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。 虽然地球内部的水的实际浓度很小，但地球内部的水总量可能与我们的海洋相当或更大。 了解水在我们星球上的分布是很重要的，因为即使是少量水的存在或缺乏也会显著影响地幔的变形和融化。 因此，地球上板块构造、大陆变形和火山活动的性质，在理论上，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，那么这意味着其他行星的动态状态也可能与水的存在/不存在有关。 然而，水的这种假设作用迄今尚未在实地得到充分检验。 这项研究的重点是描述大陆深部含水量的横向变化。 这些数据将输入预测大陆变形的模型，从而可以与观测到的大陆变形历史进行直接比较。研究人员将测量地幔捕虏体中橄榄石和辉石的含水量，这些捕虏体代表了明显较弱和较强的邻近地区（科罗拉多高原和邻近的盆地和山脉;坦桑尼亚克拉通和东非裂谷的侧翼）下面的岩石圈地幔。 本文用两种独立的方法研究了地幔捕虏体喷发过程中橄榄石中氢的扩散损失效应。首先，核心到边缘的橄榄石颗粒扩散配置文件向后建模，以获得一个粗略的估计，其喷发前的H含量。 第二，喷发前的橄榄石水含量的定量推断使用的H含量在共存的辉石（H扩散率在辉石是慢于橄榄石）和知识之间的橄榄石和辉石的H平衡分配。 沿着与岩石圈的热状态（捕虏体温压法）的约束，测得的橄榄石中的H含量被纳入本构关系的粘性蠕变的橄榄石作为H浓度的函数。 这反过来又被纳入简单的动力学模型，以量化的性质和变形程度的岩石圈地幔的湿和干的影响。 给定适当的边界条件和本构关系，这些简单的动态模型本身用于提供与观察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进行比较，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，在宽的变形区域内的未变形的岩塞）。 拟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正在开发一个高级本科/研究生课程，超越了特定领域的术语和桥梁这两个领域。 该课程将开发流体动力学和岩石学过程的物理直观，使用简单的方程，模拟实验，思想实验和物理和地球化学数据的赞赏。 额外的重点放在建立科学问题，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).