Collaborative Research: Field and Modeling-Based Tests of the Role of Water in Nominally Anhydrous Minerals in Controlling the Strength/Stability of Continental Lithospheric Mantle

地球在我们太阳系的岩石行星中可能有些独特，因为它有丰富的水。虽然地球内部水的实际浓度很小，但地球内部的总水量可能与我们海洋中的水量相当或更多。了解水在地球上的分布是很重要的，因为即使是少量的水的存在或缺乏也会对地幔的变形和融化产生重大影响。因此，从理论上讲，地球上的板块构造、大陆变形和火山活动的性质可能与含水量的微小变化密切相关。如果水的作用如此重要，那就意味着其他行星的动态状态也可能与水的存在与否有关。然而，到目前为止，水的这种假设作用还没有在实地得到充分的检验。这项研究的重点是表征大陆深处含水量的横向变化。这些数据将被输入到预测大陆变形的模型中，以便与观测到的大陆变形历史进行直接比较。研究人员将测量来自地幔捕虏体的橄榄石和辉石的含水量，这些捕虏体代表了明显强弱相邻区域的岩石圈地幔（科罗拉多高原和相邻的盆地和山脉；坦桑尼亚克拉通和东非裂谷的侧翼）。对地幔捕虏体喷发过程中橄榄石中H的扩散损失进行了两种独立的研究。首先，对橄榄石颗粒的核-边缘扩散曲线进行了反向建模，以获得其喷发前H含量的粗略估计。其次，利用共存辉石中的H含量（辉石中的H扩散率比橄榄石慢）和橄榄石和辉石之间H平衡分配的知识，定量推断橄榄石喷发前的水含量。随着对岩石圈热状态的约束（捕虏体热气压计），测量的橄榄石中H含量被纳入橄榄石黏性蠕变的本构律，作为H浓度的函数。这又被纳入简单的动力学模型，以量化岩石圈地幔的干湿作用对变形的性质和程度的影响。给定适当的边界条件和本构定律，这些简单的动态模型本身可用于提供与观测到的变形模式相匹配所需的束缚水空间分布的独立预测（也将考虑更复杂的动态模型）。将H测量和模型预测的组合与地质观测相比较，以确定岩石圈地幔的束缚H含量是否确实是大陆岩石圈横向变化变形的主要控制因素（例如，广泛变形带内的未变形塞）。建议的研究提供了岩石学/地球化学和地球动力学/地球物理学之间的协同作用。pi正在开发一门超越特定领域术语的高级本科/研究生课程，并将这两个领域联系起来。本课程将运用简单的方程式、模拟实验、思维实验以及对物理和地球化学数据的理解，培养对流体动力学和岩石学过程的物理直觉。额外的重点放在设置科学问题，识别重要变量，建立基于直觉的假设，并设计实验（自然，实验室，或数值）。

Earth may be somewhat unique among the rocky planets in our solar system in that it has an abundance of water. Although the actual concentration of water in the Earth's interior is small, the total amount of water inside Earth may be comparable to or greater than that in our ocean. Understanding how water is distributed in our planet is important because the presence or absence of even small amounts of water can significantly affect how the mantle deforms and melts. Thus, the nature of plate tectonics, continental deformation, and volcanism on Earth, in theory, may be intimately linked to small variations in water content. If the role of water is so fundamental, the implication is that the dynamic state of other planets may also be linked to the presence/absence of water. This hypothetical role of water, however, has so far not been fully tested in the field. This study is focused on characterizing lateral variations in water content in the deep parts of continents. These data will be input into models that predict the deformation of continents, allowing a direct comparison to the observed deformation histories of continents. The investigators will measure water contents in olivines and pyroxenes from mantle xenoliths representing the lithospheric mantle underlying adjacent regions that are demonstrably weak and strong (Colorado Plateau and the adjacent Basin and Range; Tanzania craton and the flanks of the East African Rift). The effects of diffusive loss of H in olivine during eruption of the mantle xenoliths is dealt with in two independent ways. First, core-to-rim diffusion profiles on olivine grains are backward modeled to obtain a rough estimate of their pre-eruptive H content. Second, pre-eruptive water contents of olivine are quantitatively inferred using the H contents in co-existing pyroxenes (H diffusivity in pyroxene is slower than in olivine) and knowledge of equilibrium partitioning of H between olivine and pyroxene. Along with constraints on the thermal state of the lithosphere (xenolith thermobarometry), measured H contents in olivines are incorporated into a constitutive law for viscous creep of olivine as a function of H concentration. This is in turn incorporated into simple dynamic models to quantify the effects of wet and dry lithospheric mantle on the nature and extent of deformation. Given appropriate boundary conditions and constitutive laws, these simple dynamic models are themselves used to provide independent predictions of the spatial distribution of bound water needed to match observed deformation patterns (more complicated dynamic models will also be considered). The combination of H measurements and model predictions are compared to geologic observations to determine whether bound H content of the lithospheric mantle is indeed a principal control on laterally varying deformation in continental lithosphere (e.g., undeformed plugs within broad deformational zones). The proposed study provides a synergy between petrology/geochemistry and geodynamics/geophysics. The PIs are developing an advanced undergraduate/graduate course that transcends field-specific terminology and bridge these two fields. The course will develop physical intuition on fluid dynamic and petrologic processes, using simple equations, analog experiments, thought experiments and an appreciation of physical and geochemical data. Additional emphasis is placed on setting up scientific problems, identifying important variables, building intuition-based hypotheses, and designing experiments (natural, laboratory, or numerical).