橄榄石相变过程原位观测实验研究

Real time observation experiment on olivine phase transformation

橄榄石是地幔中的主要造岩矿物,橄榄石到其高压相的相变对地幔动力学过程有着广泛的影响。人们通常认为地幔成分中橄榄石的α-β相变是产生410km间断面的原因,并认为橄榄石的定向排列对上地幔中地震波的各向异性有一定的影响。橄榄石的α-β相变可能对板块动力学有着影响,而相变时可能导致的流变弱化会强烈地影响到俯冲带的力学特征。本文简述了橄榄石相变研究的一些新进展,内容包括原位X射线衍射实验技术和冷却方法、实验设备、压标对实验结果精确性的影响以及成核生长和剪切引导两个主要的相变机制。本文的实验通过对(Mg_(0.9)Fe_(0.1))_2SiO_4橄榄石α-β相变过程的原位衍射实验研究确定其形核率和长大率参数,据此估算俯冲带中亚稳态橄榄石的存在状态,从而为分析研究俯冲带的动力学过程提供直接的实验依据。共进行了10组不同温压条件下的实验,分别得到了完全相变和部分相变的数据,从而得到了含Fe橄榄石原位X射线衍射相变实验的第一手重要数据,同时根据红外和光学及电子显微观测分析对实验样品的相变机制做了分析研究。

Olivine is the dominant mineral in the earth mantle. The olivine to its high pressure polymorphs phase transformation has wide implications for many mantle dynamic processes. The 410km discontinuity is usually considered to be caused by the olivine-wadsleyite transformation. And olivine's preferred orientation is important for understanding the rheology and seismic anisotropy in the upper mantle. The alpha-beta transformation of olivine possibly influences the dynamics of the subducting slab, and it may also lead to rheological weakening which strongly affects the mechanical properties of subducting slabs. The recent developments of phase transformation are reviewed in this paper including the following area: the in-situ X-ray diffraction and the quench methods; the apparatus used; the influence of press-scale to the precision of the test results; the nucleation and growth mechanism and the shear-induced mechanism. Then the experiment results of (Mg0.9Fe0.1)(2)SiO4 olivine-wadsleyite phase transformation were reported. Ten runs with different P-T conditions were performed. beta phase was obtained in some runs and a coexistence of alpha and beta phases were observed in other runs. Therefore, the first-hand in-situ X ray diffraction transformation experimental results of (Mg, Fe)(2)SiO4 olivine were obtained. And the transformation mechanisms were discussed based on FTIR, optical and electrical microscopy observations.