Spin crossover in ferropericlase and velocity heterogeneities in the lower mantle

Abstract:

Ferropericlase (Fp) is believed to be the second most abundant phase in the lower mantle. Since the discovery of the high spin (HS) to low spin (LS) crossover in iron in Fp [1], this phenomenon has been investigated extensively experimentally and theoretically. This is a broad and smooth crossover that takes place throughout most of the lower mantle and does not produce an obvious signature in radial velocity or density profiles [2]. Therefore, the spin transition has been generally considered to be invisible to seismic waves. This study, however, shows that it can produce a peculiar effect on lateral velocity heterogeneities at certain depths[3]. Deciphering the origin of seismic velocity heterogeneities in the mantle is crucial to understanding internal structures and processes at work in the Earth. The spin crossover in iron introduces unfamiliar effects on seismic velocities. First principles calculations indicate that anti-correlation between shear velocity (V_S) and bulk sound velocity (V_φ) in the mantle, usually interpreted as compositional heterogeneity, can also be produced in homogeneous aggregates containing Fp. The spin crossover also suppresses thermally induced heterogeneity in V_P but not in V_S. This effect is observed in tomographic models at conditions where the spin crossover in Fp is expected in the lower mantle. In addition, the one-of-a-kind signature of this spin crossover in the R_S/P () heterogeneity ratio might be a useful “fingerprint” to detect the presence of Fp in the lower mantle.

[1] Badro J, et al. (2003) Science 300(5620):789-791.

[2] Wu Z, Justo J. F., and Wentzcovitch R. M., (2013). Phys. Rev. Lett. 110. 228501-5

[3]Wu Z., and Wentzcovitch R. M., (2014) Proc Natl Acad Sci USA. www.pnas.org/cgi/doi/10.1073/pnas.1322427111