Manifestation of compositional heterogeneities and ferropericlase spin crossover on mantle velocities

Abstract:

The origin of lateral velocity heterogeneities is essential for our understanding of Earth’s interior. Ferropericlase’s (Fp) spin crossover (FSC) fundamentally changes the interpretation of the origin of lateral heterogeneities in the mid lower mantle. In a typical pyrolite compositional model, it induces an unusual increase in bulk sound velocity (V_ϕ) with increasing temperature at mid-lower-mantle depths [1]. This reduces the sensitivity of longitudinal velocity (V_P) to lateral temperature variations around 1700 km [1]. Here we show that FSC also dramatically impacts the manifestation of two important types of compositional heterogeneities, i.e., changes in iron concentration in Fp or in molar fraction of Fp. It enhances the sensitivity of V_ϕ and V_P to these compositional variations by several-fold at similar depths. Therefore, heterogeneities in V_P tomography models are potentially good indicators of temperature versus compositional contributions to lateral velocity changes. In fact, V_P heterogeneities at mid lower mantle depths common to multiple seismic models suggest that both lateral temperature and compositional variations may be significant in this region.

[1] Wu, Z.Q., & Wentzcovitch, R.M. (2014). Spin crossover in ferropericlase and velocity heterogeneities in the lower mantle. Proc. Natl. Acad. Sci. U. S. A. 111: 10468-10472.