core and crust formation billions of years ago to present-day volcanic activity. Quantitative models of these processes require knowledge of the structural changes and compression mechanisms that take place in liquid silicates at the high pressures and temperatures in the Earth's interior. However, obtaining such knowledge has long been impeded by the challenging nature of the experiments. In recent years, structural and density information for silica glass was obtained at record pressures of up to 100 GPa (ref. 1), a major step towards obtaining data on the molten state. Here we report the structure of molten basalt up to 60 GPa by means of in situ X-ray diffraction. The coordination of silicon increases from four under ambient conditions to six at 35 GPa, similar to what has been reported in silica glass. The compressibility of the melt after the completion of the coordination change is lower than at lower pressure, implying that only a high-order equation of state can accurately describe the density evolution of silicate melts over the pressure range of the whole mantle. The transition pressure coincides with a marked change in the pressure-evolution of nickel partitioning between molten iron and molten silicates, indicating that melt compressibility controls siderophile-element partitioning.
硅酸盐液体在地球深部演化的各个阶段都起着关键作用，从数十亿年前的核心和地壳形成到现今的火山活动。这些过程的定量模型需要了解在地球内部的高压高温下，液态硅酸盐中发生的结构变化和压缩机制。然而，长期以来，由于实验的挑战性，这种知识的获取一直受到阻碍。近年来，在高达 100GPa 的记录压力下获得了石英玻璃的结构和密度信息（参考文献 1），这是获得熔融状态数据的重要一步。在这里，我们通过原位 X 射线衍射报告了高达 60GPa 的玄武岩熔体的结构。在 35GPa 时，硅的配位数从环境条件下的四个增加到六个，类似于在石英玻璃中报道的情况。配位变化完成后熔体的压缩率低于较低压力下的压缩率，这意味着只有高阶状态方程才能准确描述整个地幔压力范围内硅酸盐熔体密度的演化。该转变压力与熔融铁和熔融硅酸盐之间镍分配的压力演化之间的明显变化相吻合，表明熔体压缩性控制亲铁元素的分配。
