Influence of the iron spin crossover in ferropericlase on the lower mantle geotherm

摘要

The iron spin crossover in ferropericlase introduces anomalies in itsthermodynamics and thermoelastic properties. Here we investigate how theseanomalies can affect the lower mantle geotherm. The effect is examined inmantle aggregates consisting of mixtures of bridgmanite, ferropericlase, andCaSiO$_3$ perovskite, with different Mg/Si ratios varying from harzburgitic toperovskitic (Mg/Si$\sim$1.5 to 0.8). We find that the anomalies introduced bythe spin crossover increase the isentropic gradient and thus the geothermproportionally to the amount of ferropericlase. The geotherms can be as much as$\sim$ 200K hotter than the conventional adiabatic geotherm at deep lowermantle conditions. Aggregate elastic moduli and seismic velocities are alsosensitive to the spin crossover and the geotherm, which impacts analyses oflower mantle velocities and composition.