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Non-Linear Bayesian partition-modeling of the Earth's mantle transition zone

Periodic Reporting for period 2 - NoLiMit (Non-Linear Bayesian partition-modeling of the Earth's mantle transition zone)

Reporting period: 2020-09-01 to 2021-08-31

The objective of NoLimit was to provide new databases, procedures, and models for analyzing and better understanding how the Earth evacuates its internal heat (convection) and how convection acts to recycle geochemical heterogeneities. This is done through analysis of seismic waves that interact with the mantle transition zone, a region of the Earth between 410 and 660 km depth where abrupt increases of seismic wave speeds are related to mineralogical phase transitions. In this context, the analysis faces two challenges that are accounting for the heterogeneous distribution and the multi-scale resolution of seismic data, and integrating experimental data for interpreting seismic observations in terms of petro-physical properties (temperature, pressure, and rock composition). NoLimit has addressed these gaps with two work packages (WP). Each WP has lead to the development of software modules that can be taken separately, or coupled to each other in a forward or inverse approach. The application of these software to massive seismic datasets has first provided estimates for the maximum temperature in the deep mantle, revealed that hot materials do not rise straight to the surface from the core-mantle boundary, and required an imperfectly mixed mantle in terms of chemistry. An inverse statistical framework has then allowed obtaining new models of mantle geochemical heterogeneities. Resulting models suggest that accumulation of oceanic crust (basalt) occurs near cold downwelling limbs of mantle convection, a view which challenges current understanding of the dynamics of geochemical recycling.
The main technological achievements of the project have been to provide databases, procedures, models, and software packages that are used by the community of researchers in Earth Sciences. The innovation output is based on the two pieces of software for seismic data processing and mineral physics modeling and their combination in a Bayesian inversion framework. These software modules have been released through data repositories satisfying open science FAIR standards (data: https://zenodo.org/record/5512035; software: https://zenodo.org/record/5512805). They are currently in use by the group lead by L. Waszek at James Cook University in Australia, and by myself in Lyon University. Current views about the composition of the mantle in the transition zone suffer from ambiguities introduced by chemical equilibration models and seismic data sampling. This MSCA project has allowed to constrain spatially continuous variations of thermo-chemistry at global scale, and to distinguish between chemical equilibration models. The research project therefore has provided a conceptual understanding about convection and recycling of geochemical heterogeneities. Our conclusions are that hot materials do not rise straight from the core-mantle boundary to the surface, and the mantle is heterogeneous and imperfectly mixed in terms of chemistry. A dominant signal in our model is basalt-enrichment 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.
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.
The Earth's mantle thermal structure
Refining Earth structure through Voronoi partition modeling