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Plastic deformation of silicate spinel under the transition-zone conditions of the Earth's mantle

机译：地幔过渡带条件下硅酸盐尖晶石的塑性变形

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摘要

The dynamics of the Earth's deep interior are controlled to a large extent by rheological properties. Until recently, however, experimental studies on the rheological properties of materials thought to be present in the Earth's deep interior have been limited to relatively low pressures. Most previous estimates of rheology have therefore been based on either large extrapolations of low-pressure experimental data or inferences from geodynamical observations. Such studies have provided only weak constraints on the complicated rheological structure expected in the transition zone of the Earth's mantle (between 410 and 660 km depth) where a series of phase transformations occur in silicate minerals. Here we report the results of a direct experimental study of deformation, under transition-zone conditions, of the spinel phase of (Mg,Fe)_2SiO_4 (ringwoodite; thought to be present in the Earth's transition zone). Relatively coarse-grained samples show evidence of dislocation creep with dislocation structures similar to those observed in oxide and germanate spinels, which have significantly higher creep strengths than olivine. In contrast, a fine-grained sample shows evidence for grain-size-sensitive creep. These observations suggest that a ringwoodite-rich layer of the transition zone is likely to have a higher viscosity than the olivine-rich upper mantle, whereas a subducting slab in the deep transition zone may lose its strength if significant grain-size reduction occurs.
机译：流变性质在很大程度上控制着地球深处内部的动力学。然而，直到最近，有关被认为存在于地球深处内部的材料的流变特性的实验研究还仅限于相对较低的压力。因此，大多数以前的流变学估计都是基于低压实验数据的大推算或来自地球动力学观测的推论。这些研究仅对预期在地球地幔过渡带（深度在410至660 km之间）的复杂流变结构提供了微弱的约束，在该过渡带中硅酸盐矿物发生了一系列相变。在这里，我们报告了在过渡区条件下（Mg，Fe）_2SiO_4的尖晶石相（菱铁矿；被认为存在于地球过渡带中）的尖晶石相变的直接实验研究的结果。相对粗粒度的样品显示出位错蠕变的证据，其位错结构类似于在氧化物和锗酸盐尖晶石中观察到的位错结构，其蠕变强度明显高于橄榄石。相反，细颗粒样品显示出对粒度敏感的蠕变的证据。这些观察结果表明，过渡带的富林木层比富橄榄石的上地幔具有更高的粘度，而深过渡带中的俯冲板可能会降低其强度，如果发生明显的晶粒尺寸减小。

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