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，一个构造板降落在另一个构造板下面，是大陆壳的形成，人类所生活的陶土和重要的矿石沉积物的形成。这两个过程的一个关键方面是地面和更深的部分之间的材料交换。俯冲带形成的岩浆通过独特的化学反映了材料的转移，表明其源区域的表面源性材料。一个显着的例子是俯冲带岩石岩石岩石的氧化还原状态，与中洋脊玄武岩相比，通常认为这是由于氧化的表面材料的俯冲而产生的。尽管可以通过更既定的方法来评估火山岩的氧化还原状态，但由于冷却速率较慢和缺乏适当的矿物组合，俯冲带的氧化还原状态可能更难评估。这项工作将通过在无处不在的岩浆，闪石中对FE氧化还原状态的详细理解以及将这种理解应用于俯冲带的俯冲带岩石岩石套件的详细了解，从而扩大我们对弧岩浆的氧化还原状态的了解。这项工作将为社区提供一个新的工具，以在俯冲带氧化还原研究中实施。除了科学贡献外，这项工作还将支持加州理工学院的女性后学者和夏季本科研究人员的科学培训。在俯冲带中爆发的火山岩通常比中山脊的火山岩更氧化。在电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个或多个：（a）源过程：通过熔融/流体携带氧化物质的熔体/流体的亚弧形氧化。 （b）地壳分化过程：在地壳中存储期间的同化和分馏；或（c）喷发和浅水水平的过程：上升和喷发期间的脱气。大多数对弧岩中的研究都集中在火山岩上，这些岩石可能经历了上述某些或全部过程。因此，火山岩是艰难的地质记录，从而解开各种氧化机制的影响。为了了解负责弧形岩浆氧化性质的过程，这项研究着重于使用闪比尔的Fe Valence状态的时间和遗传相关的弧形岩石岩石。研究目标分为三个：（1）使用高压和温度活塞缸实验的氧气中的闪石晶粒合成； （2）开发新的工作流程，以通过原位，高分辨率同步子M？ssbauer光谱法（SMS）来量化闪石中铁铁的含量状态和适应性机制； （3）将第1部分和第2部分的结果应用于吸积的塔尔基纳弧（阿拉斯加）中不同地壳深度的火成岩。尽管以前已经表征了闪石中的FE形成，主要是通过大量技术（例如湿化学）提供的，但SMS提供了高空间分辨率和精确度，这对于确定闪光片的水平量表至关重要，因为这些矿物质通常在空间上是划分的，因此可以表现出诸如subsolidus的统一范围，并包含较高的言论，并包含较大的言论，并构成了较小的言论，并包含primitions和prim的较高范围，并且可以表现出较大的言论。并被认为是通过基金会的知识分子优点和更广泛的影响审查标准来评估值得支持的。