Rheological Behavior of Clinopyroxene and Clinopyroxene-Olivine Aggregates under Hydrous Conditions: Implications for Dynamics of the Lower Crust and Upper Mantle

研究人员建议对斜辉石的高温高压流变特性进行实验室研究，斜辉石是下地壳和上地幔中的一种重要矿物。因为即使是少量的水也会大大削弱名义上的无水矿物，所以将重点放在水对斜辉石和斜辉石+橄榄石团聚体粘度的贡献上。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。然而，最近，研究人员证明了橄榄石为主的聚集体的粘度与OH浓度成反比。提出的研究建立在这一观察和最近报道的关于干斜辉石岩变形行为的实验的基础上。水的浓度将通过微傅里叶红外分析来确定，变形产生的微观结构将通过光学和电子显微镜来分析。为了从实验室推断到地质条件，一个主要目标将是确定详细的流动规律，描述粘度作为水浓度的函数，以及扩散蠕变和位错蠕变状态下的差应力、晶粒尺寸和温度。最近对大陆地震深度和地壳厚度与弹性厚度之间关系的分析得出这样的结论：至少在某些地方，下地壳比上地幔更坚固。这种观点与大陆流变学的大多数模型相反，例如果冻三明治构造，在这种构造中，弱的下地壳位于强的上地壳和强的上地幔之间。许多模型认为大陆岩石圈的力量存在于莫霍下的地幔中。当然，下地壳和上地幔的相对强度将取决于水的存在与否。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与地幔相同的岩石中。我们最近的实验表明，这两种矿物质在无水条件下具有相似的强度，但斜辉石在有水的情况下较弱。为了解决这些问题和模拟下地壳和上地幔的动力学行为，仔细研究斜辉石和斜辉石+橄榄石聚集体的粘度对水浓度的依赖性是至关重要的。

Kohlstedt0126277The investigators propose a laboratory investigation of the high-temperature, high-pressure rheological properties of clinopyroxene, an important mineral in both the lower crust and the upper mantle. Because even a small amount of water dramatically weakens nominally anhydrous minerals, emphasis will be given to the contribution of water to the viscosity of aggregates of clinopyroxene and clinopyroxene + olivine. 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. Recently, however, the investigators demonstrated that the viscosity of olivine-dominated aggregates is inversely proportional to OH concentration. The proposed research builds on this observation and on recently reported experiments on the deformation behavior of dry clinopyroxenite. Water concentration will be determined by micro-FTIR analysis, and deformation-produced microstructures will be analyzed using optical and electron microscopy. In order to extrapolate from laboratory to geologic conditions, a major goal will be to determine detailed flow laws describing viscosity as a function of water concentration as well as differential stress, grain size and temperature in both the diffusion creep and the dislocation creep regime.Recent analyses of the depth of earthquakes in continents and of the correlation between crustal thickness and elastic thickness have led to the conclusion that the lower crust, at least in some places, is stronger than the uppermost mantle. This point of view is contrary to most models of continental rheology such as the jelly-sandwich construction in which a weak lower crust lies between a strong upper crust and a strong upper mantle. Many models take the strength of the continental lithosphere to lie in the mantle beneath the Moho. Certainly, the relative strength of lower crust and upper mantle will 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 as in the mantle. Our recent experiments suggest that these two minerals have similar strengths under anhydrous conditions but that clinopyroxene is weaker when water is present. A careful investigation of the dependence of viscosity on water concentration for aggregates of clinopyroxene and clinopyroxene + olivine is critical in order to address these issues and to model the dynamical behavior of the lower crust and upper mantle.