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，以及全球范围内的潜在返回机制是理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。 俯冲板块可以携带大量的水在含水矿物，但大多数这些矿物下降到更高的压力/温度条件下之前或通过下地幔的顶部。 另一个重要的下行储层是夹带在板块附近流场中的水合地幔物质（名义上无水矿物中的水，形成低粘度通道或LVC）。 LVC形成于浅地幔楔中，是板块表面与水化固相线之间流体迁移和热分离的结果。 它的重要作用是降低了当地固体相对于周围地幔的粘度和密度。 我们将开发2-D的板状地幔流和地球化学演化的数值模型，以表征地球动力学和地球化学的影响，深部运输的LVC下地幔在俯冲带和评估这些模型使用深地幔地震速度结构和洋岛玄武岩地球化学的观察。 我们将定义粘度变化的影响，包括全球径向分量和LVC内的局部粘度降低，整体速度结构。 我们将包括深度脱水反应，评估潜在的密度对比和熔融，并将确定是否浮力的LVC将导致它从热板分离，并与周围地幔混合，从而引入流体改性微量元素和同位素特征定义的化学异质性。 我们将使用岩石学模型MELTS来评估板块和/或板块相邻材料的化学贡献，并将模型预测与现有的海洋岛屿玄武岩地球化学数据集进行比较。 本研究结合地球物理和地球化学约束条件，对深部板片地球动力学和地幔固体流场进行了综合研究。 地球内部深处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循环进入地球内部时，它们在某些矿物的结构中携带沿着大量的水。 一些水将通过脱水反应释放出来，但一个潜在的重要部分可能会留在矿物结构中，并到达最低的地幔，在那里它能够影响地幔流动模式和熔融的方式，可以通过熔岩在表面观察。 我们将开发与板块深俯冲相关的地幔流的2-D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）研究与水的深引入相关的地幔岩石熔融。 通过与地球内部的地震研究和深层熔岩的地球化学研究相比较，我们将能够对地球的深层水循环提供新的限制。 该项目由新的女性研究者领导，并将提供参与早期职业科学家尖端综合研究的宝贵经验。 从地球科学以外的领域招募的本科生的参与将允许经验的广度，并将宣传地球物理/地球化学研究到其他领域，鼓励跨学科创新，以及提供咨询经验的博士后研究员。 这两个PI都参与了对少数民族学生和当地高中的外展计划。 这项研究的结果将通过国家和国际会议以及同行评审的出版物向地球科学界广泛传播。