Viscous anisotropy of textured olivine aggregates, Part 1: Measurement of the magnitude and evolution of anisotropy
Viscous anisotropy of textured olivine aggregates, Part 1: Measurement of the magnitude and evolution of anisotropy
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DOI：
10.1016/j.epsl.2016.04.008
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发表时间：
2016-07
影响因子：
5.3
通讯作者：
L. Hansen;J. Warren;M. Zimmerman;D. Kohlstedt
中科院分区：
文献类型：
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作者：
L. Hansen;J. Warren;M. Zimmerman;D. Kohlstedt
富含橄榄石的岩石中晶体结构的发育导致上地幔粘度出现显着的各向异性，强烈影响各种大规模的地球动力学过程。大多数对上地幔粘性各向异性大小的估计都是从以数值方式预测结构和机械演化的微力学模型中得出的。不幸的是，用于对这些模型进行基准测试的数据相对较少，因此它们对地球动力学过程的适用性仍然存在疑问。在这里，我们展示了一系列实验室变形实验的结果，这些实验可以深入了解橄榄石聚集体在沿着复杂加载路径变形过程中各向异性的大小和演变。 Fo50橄榄石聚集体首先在气体介质装置中在1473 K的温度、300 MPa的围压以及各种应力和应变率下发生拉伸变形。在延伸实验的早期，样品表现出的粘度与之前确定的各向同性聚集体的粘度相似。拉伸变形伴随着[100]轴主要与拉伸轴对齐的晶体织构的形成。随后，样品在类似条件下发生扭转变形，剪切应变高达 15.5。在扭转实验的早期，样品所承受的应力比延伸实验结束时测量的应力大约 2 倍，表明粘度具有明显的各向异性。扭转实验结束时的纹理表现出[100]轴主要与剪切方向对齐，与之前的实验观察结果相当。通过检查扭转样品的径向截面，分析了由延伸产生的纹理到由扭转产生的纹理的演变。我们的结果证实，纹理在橄榄石聚集体中产生粘性各向异性，并且我们提供了一种简单的、校准的粘性各向异性参数化，用于地球动力学模型。我们的结果还为未来校准跟踪上地幔岩石各向异性演化的微机械模型提供了广泛的数据集。
The development of crystallographic textures in olivine-rich rocks leads to a marked anisotropy in viscosity of the upper mantle, strongly influencing a variety of large-scale geodynamic processes. Most estimates of the magnitude of viscous anisotropy in the upper mantle are derived from micromechanical models that predict textural and mechanical evolution numerically. Unfortunately, relatively few data exist with which to benchmark these models, and therefore their applicability to geodynamic processes remains in question. Here we present the results from a series of laboratory deformation experiments that yield insight into the magnitude and evolution of the anisotropy of olivine aggregates during deformation along complex loading paths. Aggregates of Fo50olivine were first deformed in extension in a gas-medium apparatus at a temperature of 1473 K, confining pressure of 300 MPa, and a variety of stresses and strain rates. Early in the extension experiments, samples exhibited viscosities similar to those previously determined for isotropic aggregates. Extensional deformation was accompanied by formation of crystallographic textures with [100] axes dominantly aligned with the extension axis. Samples were subsequently deformed in torsion under similar conditions to shear strains of up to 15.5. Early in the torsion experiments, samples supported stresses a factor of ∼2 larger than measured at the end of extension experiments, demonstrating a marked anisotropy in viscosity. Textures at the end of torsion experiments exhibited [100] axes dominantly aligned with the shear direction, comparable to previous experimental observations. Evolution of the textures resulting from extension to those resulting from torsion was analyzed through examination of radial sections of torsion samples. Our results confirm that texture produces viscous anisotropy in olivine aggregates, and we provide a simple, calibrated parameterization of viscous anisotropy for use in geodynamic models. Our results also provide an extensive dataset for future calibration of micromechanical models that track the evolution of anisotropy in upper mantle rocks.