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。它的地幔也是不寻常的，因为它相对于其他行星(例如，灶神星、火星)的地幔是被氧化的。地幔的氧化还原演化可能影响了当今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火山排放的气体性质发生变化的结果。理解是什么控制了地球的氧化还原演化的一个挑战是，追踪地幔氧化状态的代用品缺失或不足。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，铁在还原条件下以金属铁的形式存在，亚铁(如在橄榄石中)在中间状态下存在，铁(如在铁锈中)在氧化条件下存在。这种方法的一个困难是，铁可以很容易地从一种形式转换成另一种形式。出于这个原因，重要的是开发新的示踪剂，以了解地球地幔的氧化还原状况，这种示踪剂不太容易被抹去，而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上岩浆和氧化还原过程的示踪剂的基础工作仍然缺乏，这是这一提议的主要范围。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馏程度，该方法允许测量将铁固定在固体中的键的强度(铁键的平均力常数)。通过理论，可以根据这样的NRIXS测量来预测铁同位素可以被分馏的程度。前人的工作表明，铁的氧化还原状态和岩浆成分对铁的同位素分馏程度有很大的影响。这些研究可以解释为什么演化程度较高(硅质含量较高)的火成岩往往比硅质含量较低的岩石具有更重的铁同位素组成。然而，他们没有解释为什么洋壳相对于下地幔系统地富含铁的重同位素。这一缺陷的一个可能原因是，由于实际原因，通常由NRIXS在室温下测量的样品不能代表地幔来源的样品，这些样品的温度要高得多。事实上，更高的温度可能会影响样品的结构。如果材料表现出一些非谐性，它也可能影响化学键的强度。为了量化这一点，将在高级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)在高温下的铁键的力常数。这将通过将样品放在受控气氛中的加热金属丝上，并在不损坏样品的情况下使探测器尽可能靠近样品来实现。通过测量作为温度函数的铁键的表观力常数，将有可能恢复原子间势的四次项，并了解火成岩和熔体中的非谐和结构变化。这些测量将有助于定量了解地幔和地壳岩石中的铁同位素分馏。这是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所必需的。

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.