Iron Isotope Fractionation in the Lower Mantle

Knowledge of iron isotopes in the deep Earth is of great importance, for it provides clues for determining the history of planetary differentiation and core formation, as well as the origin of the Earth. However, iron isotope composition of the Earth's interior is rarely known due to lack of samples. Previous theoretical and experimental results are insufficient because they presume the spin state of iron or they are based on relatively low quality experimental data. Here we will use a newly developed technique --NRIXS(Nuclear Resonant Inelastic X-ray Scattering)-- to measure the force constant of lower mantle mineral candidates ferropericlase and pervoskite at relevant pressure. A reliable iron isotope fractionation factor will then be derived using SciPhon, which is a software designed specifically for the isotope fractionation factor derivation from NRIXS data. We report new high pressure 57Fe NRIXS spectra collected at beamline 3 ID-B of the Advanced Photon Source, with the aim to understand the pressure effect on iron isotope fractionation. The experiments were conducted using a 3-fold Diamond Anvil Cell and a >95% 57Fe enriched sample fp25((Fe0.25Mg0.75)O). NRIXS spectra were collected by tuning the x-ray energy range within ± 200meV around the 57Fe resonant energy of 14.4125keV. We will present the force constants and derived iron isotope fractionation factors of ferropericlase. It is expected that pressure will have an effect on the iron isotope fractionation factor and should be taken into account when modeling isotope fractionation of planetary scale and when using iron isotope systems to constrain the planetary differentiation. We will also use first-principle studies and geochemical observations to study the Fe,O,Si isotope system in the deep Earth. Our ultimate goal is to build a self-consistent geophysical and geochemical model that can be used in deciphering the mystery of Earth's history.