Volatile Recycling in Subduction Zones and Formation of Primitive Arc Magmas: A Case Study of the Trans-Mexican Volcanic Belt

EAR-0309559华勒俯冲带在全球地球化学循环中发挥着重要作用，因为它们为地壳物质循环进入地幔提供了一条途径。这些材料包括许多挥发性化合物，如水、碳、硫和氯。俯冲的地壳物质在下降时会被上覆地幔加热，挥发分至少会通过变质脱挥反应释放出来，然后迁移到俯冲板块上方的热地幔中。由于水降低了地幔岩石的融化温度，这一过程导致了上覆地幔的融化。因此，俯冲带中挥发物的循环是导致地幔中广泛形成岩浆的原因，导致了像环太平洋“火环”那样的火山链。本研究的目的是探讨挥发性元素(H、C、S、Cl)在俯冲带中如何循环，以及挥发分在俯冲板块上方地幔楔体岩浆形成中的作用。由于挥发物在喷发过程中几乎完全从岩浆中脱气，因此很难直接测量原始浓度。为了绕过这个问题，PI将分析熔体中的微小包裹体，这些包裹体在喷发时被冷却为玻璃，被困在火山岩中的橄榄石晶体中。这类包裹体通常保存其喷发前的挥发性成分，如果它们在喷发过程中保持在晶体中的话。PI将从墨西哥一些与俯冲有关的火山收集样本，并使用几种技术分析橄榄石中捕获的包裹体，以测量挥发物以及其他常量和微量元素的浓度和同位素组成。PI将利用这些结果来检验有关俯冲带中挥发分循环和岩浆生成的一些基本观点，并构建挥发分通量的定量模型。由此产生的数据集在空间复盖面方面将是世界上最广泛的，在世界上任何俯冲带区域的挥发性元素浓度和同位素比率方面将是最完整的。在这个项目中，PI将通过一名研究生和本科生参与实地工作、样品分析、地球化学建模、假设检验以及在科学会议和出版物上展示结果，将研究和教育结合起来。这些学生将学习使用一些复杂的分析技术。该项目，特别是实地考察工作，将涉及与墨西哥科学家和学生的合作，从而为他们和他们的美国同行提供互惠互利的科学和文化体验。由于这种合作，该项目得到了美国国家科学基金会国际科学与工程办公室美洲项目的部分支持。

EAR-0309559WallaceSubduction zones play an important role in global geochemical cycles because they provide a pathway for crustal materials to be recycled into the Earth's mantle. The materials include many volatile compounds such as water, carbon, sulfur, and chlorine. Subducted crustal materials become heated by the overlying mantle as they descend, and the volatiles are at least partially released by metamorphic devolatilization reactions, after which they migrate into the hot mantle above the subducted slab. This process causes the overlying mantle to melt because water lowers the melting temperature of mantle rocks. Thus recycling of volatiles in subduction zones is responsible for widespread formation of magma in Earth's mantle, resulting in chains of volcanoes such as those of the circum-Pacific "Ring of Fire". The objectives of this study are to investigate how volatile elements (H, C, S, Cl) are recycled in subduction zones and the role of volatiles in magma generation in the mantle wedge above subducting slabs. Because volatiles are almost completely degassed from magma during eruption, it is difficult to measure original concentrations directly. To circumvent this problem, the PI will analyze tiny inclusions of melt, quenched to glass upon eruption, that are trapped in olivine crystals in volcanic rocks. Such inclusions commonly preserve their pre-eruption volatile contents if they remain sealed in the crystals during eruption. The PI will collect samples from a number of subduction-related volcanoes in Mexico and analyze trapped inclusions in olivine using several techniques to measure the concentrations and isotopic compositions of volatiles, as well as other major and trace elements. The PI will use the results to test some fundamental ideas about volatile recycling and magma generation in subbduction zones and to construct quantitative models of volatile fluxes. The resulting data set will be the most extensive in terms of spatial coverage and most complete in terms of volatile element concentrations and isotope ratios for any subduction zone region in the world. In this project, the PI will integrate research and education by the involvement of a graduate and undergraduate student in fieldwork, sample analysis, geochemical modeling, hypothesis testing, and presentation of results at scientific meetings and through publications. These students will learn to use a number of sophisticated analytical techniques. The project, particularly the fieldwork, will involve collaboration with scientists and students in Mexico, thus providing a mutually beneficial scientific and cultural experience for them and their U.S. counterparts. Because of this collaboration, the project is partially supported by NSF's Americas Program of the Office of International Science and Engineering.