Collaborative Research: Voyage to the bottom of Arcs: interplay between water, deformation, and lower crustal stability

合作研究：弧底航行：水、变形和下地壳稳定性之间的相互作用

• 批准号: 1855430
• 负责人: Mark Behn
• 金额: $ 23.63万
• 依托单位: Boston College
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2019
• 资助国家: 美国
• 起止时间: 2019-07-01 至 2024-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1855430&HistoricalAwards=false
• 关键词: Collaborative; Research; Voyage; bottom; Arcs

The formation of continents is intimately linked to plate tectonics on Earth. Subduction zones are tectonic boundaries where oceanic plates sink back into the Earth's deep interior. This process triggers melting in the deep Earth, the formation of volcanic island arcs, and the building of continental crust over time. Arc volcanos are well-studied due to their exposure at the Earth's surface. However, the roots of island arcs are difficult to access and therefore, remain poorly studied. Deep arc roots play a critical role in controlling the evolution of continental crust and the Earth system as a whole. Due to their high density, arc roots are hypothesized to regularly detach (delaminate) from the shallow and less dense parts of the arc, and sink back into the Earth, modifying the composition of the bulk continental crust over time. Importantly, the viscous nature of deep arc roots, and hence their likelihood of being detached, depends on their composition, particularly water. This study will provide new constraints on how material is delaminated, or removed, from volcanic arc roots. The team will combine state-of-the-art volatile analyses with rheological measurements on natural samples, which will be integrated into numerical models. The formation and evolution of continents, without which no life on land could have developed, is an active topic of interest for the broad scientific community.The processes by which the Earth's crust, and in particular the continental crust, has been constructed over time have been debated since the observation that bulk continental crust is andesitic in composition, but mantle-derived parental melts are mostly basaltic. A number of studies have suggested that delamination, a process by which dense mafic rocks at the base of arcs regularly sink back into the mantle, could account for the chemical gap between bulk continental crust and mantle melts. Other studies have proposed that delamination alone is not sufficient to explain the discrepancy. Currently, numerical simulations of this process have two major limitations: (1) viscosities used in current models are not linked to water measurements on natural arc cumulates because these in-situ analyses have never been performed; and (2) it is assumed that the material that detaches is initially isotropic although natural cumulate samples show a wide range of textures and internal deformation. This is a novel interdisciplinary project that is focused on the chemical and rheological role of water in lower crustal cumulates from four arc settings. The goal is to examine the interplay between water, deformation, and lower crustal stability in subduction zones, using a unique combination of geochemical, rheological, and numerical approaches. This collaborative project will support two Ph.D. students and the training of a postdoctoral investigator. In addition, the PIs will design a tectonics class tailored for K-12 students at the Perkins School for the Blind (MA). They will also offer undergraduate internship opportunities and make an active effort to recruit promising minority students.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

大陆的形成与地球上的板块构造密切相关。俯冲带是海洋板块下沉到地球内部深处的构造边界。这一过程引发了地球深部的熔化，火山岛弧的形成，以及随着时间的推移大陆地壳的形成。由于弧火山暴露在地球表面，因此对其进行了充分的研究。然而，岛弧的根很难接近，因此，仍然很少研究。深弧根在控制大陆地壳和整个地球系统的演化中起着关键作用。由于其高密度，弧根被假设为定期从弧的较浅和较低密度的部分分离（分层），并沉入地球，随着时间的推移改变了大块大陆地壳的组成。重要的是，深弧根的粘性，因此它们被分离的可能性，取决于它们的成分，特别是水。这项研究将提供新的限制如何从火山弧根材料分层，或删除。该团队将联合收割机结合最先进的挥发性分析与天然样品的流变测量，这将被集成到数字模型。大陆的形成和演化是广大科学界感兴趣的一个活跃话题，没有大陆，陆地上的生命就不可能发展。地壳，特别是大陆地壳，随着时间的推移而形成的过程一直存在争议，因为观察到大块大陆地壳的成分是安山岩，但幔源母体熔体主要是玄武岩。许多研究表明，拆沉作用是弧底致密的基性岩定期沉入地幔的过程，可以解释大块大陆地壳和地幔熔体之间的化学间隙。其他研究提出，分层本身不足以解释这种差异。目前，这一过程的数值模拟有两个主要的局限性：（1）目前的模型中使用的粘度与自然弧累积的水测量无关，因为这些原位分析从未进行过;（2）尽管自然累积样品显示出广泛的纹理和内部变形，但假设分离的材料最初是各向同性的。这是一个新的跨学科项目，重点是水的化学和流变作用下地壳累积从四个弧设置。目的是研究俯冲带中水、变形和下地壳稳定性之间的相互作用，使用地球化学、流变学和数值方法的独特组合。该合作项目将支持两个博士学位。学生和博士后研究员的培训。此外，PI将为帕金斯盲人学校（MA）的K-12学生量身定制一门构造课。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

项目成果

High water content of arc magmas recorded in cumulates from subduction zone lower crust

• DOI: 10.1038/s41561-022-00947-w
• 发表时间: 2022-05
• 期刊: Nature Geoscience
• 影响因子: 18.3
• 作者: B. Urann;V. Le Roux;O. Jagoutz;O. Müntener;M. Behn;E. Chin