Fe3+/FeT Ratios in Amphiboles - A New Tool for Understanding the Redox State of Arc Magmas

俯冲带，即一个构造板块下沉到另一个板块下面的地方，是大陆地壳形成的基础，是人类赖以生存的陆地，也是重要矿床的形成之地。这两个过程的一个关键方面是地球表面和更深部分之间的物质交换。在俯冲带形成的岩浆通过独特的化学反应反映了这种物质的转移，表明它们的源区有表面源物质。一个突出的例子是，与洋中脊玄武岩相比，俯冲带火山岩的氧化还原状态升高，通常认为这是由氧化表面物质的俯冲引起的。虽然可以通过更成熟的方法来评估火山岩的氧化还原状态，但由于冷却速度较慢和缺乏适当的矿物组合，俯冲带深部岩石的氧化还原状态更难评估。通过对弧岩浆、角闪洞中普遍存在的矿物铁氧化还原状态的详细了解，并将这一认识应用于一套俯冲带深成岩，本工作将扩大我们对弧岩浆氧化还原状态的认识。本工作将为学界开展俯冲带氧化还原研究提供新的工具。除了科学贡献外，这项工作将支持一名女性博士后学者和加州理工学院的暑期本科生研究员的科学培训。在俯冲带喷发的火山岩通常比在洋中脊喷发的火山岩氧化程度更高。弧环境中岩浆氧化还原状态升高的原因是有争议的，但通常归因于以下一种或多种原因：(a)来源过程：弧下地幔通过熔体/流体携带来自板块的氧化物质氧化；(b)地壳分异过程：在地壳储存过程中的同化和分异作用；(c)喷发和浅层过程：在上升和喷发过程中脱气。对弧内岩的研究大多集中在火山岩上，火山岩可能经历了上述部分或全部过程。因此，火山岩是难以解开各种氧化机制影响的地质记录。为了了解弧岩浆氧化性的成因过程，本文利用角闪孔铁价态对不同地壳深度的弧深岩体进行了时间和成因相关性研究。研究目标有三个方面：(1)采用高压、高温活塞缸实验，在不同氧度条件下合成角闪孔颗粒；(2)开发了一种新的工作流程，通过原位高分辨率同步加速器M？斯堡尔光谱；(3)将第1部分和第2部分的结果应用于Talkeetna （Alaska）增生弧不同地壳深度的火成岩。虽然之前已经主要通过体积技术（如湿化学）对角闪洞中的铁形态进行了表征，但SMS提供了高空间分辨率和精度，这对于确定亚颗粒尺度的角闪洞铁形态至关重要，因为这些矿物通常是空间分带的，可以沿边缘和裂缝表现出亚固相蚀变，并且含有包裹体。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Subduction zones, where one tectonic plate descends beneath another, are fundamental the formation of continental crust, the terra firma upon which humans live, and the generation of important ore deposits. A critical aspect of both of these processes is the exchange of material between the surface and deeper parts of the Earth. Magmas formed at subduction zones reflect this transfer of material through distinctive chemistry suggesting surface-derived material in their source region. One salient example of this is the elevated redox state of subduction zone volcanic rocks, as compared to mid-ocean ridge basalts, which is generally thought to arise from the subduction of oxidized surface material. Although the redox state of volcanic rocks can be assessed through more established methods, the redox state of subduction zone plutonic rocks can be more difficult to assess due to slower cooling rates and lack of appropriate mineral assemblages. This work will expand our knowledge of the redox state of arc magmas through development of a detailed understanding of Fe redox state in a ubiquitous mineral in arc magmas, amphibole, and application of this understanding to a suite of subduction zone plutonic rocks. This work will provide a new tool for the community to implement in the study of subduction zone redox. In addition to scientific contributions, this work will support the scientific training of a female post-doctoral scholar and summer undergraduate researcher at Caltech. Volcanic rocks erupted in subduction zone settings are generally more oxidized than those from mid-ocean ridges. The cause of elevated magmatic redox state in arc environments is controversial but commonly attributed to one or more of the following: (a) source processes: sub-arc mantle oxidation via melts/fluids carrying oxidized species from the slab; (b) crustal differentiation processes: assimilation and fractionation during storage in the crust; or (c) eruption and shallow level processes: degassing during ascent and eruption. Most studies on in arc rocks focus on volcanic rocks, which may have experienced some or all of the above processes. Consequently, volcanic rocks are difficult geological records from which to untangle the effects of various oxidizing mechanisms. To understand the processes responsible for the oxidized nature of arc magmas, this research focuses on temporally and genetically related arc plutonic rocks from different crustal depths using Fe valence state in amphibole. The research objectives are three-fold: (1) synthesis of amphibole grains under varying oxygen fugacities using high-pressure and temperature piston-cylinder experiments; (2) development of a new workflow to quantify both Fe valence state and accommodation mechanism of ferric iron in amphibole via in-situ, high-resolution synchrotron M?ssbauer spectroscopy (SMS); and (3) application of the results of part 1 and 2 to igneous rocks from different crustal depths in the accreted Talkeetna arc (Alaska). Although Fe speciation in amphibole has been characterized previously, primarily through bulk techniques (e.g., wet chemistry), SMS affords high spatial resolution and precision which is critical in determining amphibole Fe speciation at the sub-grain scale, as these minerals are often spatially zoned, can exhibit sub-solidus alteration along rims and fractures, and contain inclusions.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.