Volatile Cycling and Electrical Conductivity in the Earth’s Mantle: The Carbonatite connection – ELECTROLITH

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Geochemical investigations on mantle-derived rocks and remote geophysical observations are powerful tools to illuminate the physico-chemical processes that govern the Earth’s mantle dynamics. Providing a model satisfying both types of observations is the cornerstone of cutting-edge research. One of the conditions for such achievement is to account for the role of fluids, fuel of mantle dynamics, and for their effects on mantle rocks properties, particularly the enhancement of electrical conductivity.

Geochemical observations have long recognized the importance of carbonatites (carbonate melts) as mantle fluids (excepted in the mantle wedge of subduction zone, clearly dominated by water). Their occurrence is favored by pressure-temperature-redox conditions prevailing in numerous mantle regions. Carbonatites facilitate, if not trigger, mantle melting at hotspot or mid-ocean-ridge and play a key role in diamond formation. However, because of the scarcity of carbonatites reaching the Earth’s surface, and due to technical difficulties in studying their unusual physical properties, the importance of carbonatites is poorly addressed by the geophysical community. The role of fluids on geophysical anomalies in mantle rocks has been essentially interpreted in terms of water defects in crystals. Based on recent developments, water ability to explain mantle geophysics is unclear and geochemical record of mantle fluids indicate that chemical complexity of the fluids needs to be increased, by taking into account at least carbon and possibly other volatiles. Recently, in a joined geochemical-geophysical perspective, the PI has revealed the exceptionally high conductivity of carbonate melts implying that conductive mantle regions might rather image percolation of carbonatites at grain boundaries.

This project aims at strengthening and broadening the carbonatite model in order to link deep volatile cycling with the physical processes responsible for mantle electrical conductivity. Theoretical physicists, specialists of molecular dynamics of magmatic liquids, experimental geophysicists, specialist of partial melts and their physical properties and field geophysicists, specialists of electromagnetic inductions and mantle conductivity will contribute to the achievement of such an ambitious goal. Each research units will bring his expertise to study mantle carbonatites, their impact on mantle rock properties, and their overall role in the fate of mantle volatiles.

This objective will be achieved by providing key constraints on 1-the physical and structural properties of carbonatite fluids, 2-their role in the storage and in the transfer of volatile species, 3-their impacts on mantle transport properties such as electrical conductivity, permeability and viscosity and how deformation might affect such properties. Then, 4-mantle electrical conductivity in different geodynamic settings where carbonatites are expected will be collected from field experiments. Then, the bulk conductivity versus depth in the mantle will be modelled with the physical and structural properties of the carbonatite fluids to infer physical properties of the mantle (temperature, permeability, amount of fluids)

By this multi-scale (atom to mantle) and multidisciplinary (atomic and thermodynamic modelling, high temperature-pressure experimentation, geophysical field measurements) approach, we anticipate a global vision of the deep mantle cycle of volatiles integrated into the broad context of mantle geodynamics.