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

Abstract:

The spin crossover in ferropericlase introduces anomalies in its thermodynamics and thermoelastic properties. Here we investigate the influence of this spin crossover in the lower mantle adiabatic geotherm. The effect is investigated for lower mantle compositions consisting of mixtures of bridgmanite, ferropericlase, and CaSiO₃-perovskite, for several Mg/Si ratios varying from perovskitic to pyrolitic compositions. The thermodynamics properties of (Mg,Fe)SiO₃ bridgmanite and of (Mg,Fe)O ferropericlase were obtained using LDA+U plus quasiharmonic approximation QHA，while LDA and the Mie-Debye-Grüneisen approach were used for Ca-perovskite. This spin crossover essentially increases the adiabatic gradient producing temperatures between those obtained by Anderson [1] and Brown and Shankland [2]. We show that aggregate moduli and seismic velocities are sensitive to the alteration in the adiabatic geotherm caused by this spin crossover, which can influence analyses of lower mantle velocities in terms of composition.

[1] O. L. Anderson (1982), The Earths core and phase diagram of iron, Phil. Trans. R. Soc. Lond. A, 306, 2135.

[2] J.M. Brown and T.J. Shankland (1981), Thermodynamic parameters in the Earth as determined from seismic profiles, Geophys. J. R. astr. Soc., 66, 579596.