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: 0635700
• 负责人: Stephen Mackwell
• 金额: $ 2.42万
• 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: 2007-01-01 至 2009-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0635700&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，地球可能是独一无二的，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海洋中的水相当或更多。了解水在地球上是如何分布的很重要，因为即使是少量水的存在或不存在也会对地幔的变形和融化产生重大影响。因此，从理论上讲，板块构造、大陆变形和地球上的火山活动的性质可能与水含量的微小变化密切相关。如果水的作用如此重要，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，到目前为止，水的这种假设作用还没有在现场得到充分的检验。这项研究的重点是描述大陆深处水分含量的横向变化。这些数据将被输入到预测大陆变形的模型中，从而可以直接与观测到的大陆变形历史进行比较。调查人员将测量来自地幔包体的橄榄石和辉石的水分含量，这些包体代表明显较弱和较强的邻近地区(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两侧)下的岩石圈地幔。在地幔包体喷发过程中，橄榄石中氢的扩散损失的影响可以用两种独立的方法来处理。首先，对橄榄石颗粒从核到边缘的扩散剖面进行反向模拟，以获得其喷发前氢含量的大致估计。其次，利用共生辉石中的氢含量(氢在辉石中的扩散速率慢于橄榄石中的氢扩散速率)和H在橄榄石和辉石之间的平衡分配知识，定量地推断了橄榄石的喷发前水含量。除了对岩石圈热状态的限制(捕虏体温压法)，测量的橄榄石中的H含量被纳入作为H浓度函数的橄榄石粘性蠕变的本构定律。这反过来又被纳入简单的动力学模型，以量化潮湿和干燥的岩石圈地幔对变形性质和程度的影响。在给定适当的边界条件和本构定律的情况下，这些简单的动态模型本身用于提供与观测到的变形模式匹配所需的束缚水的空间分布的独立预测(也将考虑更复杂的动态模型)。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结合H含量是否确实是大陆岩石圈横向变化变形(例如，大变形带内的未变形塞)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。PIS正在开发一门高级本科生/研究生课程，该课程超越了特定领域的术语，并在这两个领域之间架起了桥梁。本课程将发展对流体动力学和岩石学过程的物理直觉，使用简单的方程、模拟实验、思维实验和对物理和地球化学数据的欣赏。另外的重点是建立科学问题，确定重要的变量，建立基于直觉的假设，以及设计实验(自然的、实验室的或数值的)。