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 using thermodynamics properties from ab initio calculations. The anomalous effect is examined in mantle aggregates consisting of mixtures of bridgmanite, ferropericlase, and CaSiO3 perovskite, with different Mg/Si ratios varying from harzburgitic to perovskitic (Mg/Si ∼ 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 ∼200 K 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.
Bibliographical noteFunding Information:
The authors acknowledge Caroline Qian for her early contributions to the isentrope code. This work was supported primarily by grants NSF/EAR1319368 and 1348066 and NSF/CAREER1151738. Zhongqing Wu was supported by State Key Development Program of Basic Research of China (2014CB845905) and NSF of China (41274087). C. Houser was supported by the Earth-Life Science Institute at Tokyo Institute of Technology. Results produced in this study are available in the supporting information.
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- lower mantle
- spin crossover