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

俯冲带，即一个板块俯冲到另一个板块之下的地方，是形成陆壳、人类赖以生存的陆地和产生重要矿藏的基础。这两个过程的一个关键方面是地球表面和深处之间的物质交换。在俯冲带形成的岩浆通过独特的化学物质反映了这种物质的转移，表明它们的源区是地表衍生的物质。与大洋中脊玄武岩相比，俯冲带火山岩的氧化还原状态升高是一个突出的例子，通常认为这是由于表面氧化物质的俯冲而产生的。虽然火山岩的氧化还原状态可以通过更成熟的方法来评估，但由于冷却速度较慢和缺乏适当的矿物组合，俯冲带深成岩的氧化还原状态可能更难评估。这项工作将通过对弧岩浆、角闪石中普遍存在的矿物中铁的氧化还原状态的详细了解，并将这一理解应用于一套俯冲带深成岩，从而扩展我们对弧岩浆氧化还原状态的认识。这项工作将为社区在俯冲带氧化还原研究中提供一种新的工具。除了科学贡献，这项工作还将支持加州理工大学一名女性博士后学者和夏季本科生研究员的科学培训。在俯冲带背景下喷发的火山岩通常比来自大洋中脊的火山岩被氧化得更严重。弧环境中岩浆氧化还原状态升高的原因是有争议的，但通常归因于下列一种或多种原因：(A)源过程：通过熔体/流体携带来自板片的氧化物种进行弧下地幔氧化；(B)地壳分异过程：在地壳中储存期间的同化和分馏；或(C)喷发和浅层过程：上升和喷发期间的脱气。对弧内岩石的研究大多集中在火山岩上，火山岩可能经历了上述过程的一部分或全部。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的过程，本研究利用角闪石中的铁价态对不同地壳深度的时间和成因相关的弧深成岩进行了研究。研究目标有三个：(1)利用高压和高温活塞-圆筒实验在不同氧逸度下合成角闪石颗粒；(2)开发一种新的工作流程，通过原位高分辨率同步加速器穆斯堡尔谱(SMS)定量测定角闪石中铁的价态和铁的调节机制；以及(3)将第一部分和第二部分的结果应用于(阿拉斯加)吸积的塔尔基特纳弧(Talkeetna Arc)中不同地壳深度的火成岩。尽管角闪石中的铁形态以前主要通过散装技术(如湿化学)进行了表征，但SMS提供了高空间分辨率和精确度，这对于在亚颗粒尺度上确定角闪石的铁形态至关重要，因为这些矿物通常是空间分区的，可以沿着边缘和裂缝显示亚固相蚀变，并含有包裹体。这一裁决反映了NSF的法定使命，并通过使用基金会的智力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.