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

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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).