Mantle superplasticity and its self-made demise
Mantle superplasticity and its self-made demise
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DOI：
10.1038/nature09685
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发表时间：
2010-12-23
期刊：
影响因子：
64.8
通讯作者：
Yoshida, Hidehiro
中科院分区：
文献类型：
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作者：
Hiraga, Takehiko;Miyazaki, Tomonori;Yoshida, Hidehiro
人们在金属甚至陶瓷中发现了固体晶体材料异常的塑性变形能力（称为超塑性）(1)。几十年来，人们一直推测这种超塑性行为会发生在从表面冰盖到地球下地幔的地质材料中(2-8)。在材料科学中，当材料在大拉伸应变下变形而不失效时，超塑性被证实；然而，还没有实验研究表明岩土材料具有这种特性。在这里，我们表明，多晶镁橄榄石 + 方镁石 (9:1) 和镁橄榄石 1 顽辉石 + 透辉石 (7:2.5:0.5)（它们是地幔的良好类似物）在固相线条件下经历了高达 500% 的均匀伸长。这种超塑性变形伴随着应变硬化，这可以通过晶粒切换条件（即晶界滑动）下的超塑性和晶粒长大的晶粒尺寸敏感性得到很好的解释；晶界滑动是超塑性的主要变形机制。我们将观察到的应变-晶粒尺寸-粘度关系应用于被认为发生超塑性的地幔部分，例如上地幔中的局部剪切带以及相变后穿透到过渡带和下地幔的俯冲板片内的局部剪切带。计算表明，地幔中的超塑性流动不可避免地伴随着显着的晶粒生长，这可以带来细晶粒（
The unusual capability of solid crystalline materials to deform plastically, known as superplasticity, has been found in metals and even in ceramics(1). Such superplastic behaviour has been speculated for decades to take place in geological materials, ranging from surface ice sheets to the Earth's lowermantle(2-8). In materials science, superplasticity is confirmed when the material deforms with large tensile strain without failure; however, no experimental studies have yet shown this characteristic in geomaterials. Here we show that polycrystalline forsterite + periclase (9:1) and forsterite 1 enstatite + diopside (7:2.5:0.5), which are good analogues for Earth's mantle, undergo homogeneous elongation of up to 500 per cent under subsolidus conditions. Such superplastic deformation is accompanied by strain hardening, which is well explained by the grain size sensitivity of superplasticity and grain growth under grain switching conditions (that is, grain boundary sliding); grain boundary sliding is the main deformation mechanism for superplasticity. We apply the observed strain-grain size-viscosity relationship to portions of the mantle where superplasticity has been presumed to take place, such as localized shear zones in the upper mantle and within subducting slabs penetrating into the transition zone and lower mantle after a phase transformation. Calculations show that superplastic flow in the mantle is inevitably accompanied by significant grain growth that can bring fine grained (