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

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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.