Can we resolve 3D density heterogeneity in the lower mantle

Abstract

We reexamine the possibility of resolving 3D density structure in the mantle, by performing test inversions using a dataset of well constrained degree 2 splitting coefficients. We show that degree 2 in V3 and Vp can be resolved, and confirm that dlnV3 / DlnVp >2.5 at depths > 2000km, indicating a chemical component to large scale heterogeneity. Models with ρ positively correlated with V3 in the deepest mantle fit the data at least as well as those in which density and velocities are anti-correlated. However, some rough bounds on dlnp / dlnV3 can be inferred. Models obtained from geodynamic considerations are acceptable, in contrast to models with large negative (< -0.3) or positive (> 0.6) dlnp / dlnV3 throughout the lower mantle. The most robust feature is a marked increase in dlnp / dlnV3 at the top of the lower mantle reaching a maximum around 1500 km.