Broad Elastic Softening of (Mg,Fe)O Ferropericlase Across the Iron Spin Crossover and a Mixed‐Spin Lower Mantle

The elastic bulk modulus softening of (Mg,Fe)O ferropericlase across the iron spin crossover induces dramatic changes in its physical properties, including seismic P‐velocities and viscosity. Here, we performed compression of powders of (Mg0.8‐0.9Fe0.2‐0.1)O in a piezo‐driven dynamic Diamond Anvil Cell (dDAC) and derive the bulk modulus by differentiation of pressure and volume data, providing first data on the broadness of the elastic softening for ferropericlase with mantle‐relevant compositions. We complement our experimental results with theoretical calculations that extend previous studies by considering multiple random configurations of iron, and going beyond treating high‐ and low‐spin iron as an ideal solution. Both experiments and computations show a broad and asymmetric softening of the bulk modulus, and suggest that the softening is sensitive to the distribution of iron in the ferropericlase structure. Our high‐temperature calculations show that mixed‐spin (Mg,Fe)O dominates the lower mantle at all depths below 1,000 km. In contrast to most previous works, we find that ferropericlase will not exist in pure low‐spin state along a typical mantle geotherm. Based on our model, the physical properties of ferropericlase will show significant lateral variation at depths below 1,400 km, with the strongest effects expected between 2,000 and 2,600 km. Plain Language Summary Understanding the physical properties of deep mantle minerals is pivotal to interpret geophysical observables and constrain large‐scale geodynamic models. (Mg,Fe)O ferropericlase is the second most abundant mineral in Earth's lower mantle, ranging from 660 to 2,890 km depths. Previous works have shown that the electronic configuration of iron in ferropericlase changes at pressures corresponding to the mid‐mantle. This process, called iron‐spin crossover, markedly affects a variety of physical properties, including seismic wave speeds and viscosity. Here we combine novel experiments and theoretical calculations to provide a coherent picture of the onset depth and broadness of the spin crossover in Earth's lower mantle. We show that the spin crossover happens throughout most of the lower mantle, altering physical properties at most depths up to the core‐mantle boundary. This quantitative understanding is key to model the impacts of the spin crossover on geophysical mantle properties and mantle dynamics. Key Points We constrain the broadness of the iron spin crossover by a combination of