Shear-Induced Fabric and Weakening of Olivine and Dependence on Pressure and Water

正在进行的实验研究，以确定变形机制和结构发展的橄榄石聚集体变形的剪切几何形状在高压和已知的水含量。 橄榄石的变形机制和晶格择优取向控制着上地幔的物理性质和地震各向异性。 在大洋岩石圈干燥的条件下，橄榄石中滑移系的变形机制和相对剪切强度是众所周知的。 在气体装置中的低压（300 MPa）变形实验中产生的橄榄石的晶格择优取向和数值模拟与上地幔的原位地震各向异性很好地对应;（010）[100]和（0 kl）[100]上的剪切导致地震快速结晶a轴与海洋扩张中心的剪切方向对齐。 最近的结果进行剪切实验在高压（2 GPa）在固体介质装置表明，显着不同的橄榄石织物在潮湿条件下变形过程中发展。 在高压下形成的晶格择优取向与位错滑移主要在（010）[001]和（100）[001]上有关;据推测，这些滑移系的临界分辨剪切应力的降低是由与氢点缺陷的相互作用引起的，氢点缺陷在大于1 GPa的压力下在橄榄石中具有高溶解度。 基于这些结果，地震快的方向可以对齐垂直于剪切方向，地震各向异性的地球动力学解释的重要后果。 在一些自然变形的橄榄岩中也观察到了类似的结构，这些结构被认为是在潮湿的条件下形成的，例如在俯冲带中，下行的板块释放出流体。 然而，在高压实验中使用固体介质组件观察到的与[001]方向上的剪切相关的高压橄榄石组构在使用熔盐组件在1.6 GPa下剪切的橄榄石聚集体中没有再现。 使用熔盐组件的优点是，应力可以用固体介质组件无法达到的分辨率来测量，从而可以收集精确的力学数据并应用于地球动力学问题。 用熔融盐组件和先前实验工作获得的结果之间的另一个差异是，在熔融盐组件中测量的强度显著低于在相同温度、应变速率和水含量但不同围压（1.6对0.3GPa）下在气体装置中变形的橄榄石的强度。 为了确保在高压下测量的强度是准确的，测量熔融盐组件中的应力的方法正在被改进，并与使用金属的高分辨率三轴气体装置中的剪切应力的测量进行比较，金属的强度不依赖于压力。 这项研究是根据霍利奥克和Tullis的实验研究进行的，他们使用熔盐电池进行了高压（1.6 GPa）变形实验，并获得了与低压（300 MPa）下开发的织物相似的织物。 这些实验旨在弥合不同实验方法的不同结果之间的差距，以提高我们对地幔变形机制的理解，表征有利于不同晶格优选取向的条件，并评估控制地幔地球动力学的流变学。

An experimental study is being conducted to determine the deformation mechanisms and fabric development of olivine aggregates deformed in a shear geometry at high pressures and known water contents. The deformation mechanisms and lattice preferred orientations of olivine control the physical properties and seismic anisotropy of the upper mantle. The deformation mechanisms and relative shear strengths of slip systems in olivine are well known under the dry conditions that characterize much of the oceanic lithosphere. Lattice preferred orientations of olivine produced in low pressure (300 MPa) deformation experiments in gas apparatus and modeled numerically correspond well to in situ seismic anisotropy of the upper mantle; shear on (010)[100] and (0kl)[100] leads to seismically fast crystallographic a axes aligned with the shear direction at oceanic spreading centers. Recent results of shear experiments performed at high pressure (2 GPa) in a solid medium apparatus indicate that remarkably different olivine fabrics develop during deformation at wet conditions. Lattice preferred orientations developed at high pressure have been linked to dislocation glide dominantly on (010)[001] and (100)[001]; presumably, reductions in critical resolved shear stresses of these slip systems are brought about by interactions with hydrogen point defects that have high solubilities in olivine at pressures greater than 1 GPa. Based on these results, the seismically fast direction can be aligned perpendicular to the shear direction, with important consequences for geodynamic interpretations of seismic anisotropy. Similar fabrics have been observed in some naturally deformed peridotites, and these fabrics are thought to have been developed under wet conditions, such as in subduction zones where downgoing slabs release fluids. However, the high pressure olivine fabrics related to shear in the [001] direction observed in high pressure experiments using the solid medium assembly have not been reproduced in olivine aggregates sheared at 1.6 GPa using the molten salt assembly. The advantage of using the molten salt assembly is that stress can be measured with a resolution not attained using solid medium assemblies, so that accurate mechanical data can be collected and applied to geodynamic problems. Another difference between the results obtained with the molten salt assembly and previous experimental work is that the strengths measured in the molten salt assembly are considerably lower than strengths of olivine deformed in the gas apparatus at the same temperature, strain rate and water content, but different confining pressures (1.6 vs. 0.3 GPa). To ensure that strengths measured at high pressure are accurate, the methods of measuring stress in the molten salt assembly are being refined and compared with measurements of shear stress in a high resolution triaxial gas apparatus using metals, whose strengths are not pressure dependent. This research follows from experimental studies of Holyoke and Tullis, who performed high pressure (1.6 GPa) deformation experiments using a molten salt cell and obtained fabrics similar to those developed at lower pressure (300 MPa). These experiments are designed to bridge the gaps between disparate results of different experimental methods, to improve our understanding of the deformation mechanisms in the mantle, characterize those conditions that favor different lattice preferred orientations, and evaluate rheologies that govern the geodynamics of the mantle.