下地幔富铝相的结构和弹性性质研究及其地质意义

The deeply subduction of slabs makes the Earth’s interior chemically heterogeneous. The mineral constituents and their physical properties of the subducted slabs change with increasing the subducted depth, and it provides significant evidences to elucidate seismic signatures around the subducted slabs. Al-rich phases (NAL: new hexagonal aluminous phase, space group P63/m; CF: calcium–ferrite phase, orthorhombic, Pbnm) are believed to constitute 10-30 wt % of subducted mid-ocean ridge basalt (MORB) in the Earth’s lower mantle. As the major minerals of MORB in the lower mantle, their crystal structures and physical properties have attracted attention from researchers. However, there is no literature about the structures, elastic properties and deformation mechanism of the Fe-bearing Al-rich phases at high-pressure and high-temperature. Here we will conduct experimental research on structures and elastic properties of Al-rich phases using synchrotron radiation X-ray diffraction, nuclear forward scattering and Brillouin light scattering in conjunction with diamond anvil cells. The spin and valence states of iron, isothermal equation of state, elastic properties, seismic velocities and deformation mechanism of Al-rich phases at high-pressure and high-temperature will be derived and in turn can be used to discuss the geophysical implications of Al-rich phases on the observed seismic signatures near the subducted slabs in the lower mantle.

板块深俯冲使得地球内部在化学成分上呈现不均一性。俯冲带中矿物的组成和物性随着俯冲深度而变化，是解释许多地震波观测现象的重要依据。NAL相（六方晶系，空间群P63/m）和CF相（斜方晶系，空间群Pbnm）为下地幔俯冲带中的两种富铝相，占俯冲洋壳（MORB）的10-30 wt %。作为MORB在下地幔中的主要矿物，富铝相的结构和物性逐渐引起研究者们关注。然而，关于含铁富铝相在高温高压下的结构、弹性性质和形变机制的研究未见文献报道。本项目拟将金刚石压腔与同步辐射X射线衍射、核向前散射和布里渊散射技术相结合，开展高温高压下富铝相结构和弹性性质的实验研究，以期获得高温高压下富铝相中铁的价态和自旋态、等温状态方程、弹性性质、地震波速以及地幔温压下富铝相的形变机制，为探讨下地幔俯冲带地震波观测异常现象提供实验依据。

板块深俯冲使得地球内部在化学成分上呈现不均一性。俯冲带的矿物组成和物理性质随着俯冲深度而变化，是解释许多地震波观测现象的重要依据。NAL相（空间群：P63/m）和CF相（空间群：Pbnm）为下地幔俯冲带中的两种富铝相，占俯冲洋壳（MORB）的10-30wt%。项目通过金刚石压腔与同步辐射X射线衍射、核向前散射和布里渊散射技术相结合，在高温高压条件下，对两种富铝相的结构和弹性性质进行了系统的研究，获得的主要研究成果如下：（1）完成了高压下含铁和不含铁富铝相CF相的结构行为和弹性性质研究。结果表明，含铁CF相中八面体位置Fe3+在25-35GPa发生高自旋到低自旋转变，并伴随着体积和体波速的减小。含铁富铝相在铁的自旋转变区所表现出来的弹性异常行为可能与下地幔俯冲带附近观测到的地震波异常现象有关。（2）完成了富铝相NAL相在300K、450K、600K和800K温度下的热力学状态方程和单晶弹性性质研究。结果显示，NAL相具有较大的纵波各向异性和横波分裂因子，可能是引起下地幔上部俯冲带中地震波各向异性现象的原因之一。