Small quantity but large effect — How minor phases control strain localization in upper mantle shear zones
Small quantity but large effect — How minor phases control strain localization in upper mantle shear zones
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DOI：
10.1016/j.tecto.2014.12.008
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发表时间：
2015-03
期刊：
影响因子：
2.9
通讯作者：
J. Linckens;M. Herwegh;O. Müntener
中科院分区：
文献类型：
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作者：
J. Linckens;M. Herwegh;O. Müntener
低粘度域，如局部剪切带，对上地幔的地球动力学有重要的控制作用。晶粒尺寸减小和随后的应变局部化与从位错到扩散蠕变的转换有关，是形成低粘度域的一种机制。为了维持应变局部化，地幔矿物的粒度需要在地质时间尺度上保持较小。保持橄榄石晶粒尺寸小的一种方法是在晶粒生长期间通过其它矿物（第二相）钉扎移动的晶界。详细的微观结构研究的基础上形成的三个剪切带在不同的地球动力学设置的天然样品允许派生的橄榄石粒度依赖于第二相的内容。三个剪切带中多矿物橄榄石的粒度随应变的变化规律相似。如果第二相要钉扎移动的橄榄石晶界，则需要在晶粒生长之前将各相充分混合。我们认为，熔体岩石和变质反应是至关重要的初始相混合在地幔岩石。随着变形的进行和应变的增加，结合传质过程和晶粒成核的晶界滑动促进相混合，导致通过扩散蠕变变形的细粒多矿物混合物。由于多矿物地幔岩石中体积较小的矿物的存在，应变局部化仅在低温（<~800 °C）下的高应变变形（超糜棱岩）中才是重要的。在较小的应变和应力条件和/或较高的温度下，需要考虑其他参数，如可用于变形给定岩石体积的总能量、机械各向异性的遗传或水或熔体的存在，以解释上地幔中的应变局部化。
Low viscosity domains such as localized shear zones exert an important control on the geodynamics of the uppermost mantle. Grain size reduction and subsequent strain localization related to a switch from dislocation to diffusion creep, is one mechanism to form low viscosity domains. To sustain strain localization, the grain size of mantle minerals needs to be kept small over geological timescales. One way to keep olivine grain sizes small is by pinning of mobile grain boundaries during grain growth by other minerals (second phases). Detailed microstructural studies based on natural samples from three shear zones formed at different geodynamic settings allowed the derivation of the olivine grain-size dependence on the second-phase content. The polymineralic olivine grain-size evolution with increasing strain is similar in the three shear zones. If the second phases are to pin the mobile olivine grain boundary the phases need to be well mixed before grain growth. We suggest that melt–rock and metamorphic reactions are crucial for the initial phase mixing in mantle rocks. With ongoing deformation and increasing strain, grain boundary sliding combined with mass transfer processes and nucleation of grains promotes phase mixing resulting in fine-grained polymineralic mixtures that deform by diffusion creep. Strain localization due to the presence of volumetrically minor minerals in polymineralic mantle rocks is only important at high strain deformation (ultramylonites) at low temperatures (<~800 °C). At smaller strain and stress conditions and/or higher temperatures other parameters like overall energy available to deform a given rock volume, the inheritance of mechanical anisotropies or the presence of water or melts needs to be considered to explain strain localization in the upper mantle.