Structural evolution of liquid silicates under conditions in Super-Earth interiors

Molten silicates existing at depth play a pivotal role in planetary evolution. However, the determination of the local structure and physical properties of liquid silicates under extreme pressure and temperature conditions has been hindered by the inherent experimental complexity of such measurements. In this study, we present an in situ X-ray diffraction investigation of liquid silicates conducted at the Matter in Extreme Conditions (MEC) end-station of the Linear Coherent Linac Source (LCLS) at SLAC National Accelerator Laboratory. The combination of the ultrabright X-ray source and a high-power optical laser allowed us to successfully probe the local atomic arrangement of shock-compressed liquid (Mg,Fe)SiO3 with a range of Fe content, spanning pressures from 81(9) to 385(40) GPa. This experimental dataset is then compared to ab initio molecular dynamics performed under similar pressure-temperature conditions. Our results reveal a continuous densification of the O-O and Mg-Si networks beyond the pressure range found at the interior of the Earth, which can alter the properties of melts at these extreme conditions. This may have a considerable impact on the early stages of planetary evolution, resulting in substantial differences in the differentiation processes between smaller rocky planets, such as Earth and Venus in our solar system, and super-Earths, i.e., exoplanets with masses nearly three times larger than that of Earth.深部赋存的熔融硅酸盐在行星演化过程中发挥着至关重要的作用。然而，在极端温压条件下测定液态硅酸盐的局域结构与物理性质，受限于此类测量本身固有的实验复杂性。本研究依托SLAC国家加速器实验室直线相干直线加速器光源（Linear Coherent Linac Source, LCLS）的极端条件物质（Matter in Extreme Conditions, MEC）实验终端，开展了液态硅酸盐的原位X射线衍射（in situ X-ray diffraction）研究。超高亮度X射线源与高功率光学激光的联用，使我们得以成功探测不同铁含量的冲击压缩态液态(Mg,Fe)SiO3的局域原子排布，实验压强范围覆盖81(9)至385(40)吉帕（GPa）。随后，本实验数据集与相同温压条件下的从头算分子动力学（ab initio molecular dynamics）模拟结果进行了对比。研究结果表明，在超出地球内部的压强区间内，O-O与Mg-Si原子网络发生了持续致密化，这会改变极端条件下熔体的物理性质。这或许会对行星演化的早期阶段产生显著影响，进而导致类地行星（如太阳系内的地球与金星）与超级地球（即质量约为地球3倍的系外行星）之间的分异过程存在显著差异。