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.

The two most abundant minerals in the Earth’s lower mantle are bridgmanite and ferropericlase. The bulk modulus of ferropericlase (Fp) softens as iron d-electrons transition from a high-spin to low-spin state, affecting the seismic compressional velocity but not the shear velocity. Here, we identify a seismological expression of the iron spin crossover in fast regions associated with cold Fp-rich subducted oceanic lithosphere: the relative abundance of fast velocities in P- and S-wave tomography models diverges in the ~1,400-2,000 km depth range. This is consistent with a reduced temperature sensitivity of P-waves throughout the iron spin crossover. A similar signal is also found in seismically slow regions below ~1,800 km, consistent with broadening and deepening of the crossover at higher temperatures. The corresponding inflection in P-wave velocity is not yet observed in 1-D seismic profiles, suggesting that the lower mantle is composed of non-uniformly distributed thermochemical heterogeneities which dampen the global signature of the Fp spin crossover.

The two most abundant minerals on Earth which together make up over 90% of the Earth’s lower mantle are (Mg,Fe)O-ferropericlase (Fp) and (Mg,Fe)SiO3-bridgmanite (Bm). Iron in Fp undergoes a high-spin to low-spin (HS-LS) crossover that influences density, viscosity, elasticity, thermal conductivity, and elemental partitioning, however, the predicted effects of this transition are not apparent in global 1D seismic velocity profiles. This discrepancy suggests that the predictions are inaccurate, seismic resolution is insufficient to resolve the effects, or a substantial portion of the mid-lower mantle is relatively SiO2-rich (hence Fp poor) compared to the shallow mantle. The melt-depleted mantle lithosphere of subducted oceanic slabs that sink into the lower mantle contains 22% Fp, and thus offers the best opportunity to prospect for a spin change in Fp. Here we reveal a loss in the abundance of fast seismic velocity anomalies in compressional (P-wave) tomography models at 1,400-2,000 km depth that is opposite to the trend in shear (S-wave) models. This can be explained by the decreasing temperature sensitivity of P-velocity expected for the mixed spin state of iron in Fp at corresponding pressures7. We also observe a similar but subtle signal for seismically slow regions below 1,800 km, consistent with a pressure increase and broadening of the Fp spin transition at higher temperatures. Seismic wave raypath distribution is similar for both P- and S-waves in this depth range, therefore this signature cannot be attributed to substantial differences in data coverage. Our identification of the spin transition signal in seismically fast and slow regions indicates that the spin crossover can identify the presence of Fp in the lower mantle. The absence of a Fp spin crossover signal in global seismic profiles supports the notion that the lower mantle is chemically heterogeneous at large scales and contains SiO2-rich regions that suppress the average signature of these pressure-induced electron spin pairing transitions.

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The primary phase of the Earth’s lower mantle, (Al, Fe)‐bearing bridgmanite, transitions to the post‐perovskite (PPv) phase at Earth’s deep mantle conditions. Despite extensive experimental and ab initio investigations, there are still important aspects of this transformation that need clarification. Here, we address this transition in (Al3+, Fe3+)‐, (Al3+)‐, (Fe2+)‐, and (Fe3+)‐bearing bridgmanite using ab initio calculations and validate our results against experiments on similar compositions. Consistent with experiments, our results show that the onset transition pressure and the width of the two‐phase region depend distinctly on the chemical composition: (a) Fe3+‐, Al3+‐, or (Al3+, Fe3+)‐alloying increases the transition pressure, while Fe2+‐alloying has the opposite effect; (b) in the absence of coexisting phases, the pressure‐depth range of the Pv‐PPv transition is likely too broad to cause a sharp D” discontinuity (<30 km); (c) the average Clapeyron slope of the two‐phase regions are consistent with previous measurements, calculations in MgSiO3, and inferences from seismic data. In addition, (d) we observe a softening of the bulk modulus in the two‐phase region. The consistency between our results and experiments gives us the confidence to proceed and examine this transition in aggregates with different compositions computationally, which will be fundamental for resolving the most likely chemical composition of the D" region by analyses of tomographic images.