Redox and Structural Controls on Iron Isotopic Variations in Igneous Rocks

火成岩中铁同位素变化的氧化还原和结构控制

• 批准号: 1144429
• 负责人: Nicolas Dauphas
• 金额: $ 24.97万
• 依托单位: University of Chicago
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-01-01 至 2014-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1144429&HistoricalAwards=false
• 关键词: Redox; Structural; Controls; Iron; Isotopic

The elevated degree of oxidation present in the Earth is a major condition of its habitability, allowing the existence of free oxygen and other oxidized species used during respiration to sustain activity in human beings and simpler life forms. Earth is more oxidized than other planetary bodies such as Mars and the reason for this is not well understood. For example, Titan's atmosphere (Titan is the largest moon of Saturn) is composed primarily of nitrogen, methane and ethane. In addition, Titan's surface is covered with lakes of hydrocarbons. It is not known whether Earth was born like it is today or whether it started with a Titan-like atmosphere and the oxidized conditions were established during Earth's history through geological processes. In this study, a new tool will be developed to measure the oxidation conditions of Earth through time. The measurements will use and develop cutting edge analytical methods at a national facility; the intense X-ray source located at the Advanced Photon Source (Argonne National Laboratory). This study will provide critical constraints on why our planet is unique and it will help us understand the nature of volcanic emissions in the distant past. On long timescales, the nature of volcanic emissions has played a key role in climate regulation and prevented the Earth from going into a permanent snowball state. The iron oxidation state of magmas (i.e., Fe3+/Fe2+ ratio) is a key parameter to trace the redox evolution of the Earth. Unfortunately, geological processes such as assimilation, degassing, crystallization, and alteration can blur this record. Iron isotopes provide insight into the conditions of mantle melting that are less susceptible to these secondary processes. A team of investigators with expertise in experimental petrology, iron isotope geochemistry, and nuclear resonance vibrational spectroscopy will calibrate the effects of redox and structural conditions on equilibrium isotopic fractionation between ferrous (Fe2+) and ferric (Fe3+) iron in magmas and minerals. This will provide a solid framework for interpreting iron isotopic variations and redox conditions in igneous rocks of all ages. Silicate glass, olivine, and spinel will be studied by the Nuclear Resonant Inelastic X-ray Scattering (NRIXS) technique to get a holistic view of iron isotopic fractionation during mantle and crustal melting, as well as mafic and felsic magma differentiation. Measurements of basalts through rhyolites produced under a range of oxygen fugacities, will allow the parameterization of iron equilibrium fractionation factors of magmas taking into account parameters such as Fe3+/Fetot ratio and NBO/T (nonbridging oxygen per tetrahedrally coordinated cation, a measure of polymerization of a silicate melt) to predict equilibrium Fe isotopic fractionation between minerals and melts.

地球中存在的较高氧化程度是其宜居性的一个主要条件，允许自由氧和呼吸过程中使用的其他氧化物质的存在，以维持人类和简单生命形式的活动。地球比火星等其他行星体氧化程度更高，其原因尚不清楚。例如，泰坦的大气层（泰坦是土星最大的卫星）主要由氮气、甲烷和乙烷组成。此外，土卫六的表面覆盖着碳氢化合物湖泊。目前尚不清楚地球是像今天这样诞生的，还是它始于类似泰坦的大气层，并且氧化条件是在地球历史上通过地质过程建立的。在这项研究中，将开发一种新工具来测量地球随时间的氧化状况。测量将在国家设施中使用和开发尖端分析方法；位于先进光子源（阿贡国家实验室）的强 X 射线源。这项研究将为我们的星球为何独特提供关键限制，并将帮助我们了解遥远过去火山排放的本质。在很长一段时间内，火山排放的性质在气候调节中发挥了关键作用，并防止地球进入永久的雪球状态。岩浆的铁氧化态（即Fe3+/Fe2+比率）是追踪地球氧化还原演化的关键参数。不幸的是，同化、脱气、结晶和蚀变等地质过程可能会模糊这一记录。铁同位素可以深入了解不易受这些次生过程影响的地幔熔化条件。一组在实验岩石学、铁同位素地球化学和核磁共振振动光谱方面具有专业知识的研究人员将校准氧化还原和结构条件对岩浆和矿物中亚铁 (Fe2+) 和三价铁 (Fe3+) 之间平衡同位素分馏的影响。这将为解释各个时代火成岩中的铁同位素变化和氧化还原条件提供坚实的框架。将通过核共振非弹性 X 射线散射 (NRIXS) 技术研究硅酸盐玻璃、橄榄石和尖晶石，以获得地幔和地壳熔化过程中铁同位素分馏以及镁铁质和长英质岩浆分异过程的整体视图。通过在一系列氧逸度下产生的流纹岩测量玄武岩，将允许岩浆的铁平衡分馏因子参数化，同时考虑诸如 Fe3+/Fetot 比率和 NBO/T（每四面体配位阳离子的非桥接氧，聚合的量度）等参数硅酸盐熔体）来预测矿物和熔体之间的平衡铁同位素分馏。