Implications of Deep Transport of Slab-Adjacent Hydrated Material at Subduction Zones

水从地表到下地幔的输运，以及全球尺度上潜在的返回机制，是理解全地幔地球化学演化、全地幔动力学和水收支的重要考虑因素。俯冲板块可以携带大量的含水矿物，但这些矿物中的大多数在下地幔顶部之前或之前下降到更高的压力/温度条件下时会脱水。另外一个重要的下行储集层是水合地幔物质(水保存在名义上无水的矿物中，形成低粘度通道，或LVC)，夹带在平板相邻的流场中。低VC形成于浅地幔楔体中，是板坯表面与水化固相线之间流体迁移和热分离的结果。它具有降低相对于周围地幔的局部固体粘度和密度的重要结果。我们将建立与板块相关的地幔流动和地球化学演化的二维数值模型，以表征俯冲带低VC向下地幔深部输送的地球动力学和地球化学意义，并通过对深部地幔地震速度结构和洋岛玄武岩地球化学的观测来评估这些模型。我们将定义粘度变化对整体速度结构的影响，包括全局径向分量和LVC内的局部粘度降低。我们将包括深度脱水反应，评估潜在密度对比和熔融，并将确定LVC的浮力是否会导致其与热板分离并与周围地幔混合，从而引入由流体修饰的微量元素和同位素特征定义的化学不均质性。我们将使用岩石学模型熔体来评估板片和/或板片相邻物质的化学贡献，并将模型预测与现有的洋岛玄武岩地球化学数据集进行比较。该研究综合地球物理和地球化学约束，对深部板岩地球动力学及其相关的地幔固体流场进行了综合研究。存在于地球深处的水量是全球水循环中受限制最少的方面。随着构造板块下沉，并在俯冲带重新进入地球内部，它们在某些矿物的结构中携带了大量的水。一些水将通过脱水反应释放出来，但潜在的重要部分可能留在矿物结构中并到达最低地幔，在那里它能够影响地幔流动模式和熔融，通过表面熔岩可以观察到的方式。我们将建立与板块深俯冲有关的地幔流动的二维地球动力学模型，以(I)确定矿物结构中所含水对系统物理动力学的影响，以及(Ii)研究与深部水引入有关的地幔岩石的熔融。通过与地球内部的地震研究和深源熔岩的地球化学研究相比较，我们将能够为地球深水循环提供新的约束。该项目由新的女性研究人员领导，将提供宝贵的经验，由一位职业生涯早期的科学家参与一项尖端综合研究。从地球科学以外的领域招聘的本科生的参与将允许丰富的经验，并将向其他领域宣传地球物理/地球化学研究，鼓励跨学科创新，以及为博士后研究人员提供建议经验。这两个私人助理都参与了针对少数族裔学生和当地高中的外联计划。这项研究的结果将通过国家和国际会议以及同行评议的出版物广泛传播给地球科学界。

Water transport from the Earth's surface into the lower mantle, and potential mechanisms of return on a global scale are important considerations for understanding whole mantle geochemical evolution, whole mantle dynamics, and the water budget. Subducting slabs can carry significant amounts of water in hydrous minerals, but most of these minerals dewater as they descend into higher pressure/temperature conditions before or by the top of the lower mantle. An additional, important down-going reservoir is hydrated mantle material (water held in nominally-anhydrous minerals, forming a low-viscosity channel, or LVC) entrained in the slab-adjacent flow field. The LVC forms in the shallow mantle wedge as a consequence of fluid migration and thermal separation between the slab surface and the hydrated solidus. It has the important consequence of reducing the local solid viscosity and density relative to ambient mantle. We will develop 2-D numerical models of slab-associated mantle flow and geochemical evolution to characterize the geodynamical and geochemical implications of deep transport of the LVC to the lower mantle at subduction zones and evaluate these models using observations of deep mantle seismic velocity structure and ocean island basalt geochemistry. We will define the impact of viscosity variations, including global radial components and local viscosity reduction within the LVC, to the overall velocity structure. We will include deep dehydration reactions, evaluate potential density contrasts and melting, and will determine if the buoyancy of the LVC will lead it to separate from the thermal slab and mix with ambient mantle, thereby introducing a chemical heterogeneity defined by fluid-modified trace element and isotopic character. We will use the petrological model MELTS to evaluate the chemical contributions of slab and/or slab-adjacent material and compare model predictions with existing geochemical datasets of ocean island basalts. This research integrates geophysical and geochemical constraints for a comprehensive study of deep slab geodynamics and the associated mantle solid flow field. The amount of water present in the deep interior of the Earth is the least constrained aspect of the global water cycle. As tectonic plates sink and are recycled into the Earth's interior at subduction zones, they carry along a significant amount of water within the structure of certain minerals. Some water will be liberated through dehydration reactions but a potentially important fraction may remain within mineral structures and reach the lowermost mantle, where it is able to influence mantle flow patterns and melting in ways that can be observed through lavas at the surface. We will develop 2-D geodynamic models of mantle flow associated with the deep subduction of plates to (i) determine the impact of water held in mineral structures on the physical dynamics of the system and (ii) study melting of mantle rocks associated with the deep introduction of water. By comparison with seismic studies of the Earth's interior and geochemical studies of deeply originating lavas, we will be able to provide new constraints on the deep water cycle of the Earth. This project is led by new female investigator and will provide the valuable experience of participation in a cutting-edge integrative study by an early-career scientist. Involvement of undergraduate students recruited from areas outside the geosciences will allow for breadth of experience and will advertise geophysical/geochemical research to other fields, encouraging interdisciplinary innovation, as well as providing advising experience to a postdoctoral investigator. Both PIs are involved in outreach programs to minority students and local high schools. The results of this research will be disseminated broadly to the earth science community through national and international meetings and peer-reviewed publications.