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

角闪石中的 Fe3 /FeT 比率 - 了解弧岩浆氧化还原状态的新工具

• 批准号: 1841790
• 负责人: Claire Bucholz
• 金额: $ 29万
• 依托单位: California Institute of Technology
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2019
• 资助国家: 美国
• 起止时间: 2019-05-15 至 2023-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1841790&HistoricalAwards=false
• 关键词: Fe3+; FeT; Ratios; Amphiboles; New

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.

俯冲带是一个构造板块下降到另一个构造板块之下的地方，是大陆地壳、人类赖以生存的陆地以及重要矿床形成的基础。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，表明其源区存在地表衍生的物质。一个突出的例子是俯冲带火山岩的氧化还原态比大洋中脊玄武岩高，通常认为这是由氧化表面物质的俯冲引起的。尽管可以通过更成熟的方法评估火山岩的氧化还原状态，但由于冷却速率较慢且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以评估。这项工作将通过详细了解弧岩浆中普遍存在的矿物——角闪石中铁的氧化还原态，并将这种理解应用于一系列俯冲带深成岩，从而扩展我们对弧岩浆氧化还原态的了解。这项工作将为业界在俯冲带氧化还原研究中实施提供新的工具。除了科学贡献外，这项工作还将支持加州理工学院女性博士后学者和暑期本科研究员的科学培训。在俯冲带环境中喷发的火山岩通常比洋中脊喷发的火山岩氧化程度更高。电弧环境中岩浆氧化还原状态升高的原因存在争议，但通常归因于以下一种或多种： (a) 源过程：通过携带板块中氧化物质的熔体/流体进行弧下地幔氧化； (b) 地壳分异过程：地壳储存期间的同化和分馏； (c) 喷发和浅层过程：上升和喷发过程中的脱气。大多数弧内岩石的研究都集中在火山岩上，火山岩可能经历了上述部分或全部过程。因此，火山岩是很难阐明各种氧化机制影响的地质记录。为了了解弧岩浆氧化性质的过程，本研究利用角闪石中的 Fe 价态，重点研究来自不同地壳深度的时间和成因相关的弧深成岩。研究目标有三个：（1）利用高压高温活塞缸实验合成不同氧逸度下的角闪石颗粒； (2) 开发新的工作流程，通过原位高分辨率同步加速器穆斯堡尔光谱 (SMS) 量化角闪石中铁的价态和调节机制； (3) 将第 1 部分和第 2 部分的结果应用于增生塔尔基特纳弧（阿拉斯加）不同地壳深度的火成岩。尽管以前主要通过本体技术（例如湿化学）对角闪石中的铁形态进行了表征，但 SMS 提供了高空间分辨率和精度，这对于确定亚晶粒尺度的角闪石铁形态至关重要，因为这些矿物通常在空间上分区，可以沿着边缘和裂缝表现出亚固相线变化，并含有夹杂物。该奖项反映了 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进行评估，被认为值得支持 标准。