The influence of microstructure on deformation of olivine in the grain-boundary sliding regime
The influence of microstructure on deformation of olivine in the grain-boundary sliding regime
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DOI：
10.1029/2012jb009305
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发表时间：
2012-09
影响因子：
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通讯作者：
L. Hansen;M. Zimmerman;D. Kohlstedt
中科院分区：
文献类型：
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作者：
L. Hansen;M. Zimmerman;D. Kohlstedt
[1]虽然微结构的演变是至关重要的应变相关的过程中，在地球的地幔，纯橄榄岩的流动规律只进行了校准与低应变实验。因此，我们进行了一系列的高应变扭转实验的薄壁圆筒富铁橄榄石集合体。实验在气体介质装置中在1200°C和恒定应变速率下进行。在我们的实验中，在不同的应变速率下，观察到峰值应力，然后是显着的应变弱化。我们首先将样品变形到足够高的应变，以实现稳态微观结构，然后进行应变速率步进测试以表征具有恒定微观结构的每个样品的蠕变行为。一个全球性的数据拟合产生的应力指数为4.1和晶粒尺寸指数为0.73，值同意与以前的小应变实验进行橄榄石在位错容纳晶界滑动（GBS）制度。强的晶体学择优取向为（010）[100]位错的运动提供了支持。所观察到的应变弱化并不完全是由晶粒尺寸减小解释，因此，我们建议，其余30%的应力减少与CPO的发展有关。为了将微结构演化纳入橄榄石中GBS的本构描述中，我们（1）推导出具有稳态微结构的高应变变形的流动定律，其导致表观应力指数为5.0，以及（2）提出了一个重建所观察到的应变弱化的演化方程系统。我们的研究结果证实了低应变实验的流动规律参数和微观结构观测，并提供了一种将应变弱化纳入地球动力学模拟的方法。
[1] Although microstructural evolution is critical to strain-dependent processes in Earth's mantle, flow laws for dunite have only been calibrated with low-strain experiments. Therefore, we conducted a series of high-strain torsion experiments on thin-walled cylinders of iron-rich olivine aggregates. Experiments were performed in a gas-medium apparatus at 1200°C and constant strain rate. In our experiments, each at a different strain rate, a peak stress was observed followed by significant strain weakening. We first deformed samples to high enough strain that a steady state microstructure was achieved and then conducted strain rate stepping tests to characterize the creep behavior of each sample with constant microstructure. A global fit to the data yields a stress exponent of 4.1 and a grain-size exponent of 0.73, values which agree well with those from previous small-strain experiments conducted on olivine in the dislocation-accommodated grain-boundary sliding (GBS) regime. Strong crystallographic preferred orientations provide support for GBS accommodated by movement of (010)[100] dislocations. The observed strain weakening is not entirely explained by grain-size reduction; thus, we propose that the remaining 30% reduction in stress is related to CPO development. To incorporate microstructural evolution in a constitutive description of GBS in olivine, we (1) derive a flow law for high-strain deformation with steady state microstructure, which results in an apparent stress exponent of 5.0, and (2) present a system of evolution equations that recreate the observed strain weakening. Our results corroborate flow-law parameters and microstructural observations from low-strain experiments and provide a means for incorporating strain weakening into geodynamic simulations.