Iron is by far the most abundant transition metal in the solar system and in the Earth’s interior. Its extreme abundance and unique electronic states have shaped the evolution of the planet. Specifically, the formation of the iron core was among the biggest chemical differentiation event of the planet that has eventually led to the generation of the magnetic fields. Our planet’s formation may also leave other geochemical and isotopic signatures in Earth’s mantle. For example, geochemists have found a series of isotope anomalies from rocks available on Earth’s surface including heavy iron isotope enrichment in Mid-Ocean Ridge Basalts, the “rock factory” where Earth’s new oceanic crusts are formed. However, understanding the geochemical anomalies and physical nature of the Earth has been difficult as scientists only have very limited access to natural samples for analysis. The advent of synchrotron inelastic X-ray scattering coupled with high-pressure diamond anvil cells permits investigation of physical and chemical nature of earth materials at simulated extreme pressure-temperature conditions of the planet. In this talk, I will discuss the experimental determination of the mean force constant 〈F〉 of iron bonds in Earth’s mantle and core materials from evaluation of the phonon density of states measured by Nunclear Resonant Inelastic X-ray Scattering (NRIXS). The results are used to calculate the equilibrium iron isotope fractionation between silicate and iron at conditions relevant to Earth’s core formation. The derived fractionation factor of ~0–0.02 ‰ is small relative to the measured iron isotope enrichment in terrestrial basalts of ~+0.1‰. These synchrotron results call origin of Earth’s iron into question. I will also address recent synchrotron studies on the iron electronic spin and valence states and how these transitions can affect our understanding of the deep Earth. Future research opportunities in using synchrotron techniques to study properties of the deep Earth will be highlighted so as to stimulate Argonne scientists to collaboratively explore our enigmatic planet.

[1] J. Liu, Dauphas, N., M. Roskosz, M. Hu, H. Yang, W. Bi, J. Zhao., E. E. Alp, J. Y. Hu, and J. F. Lin, Iron isotopic fractionation between silicate mantle and metallic core under high-pressure conditions, Nature Comm., 8, 14377, , 2017.
[2] UChicago News Release: https://news.uchicago.edu/article/2017/03/16/research-proposes-new-theories-about-nature-earths-iron

Argonne Physics Division Colloquium Schedule