Elastic Softening of (Mg_0.8Fe_0.2)O Ferropericlase Across the Iron Spin Crossover Measured at Seismic Frequencies

Abstract

We experimentally determined the bulk modulus of (Mg0.8Fe0.2)O ferropericlase across the iron spin transition and in the low‐spin phase by employing a new experimental approach. In our measurements, we simulate the propagation of a compressional seismic wave (P wave) through our sample by employing a piezo‐driven dynamic diamond anvil cell that allows to oscillate pressure at seismic frequencies. During pressure oscillations, X‐ray diffraction images were continuously collected every 5–50 ms. The bulk modulus is directly calculated from these data at different pressures. Our experiments show a pronounced softening of the bulk modulus throughout the spin crossover, supporting previous single‐crystal measurements at very high frequencies and computations. Comparison of our results to previous data collected on (Mg,Fe)O with lower iron contents shows that the magnitude of softening strongly depends on iron content. Our experiments at seismic frequencies confirm that the iron spin crossover markedly affects the ratio of seismic compressional to shear wave velocities in Earth's lower mantle.

Equation of State of Polycrystalline Stishovite Across the Tetragonal‐Orthorhombic Phase Transition

“…To relate seismic scattering in the lower mantle to the presence of stishovite, a more complete understanding of how the ferroelastic phase transition interacts with seismic waves is needed. Recently, the bulk modulus of ferropericlase has been determined at seismic frequencies and pressures of the lower mantle (Marquardt et al, ). Similar experiments on sintered polycrystalline silica could demonstrate to which extent the bulk modulus is affected by the ferroelastic phase transition at seismic frequencies.…”

Bulk modulus of H2O across the ice VII–ice X transition measured by time-resolved x-ray diffraction in dynamic diamond anvil cell experiments

“…Here we perform compression experiments in combination with time-resolved XRD in order to explore the compressive behavior of H 2 O across the ice VII-ice X transition up to 180 GPa using a dynamic diamond anvil cell (dDAC) driven by a piezoelectric actuator [30,31]. This setup provides quasicontinuous volume-P data, with hundreds of diffraction patterns recorded in one experiment, and therefore allows us to compute compressibility by numerical differentiation without invoking an equation of state model to fit the data [32], a significant advance over prior static experiments with tens of data points. We track the bulk modulus evolution across the ice VII-ice X transition, and explore whether this transition as well as the pretransitional states (ice VII and ice X ) can be detected from changes in the compression behavior.…”

New dynamic diamond anvil cells for tera-pascal per second fast compression x-ray diffraction experiments

“…Recently, the potential of this new approach was demonstrated by mapping out the spin transition-induced softening of the bulk modulus for ferropericlase (Mg 0.8 Fe 0.2 )O (an iron content typical for the Earth's mantle) at 1 Hz seismic frequencies. 42 These results are highly relevant to the interpretation of seismic data of the lower mantle and will help to constrain its chemistry and thermal state. 43 In the recent experiment, powder of (Mg 0.8 Fe 0.2 )O ferropericlase was loaded along with platinum powder into symmetric piston cylinder type DAC, with 0.2 mm culets filled in the 0.1 mm wide and 0.03 mm thick gasket hole.…”