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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.

Plain Language Summary Information about the structure and composition of Earth's mantle can be derived from comparison of measured seismic wave speeds to those predicted from laboratory sound wave velocity measurements at high pressures. Ferropericlase, the second most abundant mineral in Earth's lower mantle, changes its electronic configuration at pressures corresponding to the lower mantle. Laboratory measurements carried out at very high frequencies (GHz) indicate that this so-called spin transition significantly decreases compressional wave velocities. If true, this effect can affect our interpretation of seismological observables. However, experimental results are partly in disagreement and no measurements have been conducted at typical seismic frequencies that are much lower as those typical for laboratory experiments. In this work. we directly measured the effect of the iron spin transition on the elastic response of ferropericlase at a frequency of 1 Hz. We find a significant softening effect on the bulk modulus that will lead to a decrease of compressional seismic velocities in Earth's lower mantle. Based on comparison of our results to previous work, we show that the effect strongly depends on iron content.