Magma Structure and Anharmonicity Controls on Iron Isotopic Fractionation in Igneous Rocks

在行星体中，地球是独一无二的，因为它拥有由光合作用维持的富氧大气。它的地幔也不寻常，因为相对于其他行星（如灶神星、火星）的地幔，它被氧化了。地幔的氧化还原演化可能影响了当前大气的氧化还原状态，因为大约20亿年前大气中氧含量的增加可能是由火山释放的气体性质的变化引起的。了解控制地球氧化还原演化的一个挑战是，追踪地幔氧化状态的代用物缺失或不充分。测量地球氧化还原状态的一种方法是研究铁的氧化还原状态，铁可以在还原性条件下以金属铁的形式存在，亚铁（如橄榄石）在中间条件下存在，铁（如铁锈）在氧化性条件下存在。这种方法的一个困难是铁可以很容易地从一种形态切换到另一种形态。因此，开发新的不容易湮没的地幔氧化还原条件示踪剂是很重要的，铁同位素可能就是这样的示踪剂。然而，建立铁同位素作为地球岩浆和氧化还原过程示踪剂的基础仍然缺失，这是本提案的主要范围。可以使用核共振非弹性x射线散射（NRIXS）的同步加速器方法来预测岩石和岩浆中铁同位素的分选程度，该方法可以测量使铁在固体中固定位置的键的强度（铁键的平均力常数）。根据NRIXS的测量结果，理论上可以预测铁同位素的分离程度。前人的研究表明，铁的氧化还原状态和岩浆组成对铁同位素分馏程度有很强的影响。这些研究可以解释为什么更演化（硅化程度更高）的火成岩往往比硅化程度更低的岩石具有更重的铁同位素组成。然而，他们未能解释为什么海洋地壳相对于地幔的重铁同位素有系统地富集。造成这一缺陷的一个可能原因是，由于实际原因，NRIXS通常在室温下测量的样品不能代表温度高得多的地幔衍生样品。事实上，更高的温度会影响样品的结构。如果材料表现出一些不和谐，也会影响化学键的强度。为了量化这一点，将在先进光子源同步加速器上进行实验，以测量与地质相关的材料（玻璃、熔体、橄榄石）在高温下的铁键力常数。这将通过将样品放置在受控气氛中的加热导线上并使探测器尽可能靠近样品而不损坏它们来完成。通过测量铁键的表观力常数作为温度的函数，将有可能检索原子间势的四次项，并了解火成岩和熔体的非调和和结构变化。这些测量将有助于发展对地幔和地壳岩石中铁同位素分异的定量理解。这需要利用火成岩中的铁同位素变化作为地球氧化还原状态和岩浆演化的示踪剂。

Among planetary bodies, Earth is unique in that it possesses an oxygen-rich atmosphere sustained by photosynthesis. Its mantle is also unusual in that it is oxidized relative to the mantles of other planetary bodies (e.g., Vesta, Mars). The redox evolution of the mantle may have influenced the redox state of the present atmosphere as the raise in the oxygen content of the atmosphere approximately two billion years ago could have resulted from a change in the nature of the gases emitted by volcanoes. One challenge with understanding what controlled the redox evolution of Earth is that proxies to trace the oxidation state of the mantle are missing or inadequate. One way to measure the redox state of the Earth is to study the redox state of iron, which can exist as metallic iron under reducing condition, ferrous iron (as in olivine) in intermediate condition, and ferric (as in rust) in oxidizing condition. One difficulty with this approach is that iron can easy switch from one form to another. For this reason, it is important to develop new tracers of the redox condition of the Earth's mantle that are less easily obliterated, and iron isotopes may be such tracers. However, the groundwork to establish iron isotopes as tracers of magmatic and redox processes in the Earth is still missing, which is the main scope of this proposal.The extent to which iron isotopes can be fractionated in rocks and magmas can be predicted using the synchrotron method of Nuclear Resonant Inelastic X-ray Scattering (NRIXS), which allows measuring the strength of the bonds that hold iron in position in a solid (the mean force constant of iron bonds). Through theory, it is possible to predict the extent to which iron isotopes can be fractionated based on such NRIXS measurements. Previous work showed that there was a strong influence of the redox state of iron and magma composition on the degree of iron isotopic fractionation. These studies could explain why more evolved (more silicic) igneous rocks tend to have heavier Fe isotopic compositions than less silicic rocks. They however fell short of explaining why the oceanic crust is enriched systematically in the heavy isotopes of iron relative to the underlying mantle. One possible reason for this shortcoming is that the samples that are usually measured by NRIXS at room temperature for practical reasons are not representative of mantle-derived samples, which are at much higher temperatures. Indeed, higher temperatures could affect the structure of the samples. It could also affect the strength of the chemical bonds if the material shows some anharmonicity. To quantify this, experiments will be done at the Advanced Photon Source synchrotron to measure the force constant of iron bonds of materials of geological relevance (glasses, melts, olivine) at high temperature. This will be done by placing the samples on a heated wire in a controlled atmosphere and bringing the detectors as close as possible to the samples without damaging them. By measuring the apparent force constant of iron bonds as a function of temperature, it will be possible to retrieve the quartic term of the interatomic potential and learn about anharmonic and structural changes in igneous rocks and melts. These measurements will help develop a quantitative understanding of iron isotopic fractionation in mantle and crustal rocks. This is needed to use iron isotopic variations in igneous rocks as tracers of the redox state and magmatic evolution of the Earth.