Probabilistic Tomography Maps Chemical Heterogeneities Throughout the Lower Mantle

Abstract

We obtained likelihoods in the lower mantle for long-wavelength models of bulk sound and shear wave speed, density, and boundary topography, compatible with gravity constraints, from normal mode splitting functions and surface wave data. Taking into account the large uncertainties in Earth's thermodynamic reference state and the published range of mineral physics data, we converted the tomographic likelihoods into probability density functions for temperature, perovskite, and iron variations. Temperature and composition can be separated, showing that chemical variations contribute to the overall buoyancy and are dominant in the lower 1000 kilometers of the mantle.

The stability and structure of primordial reservoirs in the lower mantle: insights from models of thermochemical convection in three-dimensional spherical geometry

“…Our study indicates that such structures may be obtained with a buoyancy ratio between 0.2 and 0.36 (which, if taking a superadiabatic jump of 2500 K, corresponds to a chemical density difference between 80 and 145 kg m −3 ). This is in agreement with current estimates of density anomalies in the lower mantle (Trampert et al 2004;Mosca et al 2012). A large thermal viscosity contrast also helps in building long wavelength structures dominated by spherical harmonic degree 2.…”

“…S2; see next paragraph for details of the calculations). For cases in which reservoirs of dense material are maintained, temperature anomalies are typically around ±500 K. Assuming that the reservoirs of dense material are further enriched in iron by about 3 per cent compared to the surrounding material, as suggested by probabilistic tomography (Trampert et al 2004;Mosca et al 2012), and converting the thermochemical structure with sensitivities calculated by Deschamps et al (2012) from an appropriate mineral physics data set, this would lead to shear wave velocity anomalies around −2.5 per cent, typical of what is observed in LLSVP.…”

“…It was recently suggested that this anticorrelation may result from wave propagation effects (Schuberth et al 2012b), or might be related to the phase change from perovskite to post-perovskite in the lowermost mantle . Because the anticorrelation between V S and V anomalies is also observed by normal mode data (Ishii & Tromp 1999;Trampert et al 2004;Mosca et al 2012), which are not affected by wave propagation effects and further show a correlation between density and seismic velocity anomalies, and because the post-perovskite hypothesis fails to predict large-scale structures, the thermochemical hypothesis appears more likely. The detailed nature of chemical heterogeneities is still under debate, the two main hypothesis being that they result either from the evolution of primordial reservoirs of dense materials formed by differentiation early in the Earth's early history (e.g.…”

“…Deschamps & Trampert 2003) prefer a thermochemical interpretation. Several lines of evidence from various data sets, including the anticorrelation between bulk-sound and shear wave velocity anomalies (Ishii & Tromp 1999;Masters et al 2000;Trampert et al 2004;Mosca et al 2012), support a thermochemical origin. It was recently suggested that this anticorrelation may result from wave propagation effects (Schuberth et al 2012b), or might be related to the phase change from perovskite to post-perovskite in the lowermost mantle .…”

Morphology of seismically slow lower-mantle structures

“…Together and individually, these observations have been interpreted to indicate the presence of compositional heterogeneity (e.g. Wen et al 2001;Trampert et al 2004).…”