Amorphization and Plasticity of Olivine During Low‐Temperature Micropillar Deformation Experiments
Amorphization and Plasticity of Olivine During Low‐Temperature Micropillar Deformation Experiments
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DOI：
10.1029/2019jb019242
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发表时间：
2020-05
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通讯作者：
Kelly Kranjc;A. Thind;A. Borisevich;Rohan Mishra;K. Flores;P. Skemer
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作者：
Kelly Kranjc;A. Thind;A. Borisevich;Rohan Mishra;K. Flores;P. Skemer
由于橄榄石在这些条件下容易因脆性机制而变形，因此通过实验量化浅层岩石圈高应力和低温下橄榄石的粘塑性流变性具有挑战性。在这项研究中，我们使用微尺度单轴压缩测试来研究橄榄石单晶在室温和压力下的流变性。使用聚焦离子束铣削技术制备标称直径为 1.25 μm 的支柱，并承受数千兆帕的持续轴向应力。大多数支柱在停留几秒到几小时后就失效了。然而，一些支柱在负载下 4-8 小时后表现出明显的塑性变形迹象，但没有失效。相应的蠕变应变率估计约为 10−6 至 10−7 s−1。实现这种变形所需的单轴应力 (4.1–4.4 GPa) 与使用纳米压痕技术获得的补充数据非常一致。扫描透射电子显微镜观察表明，变形沿着变形柱内的非晶剪切带发生。电子能量损失光谱测量表明，这些带富集了 Fe，贫化了 Mg。我们认为橄榄石中阳离子分布的不均匀性会集中应力并促进富铁区域的非晶化。灾难性失效事件的时间依赖性表明，非晶带必须生长到某个临界长度尺度才能产生不稳定的缺陷，例如剪切裂纹。
Experimentally quantifying the viscoplastic rheology of olivine at the high stresses and low temperatures of the shallow lithosphere is challenging due to olivine's propensity to deform by brittle mechanisms at these conditions. In this study, we use microscale uniaxial compression tests to investigate the rheology of an olivine single crystal at room pressure and temperature. Pillars with nominal diameters of 1.25 μm were prepared using a focused ion beam milling technique and were subjected to sustained axial stresses of several gigapascal. The majority of the pillars failed after dwell times ranging from several seconds to a few hours. However, several pillars exhibited clear evidence of plastic deformation without failure after 4–8 hr under load. The corresponding creep strain rates are estimated to be on the order of 10−6 to 10−7 s−1. The uniaxial stresses required to achieve this deformation (4.1–4.4 GPa) are in excellent agreement with complementary data obtained using nanoindentation techniques. Scanning transmission electron microscopy observations indicate that deformation occurred along amorphous shear bands within the deformed pillars. Electron energy loss spectroscopy measurements revealed that the bands are enriched in Fe and depleted in Mg. We propose that inhomogeneities in the cation distribution in olivine concentrate stress and promote the amorphization of the Fe‐rich regions. The time dependence of catastrophic failure events suggests that the amorphous bands must grow to some critical length scale to generate an unstable defect, such as a shear crack.