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

合作研究：名义无水矿物中的水在控制大陆岩石圈地幔强度/稳定性方面的作用的现场和基于模型的测试

• 批准号: 0802652
• 负责人: Anne Peslier
• 金额: $ 11.28万
• 依托单位: Universities Space Research Association
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-10-01 至 2011-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0802652&HistoricalAwards=false
• 关键词: Collaborative; Research; Field; Modeling; Based

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).

在太阳系的岩石行星中，地球可能有些独特，因为它拥有丰富的水。 尽管地球内部的实际水浓度很小，但地球内部的水总量可能与海洋中的水量相当或更多。 了解水在地球上的分布非常重要，因为即使是少量的水的存在或不存在也会显着影响地幔的变形和融化方式。 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，那么这意味着其他行星的动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为止尚未在现场得到充分测试。 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，从而可以与观测到的大陆变形历史直接进行比较。研究人员将测量来自地幔捕虏体的橄榄石和辉石中的水含量，这些地幔捕虏体代表了邻近区域（科罗拉多高原和邻近盆地和山脉；坦桑尼亚克拉通和东非裂谷的侧翼）下方的岩石圈地幔，这些区域明显较弱和较强。 地幔捕虏体喷发过程中橄榄石中氢扩散损失的影响可以通过两种独立的方式来处理。首先，对橄榄石颗粒的核心到边缘的扩散剖面进行逆向建模，以获得其喷发前氢含量的粗略估计。 其次，利用共存辉石中的 H 含量（辉石中的 H 扩散速度慢于橄榄石中的 H）以及橄榄石和辉石之间 H 的平衡分配知识，定量推断橄榄石喷发前的水含量。 除了对岩石圈热状态的限制（捕虏体热压法）之外，橄榄石中测得的 H 含量也被纳入橄榄石粘性蠕变作为 H 浓度函数的本构定律中。 这又被纳入简单的动力学模型中，以量化潮湿和干燥岩石圈地幔对变形性质和程度的影响。 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（还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结合 H 含量是否确实是大陆岩石圈横向变化变形（例如，广泛变形区内的未变形岩塞）的主要控制因素。 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 正在开发一门高级本科生/研究生课程，该课程超越特定领域的术语并在这两个领域之间架起桥梁。 该课程将使用简单的方程、模拟实验、思想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。 另外还强调提出科学问题、识别重要变量、建立基于直觉的假设以及设计实验（自然、实验室或数值）。