MARGINS: Collaborative Research: The Oxidation State of Mariana Arc Magmas and its Relationship to Subduction Volatile and Mass Cycling

合作研究：马里亚纳弧岩浆的氧化状态及其与俯冲、挥发分和物质循环的关系。地球内部和外部之间的地球化学交换，通过在山脊形成地壳和在俯冲带破坏地壳，随着时间的推移改变了地表和内部储集层的组成。随着年龄的增长，洋壳逐渐被氧化和水合，将俯冲板块中的水和氧化态联系起来。由于地幔的缓冲能力，这种联系可能在俯冲带内脱钩，也可能持续到深部。来自俯冲板块的氧化或氧化组分可能改变地幔和岩浆的氧化状态，从而影响元素分异、岩浆分异和脱气，以及地幔中氧气有效性的长期演化。最近对氧化状态的块状岩石和矿物学替代物(如氧化还原敏感的V配分、尖晶石成分、全岩Fe+3/Fe)的研究，在现代构造背景和整个地球历史上产生了相互矛盾的观点。现代俯冲带为研究地幔和水圈的氧化还原条件是否以及如何共同演化提供了理想的环境。本研究旨在开发一种新的测量玄武岩岩浆氧化状态的方法，利用基于同步加速器的微束技术(Micro-XANES)无损地原位测量玻璃中氧化还原敏感的Fe+3/Fe比值。这项技术将使熔体氧化态与原始、未脱气的岩浆液体(即天然枕状玻璃和熔融包裹体)和实验玻璃中的主要、挥发性和微量元素成分在~10米取样尺度上进行直接比较。将分析来自马里亚纳弧/海槽系统和模拟活塞-圆柱体实验的天然玻璃，目的是(1)测试岩浆Fe+3/Fe比值与熔体和地幔氧化状态的替代指标；(2)评估岩浆过程、挥发分和板片衍生成分在影响氧化状态中的作用；以及(3)模拟俯冲循环对地球内部氧化还原条件长期演变的影响。马里亚纳弧型熔岩是这项研究的理想选择，因为它们携带原始的、未脱气的玄武岩熔体包裹体，记录了一系列岩浆水分含量和板岩衍生的化学特征。此外，还可以分析马里亚纳海槽弧后盆地全长的海底玻璃样品，以提供挥发分含量和臭氧状态的空间变化信息。该项目将开发一种具有潜在变革性的新的显微分析技术，用于定量测定天然玻璃和实验玻璃中的氧化态。结果将通过同行评议的出版物和在线数据库传播。这些结果将产生广泛的结果，与边缘、山脊和深地科学界直接相关。该项目还将促进两名职业生涯早期的女性个人投资主管的职业发展，她们将利用互补的技能来应对需要高度协作的多学科团队的新问题。这两家私人投资机构坚定地致力于指导地球科学中代表性不足的少数群体，并在这一项目的试点工作期间与这些学生建立了良好的记录。最后，这一奖项将资助一名博士生在两位PI的共同指导下，因此接触到广泛的实验和分析技术，以开始在地球科学的职业生涯。

Collaborative Research: The oxidation state of Mariana arc magmas and its relationship to subduction volatile and mass cycling Intellectual Merit. Geochemical exchange between the earth's interior and exterior, via crust formation at ridges and crust destruction at subduction zones, has modified the composition of both the surface and interior reservoirs through time. The oceanic crust becomes progressively oxidized and hydrated as it ages, linking water and oxidation state in the subducting plate. This link could decouple within subduction zones due to the mantle's buffering capacity, or it may persist to depth. Oxidized or oxidizing components from the subducted slab may modify mantle and magmatic oxidation states, thereby influencing element partitioning, magmatic differentiation and degassing, and the long-term evolution of oxygen availability in the mantle. Several recent studies of bulk-rock and mineralogical proxies for oxidation state (e.g., redox-sensitive V partitioning, spinel composition, whole-rock Fe+3/_Fe) have yielded contradictory views of mantle oxidation state in modern tectonic settings and throughout earth history. Modern subduction zones provide an ideal setting to investigate if and how redox conditions of the mantle and hydrosphere have co-evolved. This study aims to develop a new method of measuring the oxidation state of basaltic magmas, using a synchrotron-based microbeam technique (micro-XANES) for non-destructive, in situ measurement of the redox-sensitive Fe+3/_Fe ratio in glasses. This technique will allow direct comparison of melt oxidation state to major, volatile, and trace element composition in primitive, undegassed magmatic liquids (i.e., natural pillow glasses and melt inclusions) and experimental glasses at a ~10 _m sampling scale. Both natural glasses from along and across the Mariana arc/trough system and synthetic glasses from analog piston-cylinder experiments will be analyzed, with the goals of (1) testing magmatic Fe+3/_Fe ratios against alternative proxies of both melt and mantle oxidation state (2) evaluating the role of magmatic processes, volatiles, and slab-derived components in influencing oxidation state, and (3) modeling the effects of the subduction cycle on the long-term evolution of redox conditions in the earth's interior. Mariana arc lavas are ideal for this study because they carry primitive, undegassed, basaltic melt inclusions that record a range of magmatic water contents and slab-derived chemical signatures. Also, samples of submarine glass from the full length of the Mariana trough back-arc basin, can be analyzed to provide information regarding spatial variations in volatile content and ozidation state.Broader Impacts. This project will develop a potentially transformative new micro-analytical technique for the quantification of oxidation state in natural and experimental glasses. Results will be disseminated through peer-reviewed publications and on-line databases. These results will have broad-reaching results with direct relevance to the MARGINS, RIDGE, and deep earth science communities. This project also will foster the careers of two early-career female PIs who will bring their complementary skills to bear on a new problem requiring a highly collaborative, multidisciplinary team. The two PIs are deeply committed to mentoring under-represented minorities in the earth sciences and have established a track record with such students during the pilot work for this project. Finally, this award will fund a Ph.D. student to be jointly mentored by the two PIs, and therefore exposed to a broad palate of experimental and analytical techniques with which to launch a career in the earth sciences.