法国里尔大学 Cordier, Patrick及其研究团队的最新研究探明了在地幔条件下，方镁石的变形比布氏岩慢。这一研究成果于2023年1月11日发表在国际顶尖学术期刊《自然》上。

在这项工作中，研究团队以2.5维错位力度变化为基础，构建低地幔压力和温度下，方镁石中氧化镁的低应力蠕变的模型。研究人员表明，方镁石在这些条件下变形非常缓慢，特别是比纯蠕变变形的布氏岩变形慢得多。这是由于方镁石在压力下氧扩散缓慢所致。在该组合中，这种次生相几乎不参与变形，因此下地幔的流变学可以很好地用布氏岩的流变学来描述。他们的结果表明，变形机制的剧烈变化可以作为应变率的函数发生。

研究人员表示，来自地球内部的热量传输驱动了地幔中的对流，这涉及到数十亿年来固体岩石的变形。地球下地幔主要由含铁的布氏岩中的硅酸镁和体积约25%的方镁石中的氧化镁(也含有一些铁)组成。人们普遍认为铁方镁石比布氏岩弱。近年来，有关压力和温度条件下下地幔组合的研究取得了相当大的进展。然而，自然应变率比实验室低8到10个数量级，实验室条件仍然无法实现。一旦确定了岩石及其组成矿物的变形机制，就有可能通过多尺度数值模拟克服这一限制，并确定难以达到的应变率的流变特性。

附：英文原文

Title: Periclase deforms more slowly than bridgmanite under mantle conditions

Author: Cordier, Patrick, Gouriet, Karine, Weidner, Timmo, Van Orman, James, Castelnau, Olivier, Jackson, Jennifer M., Carrez, Philippe

Issue&Volume: 2023-01-11

Abstract: Transport of heat from the interior of the Earth drives convection in the mantle, which involves the deformation of solid rocks over billions of years. The lower mantle of the Earth is mostly composed of iron-bearing bridgmanite MgSiO₃ and approximately 25% volume periclase MgO (also with some iron). It is commonly accepted that ferropericlase is weaker than bridgmanite. Considerable progress has been made in recent years to study assemblages representative of the lower mantle under the relevant pressure and temperature conditions. However, the natural strain rates are 8 to 10 orders of magnitude lower than in the laboratory, and are still inaccessible to us. Once the deformation mechanisms of rocks and their constituent minerals have been identified, it is possible to overcome this limitation thanks to multiscale numerical modelling, and to determine rheological properties for inaccessible strain rates. In this work we use 2.5-dimensional dislocation dynamics to model the low-stress creep of MgO periclase at lower mantle pressures and temperatures. We show that periclase deforms very slowly under these conditions, in particular, much more slowly than bridgmanite deforming by pure climb creep. This is due to slow diffusion of oxygen in periclase under pressure. In the assemblage, this secondary phase hardly participates in the deformation, so that the rheology of the lower mantle is very well described by that of bridgmanite. Our results show that drastic changes in deformation mechanisms can occur as a function of the strain rate.

DOI: 10.1038/s41586-022-05410-9

Source: https://www.nature.com/articles/s41586-022-05410-9