Seismological expression of the iron spin crossover in ferropericlase in the Earth’s lower mantle

• Published in: Nature communications Vol. 12; no. 1; p. 5905
• Main Authors: Shephard, Grace E., Houser, Christine, Hernlund, John W., Valencia-Cardona, Juan J., Trønnes, Reidar G., Wentzcovitch, Renata M.
• Format: Journal Article
• Language: English; Norwegian
• Published: London Nature Publishing Group 08.10.2021; Nature Portfolio; Nature Publishing Group UK
• Subjects: Bulk modulus; Cold spinning; Earth mantle; Electron spin; Iron; Lithosphere; Lower mantle; Minerals; P waves; Relative abundance; S waves; Seismic analysis; Seismology; Subduction (geology); Temperature; Tomography; Velocity; Wave velocity
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-021-26115-z

Abstract 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.