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Tappan, Jacqueline
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document
Spin State of Iron in Dynamically Compressed Olivine Melt
Abstract
The density contrast between silicate melts and solids is essential for understanding early differentiation of rocky planets and the origin of the present-day low-velocity structures in the Earths deep interior. Studies have found that the electronic spin state of Fe impacts the density of silicates by altering their volume and solid/liquid Fe partitioning at the pressure-temperature conditions expected for the Earths deep mantle. Previous static compression studies indicate that high-spin Fe is dominant up to the pressures of the Earths core-mantle boundary in the most abundant lower-mantle phase, bridgmanite. However, the spin behavior of Fe in molten silicates is poorly known at deep mantle conditions due to experimental challenges. We have conducted simultaneous measurements of X-ray diffraction (XRD) and X-ray emission spectroscopy (XES) on dynamically compressed olivine melt at the Matter in Extreme Conditions (MEC) beamline of the Linac Coherent Light Source (LCLS), SLAC National Accelerator Laboratory. XRD data showed that the local structure of the molten olivine is similar to those found in silicate glasses. XES spectra show low-spin Fe in olivine melt at pressures between 140 and 280 GPa at temperatures over 4000 K. The dominant low-spin state of Fe in the melt is in sharp contrast with the reported spin state of Fe in compressed silicate glasses under cold static compression where a significant fraction of Fe remains high spin even at the pressures relevant for the Earths core-mantle boundary. The contrasting spin behavior suggests the importance of thermal relaxation of local structures in melt for the stability of low-spin Fe at high pressures. The dominant low-spin Fe in olivine melt supports the strong partitioning of Fe into the melt and higher silicate melt densities. This would result in negatively buoyant silicate melts in the deep interior of the crystallizing early magma ocean and the observed low-velocity structures found near the present-day Earths core-mantle boundary....