Continued Investigation of Rheological Behavior of Clinopyroxene and Clinopyroxene-Olivine Aggregates Under Hydrous Conditions: Implications for Crustal and Mantle Dynamics

本文对下地壳和上地幔的重要矿物斜辉石和斜辉石+橄榄石的团聚体的高温高压强度进行了实验研究。最近对大陆地震深度分布和重力异常的分析得出结论，一般来说，下地壳比上地幔强。这种观点与大陆岩石圈的果冻三明治模型相反，在这种模型中，弱的下地壳位于强的上地壳和强的上地幔之间，因此大陆岩石圈的强度在于地幔而不是地壳。本研究项目探讨了下地壳和上地幔的相对强度取决于水的存在与否这一命题。人们对斜辉石相对于橄榄石的强度知之甚少，即使这两种相存在于同一块岩石中。最近的实验表明，斜辉石在无水条件下比橄榄石强，而在有水环境下比橄榄石弱。仔细研究斜辉石和斜辉石+橄榄岩粘度对水浓度的依赖关系对于解决这些问题以及模拟下地壳和上地幔的地球动力学和地球化学行为和演化至关重要。强调水对斜辉石和斜辉石+橄榄石组成的岩石粘度的影响，因为即使少量的水也会大大削弱名义上的无水矿物。在大多数研究中，水削弱效应被视为一个“开-关”过程——如果有水存在，岩石就会变弱；如果没有水，岩石就很坚固。最近的实验表明，在富含水的环境中（例如，俯冲板块上方的地幔楔），富含橄榄石的岩石的粘度比在饱和水环境中（例如，洋中脊下的地幔）的粘度小10倍以上。我们提出的研究是建立在这一观察、已发表的关于富含斜辉石的干燥岩石变形行为的研究，以及我们的初步结果的基础上的。我们的初步结果显示，斜辉石中的水弱化作用明显大于橄榄石。为了将实验室结果应用于发生在地球深处的过程，详细的流动规律是必不可少的，以便描述应变率（即粘度）作为水浓度的函数，以及扩散和位错蠕变状态下的差应力、晶粒尺寸、温度和压力。用微红外分析测定了水的浓度，用光学和电子显微镜分析了变形产生的微观结构，用电子背散射衍射测量了织物。

++This proposal involves an experimental investigation of the high-temperature, high-pressure strength of aggregates of clinopyroxene, an important mineral in both the lower crust and the upper mantle, and of aggregates of clinopyroxene plus olivine. Recent analyses of earthquake depth distributions and gravity anomalies on continents conclude that the lower crust is, in general, stronger than the uppermost mantle. This point of view is contrary to the jelly-sandwich model of the continental lithosphere in which a weak lower crust lies between a strong upper crust and a strong upper mantle such that the strength of the continental lithosphere lies in the mantle rather than in the crust. This research projects explores the proposition that the relative strengths of lower crust and upper mantle depend on the presence or absence of water. Little is known about the strength of clinopyroxene relative to that of olivine, even when the two phases are present in the same rock. Recent experiments suggest that clinopyroxene is stronger than olivine under anhydrous conditions but weaker than olivine in hydrous environments. A careful investigation of the water concentration dependence of the viscosity of clinopyroxene and clinopyroxene + olivine rocks is critical in order to address these issues and to model the geodynamical and geochemical behavior and evolution of the lower crust and upper mantle. The effect of water on the viscosity of rocks composed of clinopyroxene and clinopyroxene + olivine is emphasized, because even a small amount of water dramatically weakens nominally anhydrous minerals. In most studies, the water-weakening effect has been treated as an 'on-off' process - if water is present, rocks are weak; if water is absent, rocks are strong. Recent experiments demonstrate that the viscosity of olivine-rich rocks is a factor of more than 10 smaller in water-rich environments (e.g., the mantle wedge above a subducting slab) than in water-under saturated settings (e.g., mantle beneath a mid-ocean ridge). The proposed research builds on this observation, on published investigations of the deformation behavior of dry clinopyroxene-rich rocks, and on our initial results that reveal a significantly larger water-weakening effect in clinopyroxene than in olivine. To apply laboratory results to processes occurring at depth in the Earth, detailed flow laws are essential in order to describe strain rate (i.e., viscosity) as a function of water concentration as well as differential stress, grain size, temperature, and pressure in both the diffusion and the dislocation creep regimes. Water concentrations are determined by micro-infrared analyses, microstructures produced by deformation are analyzed using optical and electron microscopy, and fabrics are measured using electron backscattered diffraction.