Constraining the release of oxidizing fluids from the mafic slab during subduction

俯冲带是地质活动的关键区域，在这里，两个板块发生碰撞，一个板块被压在另一个板块之下，并被俯冲到地幔深处。俯冲带是一些最重要的全球地质事件的罪魁祸首，并通过引发地震和形成大型火山弧(如太平洋“火环”)对人类和人口稠密地区产生重大影响。俯冲带火山活动在多大程度上受到俯冲板块和上覆地幔岩石之间的物质和流体交换的控制，存在不确定性。研究这种过程的一种方法是分析以前被俯冲到很深的地方，但后来被挖掘出来并返回地球表面的岩石。该项目的目的是调查西阿尔卑斯山(意大利/瑞士)俯冲带岩石中单个石榴石晶体中所含流体在俯冲过程中的释放记录和特征。这一研究将进一步加深我们对俯冲流体在控制俯冲板块和上覆地幔中岩石的化学氧化或还原作用方面的重要性的理解，这将为深入了解它们在控制火山弧岩浆活动中的作用提供依据。该项目将支持波士顿学院的一名博士后研究员，他将与本科生一起工作，为他们提供分析技术和研究经验方面的培训。波士顿学院的研究团队将与来自欧洲和美国的大型合作者网络合作。该项目的一个重要组成部分将包括本科生前往欧洲，在欧洲合作者的帮助下进行自己的研究。关于俯冲板块脱水过程中释放的变质流体在控制上覆弧下地幔的氧化还原条件以及解释火山弧岩浆氧化和富含挥发分的地球化学特征方面的作用，存在着很大的争议。S、C和Fe等多价氧化还原敏感元素从俯冲板块的转移代表了产生弧下地幔氧化的一种潜在机制。这项拟议的研究将检验这样一种假设，即在俯冲过程中板块镁铁质成分脱水过程中，氧化物质在流体中释放。这项研究包括建立镁铁质地壳俯冲过程中流体释放的现场记录，检查这些流体是否含有氧化物种，以及评估俯冲镁铁质板块和上覆弧下地幔内氧化还原条件变化的影响。为了实现这一目标，该项目将利用从西阿尔卑斯山挖掘出的高压变质岩中单个石榴石晶体内生长带的首次铁和锌同位素分析。这将与精确的Sm/ND分带石榴石年代学相结合，以提供对氧化还原控制变质反应的速率和时间尺度的洞察。在国际合作者网络的帮助下，该项目旨在提供一个独特的高分辨率地球化学和地质年代学数据集，以评估俯冲带界面上发生的过程及其与弧下地幔和火山弧岩浆氧化的关系。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Subduction zones are key areas of geologic activity in which two plates collide and one plate is forced beneath the other and subducted deep into the Earth’s mantle. Subduction zones are responsible for some of the most important global geologic events and significantly impact humans and populated areas by causing earthquakes and creating large volcanic arcs such as the Pacific ‘Ring of Fire’. Uncertainty exists in the degree to which subduction zone volcanism is controlled by the exchange of material and fluids between the subducting plate and the overlying mantle rocks. One way to study such processes, is to analyze rocks that were previously subducted to great depths but have since been exhumed and returned to the Earth’s surface. The aim of this project is to investigate the record of the release and characteristics of fluids produced during subduction contained within individual garnet crystals in exhumed subduction zone rocks from the Western Alps (Italy/Switzerland). This study will further our understanding into the importance of subduction-derived fluids in controlling whether rocks within the subducting plate and the overlying mantle are more chemically oxidized or reduced, which will provide insights into their role in controlling volcanic arc magmatism. This project will support a post-doctoral researcher at Boston College, who will work with undergraduate students, providing them with training in analytical techniques and research experience. The research team from Boston College will work with a large network of collaborators from Europe and the US. An important component of this project will include undergraduate students travelling to Europe to conduct their own research with the help of European collaborators. Much debate exists around the role of metamorphic fluids, released during dehydration of the subducting slab, in controlling the redox conditions of the overlying sub-arc mantle and in explaining the oxidized and volatile-enriched geochemical signatures of volcanic arc magmas. The transfer of multi-valent, redox-sensitive elements such as S, C, and Fe from the subducting slab, represents a potential mechanism to produce oxidation of the sub-arc mantle. The proposed research will test the hypothesis that oxidizing species are released within fluids during dehydration of the mafic component of the slab during subduction. This study involves creating a field-based record of the release of fluids during subduction of the mafic crust, examining whether these fluids contain oxidizing species, and assessing the implications for changing redox conditions within the subducting mafic slab and overlying sub-arc mantle. To achieve this, the project will utilize the first coupled Fe and Zn isotopic analyses of growth zones within individual garnet crystals from exhumed high-pressure metamorphic rocks from the Western Alps. This will be combined with precise Sm/Nd zoned garnet geochronology to provide insights into the rates and timescales of redox-controlling metamorphic reactions. With the help of an international network of collaborators, this project aims to provide a uniquely high-resolution geochemical and geochronological dataset to assess processes occurring at the subduction zone interface and their relationship to oxidation of the sub-arc mantle and volcanic arc magmatism.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.