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

俯冲带邻近板片的水合物质的深层传输的意义

• 批准号: 0944157
• 负责人: Laurent Montesi
• 金额: $ 16.58万
• 依托单位: University of Maryland, College Park
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-01-01 至 2013-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0944157&HistoricalAwards=false
• 关键词: Implications; Deep; Transport; Slab; Adjacent

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.

从地球表面到下地幔的水输送以及全球范围内潜在的返回机制是了解整个地幔地球化学演化、整个地幔动力学和水收支的重要考虑因素。 俯冲板块可以在含水矿物中携带大量的水，但大多数这些矿物在下地幔顶部之前或附近下降到较高压力/温度条件时会脱水。 另一个重要的下行储层是板片相邻流场中夹带的水合地幔物质（名义上无水矿物中的水，形成低粘度通道，或 LVC）。 由于板片表面和水合固相线之间的流体迁移和热分离，LVC 在浅地幔楔中形成。 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后果。 我们将开发与板片相关的地幔流和地球化学演化的二维数值模型，以表征俯冲带LVC向下地幔深部迁移的地球动力学和地球化学影响，并利用深部地幔地震速度结构和观测结果来评估这些模型。大洋岛玄武岩地球化学。 我们将定义粘度变化（包括 LVC 内的全局径向分量和局部粘度降低）对整体速度结构的影响。 我们将包括深度脱水反应，评估潜在密度对比和熔化，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，从而引入由流体修改的微量元素定义的化学异质性和同位素特征。 我们将使用岩石学模型 MELTS 来评估板片和/或板片邻近材料的化学贡献，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。 该研究整合了地球物理和地球化学约束，对深部板片地球动力学和相关地幔固体流场进行了综合研究。 地球内部深处存在的水量是全球水循环中受限制最少的方面。 当构造板块下沉并在俯冲带回收到地球内部时，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会保留在矿物结构内并到达最底层的地幔，在那里它能够影响地幔流动模式和融化，其方式可以通过地表熔岩观察到。 我们将开发与板块深俯冲相关的地幔流的二维地球动力学模型，以（i）确定矿物结构中的水对系统物理动力学的影响，以及（ii）研究与板块深俯冲相关的地幔岩石融化水的深度引入。 通过与地球内部的地震研究和深层熔岩的地球化学研究进行比较，我们将能够对地球的深水循环提供新的限制。 该项目由新任女性研究员领导，将为早期职业科学家提供参与尖端综合研究的宝贵经验。 从地球科学以外领域招收的本科生的参与将带来广泛的经验，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，并为博士后研究人员提供咨询经验。 两位 PI 都参与了面向少数族裔学生和当地高中的外展项目。 这项研究的结果将通过国家和国际会议以及同行评审出版物广泛传播给地球科学界。