Université Lille 1, Unité Matériaux et Transformations, UMR CNRS 8207, F-59650 Villeneuve d'Ascq, France.
Nature. 2012 Jan 11;481(7380):177-80. doi: 10.1038/nature10687.
Plate tectonics, which shapes the surface of Earth, is the result of solid-state convection in Earth's mantle over billions of years. Simply driven by buoyancy forces, mantle convection is complicated by the nature of the convecting materials, which are not fluids but polycrystalline rocks. Crystalline materials can flow as the result of the motion of defects--point defects, dislocations, grain boundaries and so on. Reproducing in the laboratory the extreme deformation conditions of the mantle is extremely challenging. In particular, experimental strain rates are at least six orders of magnitude larger than in nature. Here we show that the rheology of MgO at the pressure, temperature and strain rates of the mantle is accessible by multiscale numerical modelling starting from first principles and with no adjustable parameters. Our results demonstrate that extremely low strain rates counteract the influence of pressure. In the mantle, MgO deforms in the athermal regime and this leads to a very weak phase. It is only in the lowermost lower mantle that the pressure effect could dominate and that, under the influence of lattice friction, a viscosity of the order of 10(21)-10(22) pascal seconds can be defined for MgO.
板块构造塑造了地球的表面，它是地幔中数十亿年来固态对流的结果。地幔对流仅由浮力驱动，但由于对流物质的性质而变得复杂，这些物质不是流体，而是多晶岩石。由于缺陷（点缺陷、位错、晶界等）的运动，晶体材料可以流动。在实验室中重现地幔的极端变形条件极具挑战性。特别是，实验应变率至少比自然界高六个数量级。在这里，我们表明，通过从第一性原理出发、没有可调参数的多尺度数值模拟，可以得到地幔压力、温度和应变率下的 MgO 流变学。我们的结果表明，极低的应变速率可以抵消压力的影响。在地幔中，MgO 在非热状态下变形，这导致其非常脆弱。只有在最下部的下地幔中，压力效应才可能占主导地位，并且在晶格摩擦的影响下，MgO 的粘度可以达到 10(21) 到 10(22) 帕秒的量级。
