Seismic and mineralogical structures of the lower mantle from probabilistic tomography

Abstract

One of the most powerful approaches for understanding the 3‐D thermo‐chemical structure of the lower mantle is to link tomographic models with mineral physics data. This is not straightforward because of strong trade‐offs between thermal and chemical structures and their influence on seismic structures. They can be reduced by mapping simultaneously perturbations of wave speeds and density anomalies and by the quantitative assessment of the accuracy and uniqueness of seismic and mineralogical data. Here, we present new tomographic maps of low order even‐degree seismic structures which are an improvement on earlier models. They satisfy constraints from body wave, surface wave and normal mode data simultaneously, thereby enhancing the spatial resolution. Furthermore, the seismic structure at a given location is represented by a probability density function (pdf) which takes into account the uncertainty and non‐uniqueness of the solution due to modeling and data restrictions. Following a robust statistical procedure, we fit heterogeneity of wave speeds and density from hypothetical thermo‐chemical models to those of our tomographic maps. We thereby constrain lateral variations of temperature as well as iron, silica and post‐perovskite concentration in terms of pdfs. Our work shows that large scale chemical variations are likely everywhere in the lower mantle. In most of the D″ region post‐perovskite is most abundant in the Circum‐Pacific belt, but near the core its lateral variation is more complex. Furthermore, post‐perovskite concentration trades off with the amplitudes of temperature and silicate variations, but not with their lateral distribution. This might be the reason why temperature and silicate variations appear not constrained by our data in the lowermost few hundred km 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 .…”

Global Imaging of the Earth's Deep Interior: Seismic Constraints on (An)isotropy, Density and Attenuation

“…Using mineral physics relations between seismic properties, temperature and composition, Trampert et al 2004 were able to discriminate between thermal and compositional contributions to observed density variations, suggesting that high-density anomalies in the deep mantle (2000-2891 km depth) are most likely of compositional origin. Recently, the studies on probabilistic tomography were extended by Mosca et al (2012) who incorporated improved splitting function measurements, as well as a large collection of body wave travel times (figure 11.5).…”