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

含水条件下单斜辉石和单斜辉石-橄榄石聚集体的流变行为：对下地壳和上地幔动力学的启示

• 批准号: 0126277
• 负责人: David Kohlstedt
• 金额: $ 32.02万
• 依托单位: University of Minnesota-Twin Cities
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2002
• 资助国家: 美国
• 起止时间: 2002-01-01 至 2005-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0126277&HistoricalAwards=false
• 关键词: Rheological; Behavior; Clinopyroxene; Olivine; Aggregates

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.

Kohlstedt 0126277研究人员建议对单斜辉石的高温高压流变特性进行实验室研究，单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显着削弱名义上的无水矿物，重点将被赋予水的贡献，单斜辉石和单斜辉石+橄榄石的聚集体的粘度。在大多数研究中，水弱化效应被视为一个“开-关”的过程-如果有水，岩石是脆弱的;如果没有水，岩石是强大的。然而，最近的研究表明，橄榄石为主的聚集体的粘度是成反比的OH浓度。建议的研究建立在这一观察和最近报道的干单斜辉石岩的变形行为的实验。将通过显微FTIR分析确定水浓度，并使用光学和电子显微镜分析变形产生的微观结构。为了从实验室推断地质条件，一个主要目标是确定详细的流动定律，描述粘度作为水浓度和差应力的函数，最近对大陆地震深度和地壳厚度与弹性厚度之间的相关性的分析得出的结论是，下地壳，至少在某些地方，比上地幔更坚固。这一观点与大陆流变学的大多数模型相反，如夹层结构，其中软弱的下地壳位于强大的上地壳和强大的上地幔之间。许多模型认为大陆岩石圈的强度位于莫霍面之下的地幔中。当然，下地壳和上地幔的相对强度取决于水的存在与否。关于单斜辉石相对于橄榄石的强度知之甚少，即使这两个相存在于地幔中的同一岩石中。我们最近的实验表明，这两种矿物在无水条件下具有相似的强度，但当有水存在时，单斜辉石较弱。为了解决这些问题，并模拟下地壳和上地幔的动力学行为的粘度对水浓度的依赖性的单斜辉石和单斜辉石+橄榄石聚集体的仔细调查是至关重要的。