Redox and Structural Controls on Iron Isotopic Variations in Igneous Rocks

地球上存在的氧化程度的提高是其宜居的一个主要条件，允许存在自由氧和呼吸过程中用于维持人类活动和更简单的生命形式的其他被氧化物种。与火星等其他行星体相比，地球的氧化程度更高，其原因尚不清楚。例如，土卫六的大气层(土卫六是土星中最大的卫星)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。目前还不知道地球是像今天一样诞生的，还是从类似泰坦的大气层开始的，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开发尖端分析方法；位于高级光子源(阿贡国家实验室)的高强度X射线源。这项研究将对为什么我们的星球是独一无二的提供关键的限制，它将帮助我们了解遥远过去火山排放的性质。在长时间尺度上，火山排放的性质在气候调节中发挥了关键作用，防止了地球进入永久的滚雪球状态。岩浆的铁氧化状态(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可能会模糊这一记录。铁同位素提供了对地幔熔融条件的洞察，这些条件不太容易受到这些次级过程的影响。一组在实验岩石学、铁同位素地球化学和核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铁(Fe2)和铁(Fe3)之间平衡同位素分馏的影响。这将为解释所有年龄的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。硅酸盐玻璃、橄榄石和尖晶石将通过核共振非弹性X射线散射(NRIXS)技术进行研究，以获得地幔和地壳熔融期间铁同位素分馏的整体情况，以及镁铁质和长英质岩浆的分异。通过在一定氧逸度下产生的流纹岩测量玄武岩，将允许将岩浆的铁平衡分馏系数参数化，并考虑Fe3/Fetot比和NbO/T(每四面体配位阳离子的非桥氧，硅酸盐熔体聚合的量度)等参数，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.