MARGINS: Cl Isotope Systematics of Neogene Izu Arc Volcanic Rocks

通过深层内部（地幔）和外部储层（大气、海洋、大陆和海洋地壳）之间不断的化学交换，地球仍然是一个适宜居住的星球。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”始于环绕全球的大洋中脊下地幔融化形成的海洋地壳。海洋地壳和下面地幔的最上层形成了一个厚达10公里的“板块”，在深海海沟处“俯冲”回深地幔。“俯冲带”是地球系统的重要组成部分。在陆地上，它们可以从火山链中辨认出来，形成与深海海沟平行的“弧”（如喀斯喀特山脉、阿留申群岛、印度尼西亚）。与俯冲有关的灾害，如地震、海啸和猛烈的爆炸性喷发，可对人类生命和经济造成毁灭性的破坏，并已知会影响全球气候(例如：1815年印度尼西亚坦博拉火山爆发后的1816年被称为“没有夏天的一年”)。这些灾害与俯冲带的水处理直接相关。在俯冲之前，水在大洋板块中变得丰富，随后在俯冲期间从板块中流失（连同其他挥发性元素，如Cl， CO2）。俯冲带的水作用可能导致上地幔广泛的“蛇纹岩化”，即干脆橄榄岩转变为含水、延展性的蛇纹岩。这种蛇纹岩可能储存大量的气候活性挥发物（例如H2O），这些挥发物通过弧状火山的爆发注入或再循环到大气中。边际项目的“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挥发性通量的问题。然而，蛇纹岩形成的直接地质证据只存在于~20-25公里的深度，在这个深度之外的调查必须依靠弧火山的化学性质。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来源组分的影响，其中需要识别蛇纹岩的特征。由于蛇纹岩是高流动性Cl的汇，其同位素（35Cl、37Cl）在示踪俯冲蛇纹岩方面可能具有独特的潜力。该项目旨在通过对玻璃状火山灰碎片（1毫米）的分析，确定Cl同位素作为伊豆波宁弧（西北太平洋）新近纪（高达2000万年）火山岩中俯冲蛇纹岩示踪剂的潜力。Susanne Straub先前的研究表明，伊豆波宁弧熔体的Cl丰度很高，Cl可能来自弧下的蛇纹岩化地幔。火山玻璃颗粒中的Cl同位素分析将采用新的SIMS（二次离子质谱）技术。该研究将测试熔体分化（例如脱气）可能影响本征同位素比率的影响。作为第一个在弧环境中系统研究Cl同位素，这项工作将促进对Cl同位素分馏以及Cl通量的理解，并可能在更广泛的地球化学循环中了解H2O。更广泛的影响：需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，也是边缘俯冲工厂倡议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，并进一步支持地球科学领域的女性。Susanne Straub是一位残疾的女性研究人员，她已经对伊豆冰原岩浆作用做出了实质性的贡献，这是她研究的重点。一名本科生也将参与这个项目，因为这为研究提供了一个很好的介绍。

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.