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Introduction(Mg,Fe)O ferropericlase is the second most abundant mineral in Earth's lower mantle (e.g., Irifune et al., 2015). At room pressure, ferrous iron (Fe 2+ ) in ferropericlase adopts the high-spin electronic configuration where the six 3d-electrons partially fill the t 2g and e g orbitals, maximizing the number of unpaired electrons. At a pressure of about 40 GPa, a distortion of the iron-containing octahedral sites in the crystal structure triggers a change in iron electronic configuration to the low-spin state, where electron pairing in the t 2g orbitals is favored (Badro et al., 2003). Several studies have shown that the properties of ferropericlase in mixed-spin state are markedly different from those in either high-or low-spin state. In particular, the spin crossover is accompanied by a reduction in the Fe 2+ ionic radius (Shannon, 1976) enhancing the compressibility of ferropericlase over the pressure range where the spin crossover occurs (

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The iron spin crossover in ferropericlase, the second most abundant mineral in Earth’s lower mantle, causes changes in a range of physical properties, including seismic wave velocities. Understanding the effect of temperature on the spin crossover is essential to detect its signature in seismic observations and constrain its occurrence in the mantle. Here, we report the first experimental results on the spin crossover-induced bulk modulus softening at high temperatures, derived directly from time-resolved x-ray diffraction measurements during continuous compression of (Mg0.8Fe0.2)O in a resistive-heated dynamic diamond-anvil cell. We present new theoretical calculations of the spin crossover at mantle temperatures benchmarked by the experiments. Based on our results, we create synthetic seismic tomography models to investigate the signature of the spin crossover in global seismic tomography. A tomographic filter is applied to allow for meaningful comparisons between the synthetic models and data-based seismic tomography models, like SP12RTS. A negative anomaly in the correlation between Vs variations and Vc variations (S-C correlation) is found to be the most suitable measure to detect the presence of the spin crossover in tomographic models. When including the effects of the spin crossover, the misfit between the synthetic model and SP12RTS is reduced by 63%, providing strong evidence for the presence of the spin crossover, and hence ferropericlase, in the lower mantle. Future improvement of seismic resolution may facilitate a detailed mapping of spin state using the S-C correlation, providing constraints on mantle temperatures, thanks to the temperature sensitivity of the spin crossover.