The main impact of the project is to provide a better description of convective processes occuring in the mantle. These processes are at the origin of volcanism at the surface of the Earth, which provides a non-negligible component of hazard for populations, as well as an important contribution to the CO2 in the atmosphere. Progresses beyond the state of the art have been:
(1) The development of a complete workflow for predicting seismic waveforms from mineralogy and realistic pressure and temperature conditions, and the possibility of doing a spatial analysis of these synthetic data accounting for real data acquisition geometry and realistic processing
(2) The ability to provide from observed seismic data a more accurate description of the scale of variations of the mantle structure, including uncertainties
(3) The possibility of accounting for the effect of mantle mixing. The integration of multiple datatypes indeed suggests that unperfect mixing is the rule at ~500 km and longer length-scale.
The addition of a Bayesian component into the main analysis framework has allowed:
(i) Inverting seismic data for the first-order mantle thermo-chemistry, provide uncertainties on inverted model parameters, and provide for the first time quantitative estimates on mantle mixing processes (length-scale)
(ii) Self-consistently integrating seismic data of different natures (converted and reflected body-waves)
The project has demonstrated that the mantle is more heterogeneous than previously suggested by smooth models of the Earth's interior based on seismic tomography. Heterogeneities exist at multiple scales in the vicinity of the main phase transitions between upper and lower mantle, and this heterogeneity is partly related to phenomena associated to phase transitions (stability of multiple phase assemblages, partial melting and compositional segregation). We have been able to anchor thermal profiles of the mantle down to the base of the transition zone, showing that a ~2100 Kelvin limit exists at ~660 km depth. This temperature can locally be exceeded in very small regions of the mantle (0.6%), in mantle plumes, and this must occur below a ~500 km wavelength. Large regions with a temperature of 1700-1800 Kelvin also exist, in particular beneath the Pacific basin. They appear related with upwelling material, but are not consistent with narrow plumes originating from the core mantle boundary. A dominant signal in our model is basalt-enrichment (up to 60%) associated with downwelling streams associated with paleo-zones of subduction. The mechanism leading to segregation of oceanic crust from subducted plates in the transition zone now needs to be re-evaluated and clarified.
