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

TL;DR

This study examines how the iron spin crossover in ferropericlase affects the lower mantle geotherm, revealing that it can significantly increase the temperature gradient and seismic properties, influencing mantle dynamics and composition analysis.

Contribution

It provides a detailed analysis of the impact of iron spin crossover anomalies on the lower mantle geotherm and seismic properties in realistic mantle compositions.

Findings

Ferropericlase spin crossover increases the geotherm by up to 200K.

Anomalies affect elastic moduli and seismic velocities.

Geotherm sensitivity depends on ferropericlase content.

Abstract

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