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.

地球仍然是一个可居住的星球，通过内部深处（地幔）和外部水库（大气，海洋，大陆和海洋地壳）之间的持续化学交换。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。“固体地球循环”始于环绕地球的洋中脊下地幔熔融形成洋壳。大洋地壳和下地幔的最上部形成了一个厚达数十公里的“板块”，在深海海沟处“俯冲”回地幔深处。“俯冲带”是地球系统的重要组成部分。在陆地上，它们可以从形成平行于深海海沟的“弧”的火山链中识别出来（例如印度尼西亚阿留申群岛的瀑布）。与俯冲有关的灾害，如地震、海啸和猛烈的爆炸性喷发，可能对人类生活和经济造成毁灭性的破坏，并已知会影响全球气候（例如，1815年火山爆发后，1816年出现“无夏之年”）。这些灾害与俯冲带的水处理直接相关。水在俯冲前在大洋板块中富集，随后在俯冲过程中从板块中损失（沿着其他挥发性元素，如Cl、CO2）。俯冲带中的水作用可能导致上地幔广泛的“蛇纹石化”，即干脆的橄榄岩转变为含水的韧性蛇纹岩。这种蛇纹岩可能储存大量的气候活性挥发物（如H2O），这些挥发物通过弧火山的爆发注入或再循环到大气中。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俯冲带中挥发物通量的问题。然而，蛇纹岩形成的直接地质证据仅存在于~20-25公里的深度，超过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，因为弧的化学性质反映了几种源成分的影响，其中蛇纹岩的特征需要确定。由于蛇纹岩是高流体流动性Cl的汇，其同位素（35 Cl，37 Cl）可能在追踪俯冲蛇纹化作用方面具有独特的潜力。本项目旨在通过分析玻璃质火山灰碎片（1 mm），确定Cl同位素作为伊豆小笠弧（西北太平洋）新近纪（2000万年前）火山岩中俯冲蛇纹岩示踪剂的潜力。共同PI Susanne Straub先前的研究显示，伊豆小笠原弧熔体中Cl丰度较高，Cl可能来自弧下的蛇纹状地幔。火山玻璃颗粒中的Cl同位素分析将利用新的西姆斯（二次离子质谱）技术。这项研究将测试可能影响固有同位素比的熔体分化（例如脱气）的影响。作为首次对弧背景下的Cl同位素进行系统研究，这项工作将促进对Cl同位素分馏以及Cl通量的理解，并可能促进更广泛的地球化学循环中的H2O。有待解决的问题是目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本科学主题之一。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，也将进一步支持地球科学领域的妇女。Susanne Straub是一位残疾女性研究员，她已经对伊豆小笠原岩浆活动做出了重大贡献，这是她研究的一个重点。一名本科生也将参与这个项目的工作，因为这提供了一个很好的介绍研究。