Redox and Structural Controls on Iron Isotopic Variations in Igneous Rocks

地球上存在的高氧化程度是其可居住性的主要条件，允许自由氧和其他氧化物质的存在，这些物质在呼吸过程中用于维持人类和更简单的生命形式的活动。地球比火星等其他行星体更氧化，其原因尚不清楚。例如，土卫六的大气层（土卫六是土星最大的卫星）主要由氮、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，还是从类似泰坦的大气层开始，并且在地球历史上通过地质过程建立了氧化条件。在这项研究中，将开发一种新的工具来测量地球随时间的氧化条件。这些测量将在一个国家设施中使用和发展最先进的分析方法；位于先进光子源（阿贡国家实验室）的强x射线源。这项研究将提供关键的约束，为什么我们的星球是独一无二的，它将帮助我们了解在遥远的过去火山排放的性质。从长期来看，火山喷发的性质在气候调节中发挥了关键作用，阻止了地球进入永久的雪球状态。岩浆中的铁氧化态（即Fe3+/Fe2+比值）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会模糊这一记录。铁同位素提供了对不太容易受这些次级过程影响的地幔融化条件的深入了解。一个具有实验岩石学、铁同位素地球化学和核共振振动光谱专业知识的研究小组将校准氧化还原和结构条件对岩浆和矿物中亚铁（Fe2+）和三铁（Fe3+）铁平衡同位素分异的影响。这将为解释各时代火成岩中的铁同位素变化和氧化还原条件提供一个坚实的框架。利用核共振非弹性x射线散射（NRIXS）技术对硅酸盐玻璃、橄榄石和尖晶石进行研究，以全面了解地幔和地壳熔融过程中的铁同位素分异，以及基性和长英质岩浆分异。通过在一定氧逸度下产生的流序岩对玄武岩进行测量，将考虑Fe3+/Fetot比和NBO/T（每四面体配位阳离子的非桥接氧，硅酸盐熔体聚合的测量）等参数，从而可以参数化岩浆的铁平衡分馏因素，以预测矿物和熔体之间的平衡铁同位素分馏。

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.