MARGINS: Cl Isotope Systematics of Neogene Izu Arc Volcanic Rocks

边缘：新近纪伊豆弧火山岩的 Cl 同位素系统学

• 批准号: 0549055
• 负责人: Susanne Straub
• 金额: --
• 依托单位: Columbia University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2006
• 资助国家: 美国
• 起止时间: 2006-03-01 至 2009-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0549055&HistoricalAwards=false
• 关键词: MARGINS; Cl; Isotope; Systematics; Neogene

The Earth remains a habitable planet through continual chemical exchanges between the deep interior (the mantle) and exterior reservoirs (atmosphere, oceans, continental and oceanic crust). Understanding the components of the dynamic Earth System is a central goal of modern research in the geosciences. The 'solid Earth cycle' begins with formation of oceanic crust by mantle melting beneath the globe-encircling mid-ocean ridges. The oceanic crust and the uppermost part of the underlying mantle form a 'slab' of several 10 kilometer thickness that is 'subducted' back to the deep mantle at the deep-sea trenches.'Subduction zones' are a vital part of the Earth system. On land they are recognizable from chains of volcanoes forming 'arcs' parallel to the deep-sea trenches (e.g. the Cascades, Aleutians, Indonesia). Subduction related hazards such as earthquakes, tsunamis and violent explosive eruptions can cause devastating destruction to human life and economics and are known to influence the global climate (e.g. 'year without summer' in 1816 following the 1815 eruption of Mt. Tambora in Indonesia).Such disasters are related directly to the processing of water in subduction zones. Water becomes enriched in the oceanic plate prior to subduction, and is subsequently lost from the slab (along with other volatile elements; e.g. Cl, CO2) during subduction. The processing of water in subduction zones may result in wide-spread 'serpentinization' of the upper mantle, which is the transformation of the dry and brittle peridotite into a hydrous, ductile serpentinite. Such serpentinite rocks may store large amounts of climatically active volatiles (e.g. H2O) that are injected or recycled to the atmosphere through explosive eruptions in arc volcanoes.The Subduction Factory portion of the MARGINS Program addresses the question of how serpentinite formation and destruction influences the volatile fluxes in subduction zones. However, direct geological evidence for serpentinite formation exists only to ~20-25 km depth, and investigation beyond these depths must rely on the chemistry of arc volcanoes. This is a challenging task because the chemistry of arcs reflects the influence of several source components, among which the signature of the serpentinite needs to be identified. Because serpentinite is a sink for the highly fluid-mobile Cl, its isotopes (35Cl, 37Cl) may have unique potential in tracing subduction serpentinization.This project aims to determine the potential of Cl isotopes as tracers of subduction serpentinite in Neogene (up to 20 million year old) volcanic rocks of the Izu Bonin arc (NW Pacific) via analysis of glassy volcanic ash fragments (1 mm). Prior studies by co-PI Susanne Straub revealed high Cl abundances the Izu Bonin arc melts, with the Cl likely derived from the serpentinized mantle below the arc. Cl isotopic analysis in volcanic glass particles will utilize new SIMS (secondary ion mass spectrometry) technology. The study will test for the effects of melt differentiation (e.g. degassing) that might affect the intrinsic isotope ratios. As the first systematic study of Cl isotopes in an arc setting, this work will promote the understanding of Cl isotope fractionation as well as the fluxes of Cl, and likely H2O in the broader geochemical cycle.Broader Impacts: The issues to be addressed are at the core of current research activities on the role of water in subduction zones, and of one of the three fundamental science themes of the MARGINS Subduction Factory Initiative. The project will promote scientific partnership between three key science research institutions in the U.S. Support for women in the geosciences also will furthered. Susanne Straub, a female researcher with a disability, has already made substantial contributions to Izu Bonin magmatism that is a key focus of her research. An undergraduate student will also be involved to work on this project, as this provides an excellent introduction to research.

通过内部深处（地幔）和外部储层（大气、海洋、大陆和海洋地壳）之间持续的化学交换，地球仍然是一颗宜居行星。了解动态地球系统的组成部分是现代地球科学研究的中心目标。 “固体地球周期”始于环绕地球的洋中脊下方地幔融化形成洋壳。洋壳和底层地幔的最上部形成了一个厚度达10公里的“板块”，在深海海沟处“俯冲”回深部地幔。“俯冲带”是地球系统的重要组成部分。在陆地上，可以通过形成与深海海沟平行的“弧形”的火山链（例如印度尼西亚阿留申群岛的喀斯喀特群岛）来识别它们。与俯冲有关的灾害，如地震、海啸和猛烈的爆炸性喷发，可能对人类生命和经济造成毁灭性的破坏，并且会影响全球气候（例如，1815 年印度尼西亚坦博拉山喷发后的 1816 年出现“无夏之年”）。此类灾害与俯冲带的水处理直接相关。在俯冲之前，水在海洋板块中变得丰富，随后在俯冲过程中从板块中流失（以及其他挥发性元素；例如 Cl、CO2）。俯冲带中的水处理可能会导致上地幔广泛的“蛇纹石化”，即干燥且脆的橄榄岩转变为含水的、有延展性的蛇纹岩。这种蛇纹岩可能储存大量的气候活跃挥发物（例如H2O），这些挥发物通过弧火山的爆炸性喷发注入或循环到大气中。MARGINS计划的俯冲工厂部分解决了蛇纹岩的形成和破坏如何影响挥发物的问题俯冲带的通量。然而，蛇纹岩形成的直接地质证据仅存在于约 20-25 公里深度，超出这些深度的调查必须依赖于弧火山的化学性质。这是一项具有挑战性的任务，因为弧的化学反应反映了多种源成分的影响，其中需要识别蛇纹岩的特征。由于蛇纹岩是高度流体流动的 Cl 的汇，其同位素 (35Cl、37Cl) 在追踪俯冲蛇纹石化方面可能具有独特的潜力。本项目旨在确定 Cl 同位素作为新近纪俯冲蛇纹岩示踪剂的潜力（最多 20通过对玻璃状火山灰碎片（1 毫米）的分析，了解伊豆小笠原岛弧（西北太平洋）的火山岩。联合首席科学家苏珊娜·斯特劳布 (Susanne Straub) 之前的研究表明，伊豆博南岛弧熔化物中存在高 Cl 丰度，其中 Cl 很可能来自弧下方的蛇纹石化地幔。火山玻璃颗粒中的 Cl 同位素分析将利用新的 SIMS（二次离子质谱）技术。该研究将测试可能影响固有同位素比率的熔体分化（例如脱气）的影响。作为弧环境中 Cl 同位素的首次系统研究，这项工作将促进对 Cl 同位素分馏以及更广泛的地球化学循环中 Cl 和可能的 H2O 通量的理解。 更广泛的影响：要解决的问题是当前关于水在俯冲带中的作用的研究活动的核心，也是 MARGINS 俯冲工厂计划的三个基础科学主题之一。该项目将促进美国三个主要科学研究机构之间的科学伙伴关系，也将进一步加强对地球科学领域女性的支持。苏珊娜·斯特劳布 (Susanne Straub) 是一名残疾女性研究员，她已经对伊豆博宁岩浆作用做出了重大贡献，这是她研究的重点。一名本科生也将参与该项目，因为这为研究提供了极好的介绍。