Composition versus temperature induced velocity heterogeneities in a pyrolitic lower mantle

Interpretation of lateral velocity heterogeneities is essential for our understanding of Earth's interior. Ferropericlase's (Fp) spin crossover (FSC) fundamentally changes their interpretation in the mid lower mantle. In a typical pyrolitic aggregate, FSC induces an unusual increase in bulk sound velocity (V_ϕ) with increasing temperature at mid-lower-mantle depths. This reduces the sensitivity of longitudinal velocity (V_P) to lateral temperature variations around 1700 km. Here we show that FSC also dramatically impacts the manifestation of two important types of compositional heterogeneities: i) variations in iron concentration in Fp, e.g., caused by changes in iron partitioning; ii) variation in molar fraction of Fp, as expected in slab subduction regions. FSC enhances the sensitivity of V_ϕ and V_P to these compositional variations by several-fold at similar depths. The opposite effects of lateral variations of temperature and composition on V_P is critical for distinguishing the possible physical origin of heterogeneities in tomographic P-models. Temperature and composition variations also produce opposite types of correlation between V_ϕ and shear velocity (V_S) heterogeneities and between V_S and density (ρ) heterogeneities. Only lateral temperature variations can produce anti-correlation between V_ϕ and V_S at mid lower mantle depths, while only these compositional variations can produce anti-correlation between V_S and ρ in the spin crossover region and at greater depths. Together these effects suggest that heterogeneities in V_P in the mid lower mantle common to multiple seismic models could originate in simultaneous lateral temperature and compositional variations in this region.