Melting in the Earth’s mantle is a fundamental process in the deep volatile cycles because it produces liquids that concentrate and redistribute volatile species. Such redistribution can trigger volcanic degassing, magma emplacement in the crust and hydrothermal circulation, and other sorts of chemical redistribution within the mantle (metasomatism). Melting also affects mantle viscosities and therefore impacts on global geodynamics. So far, experimental petrology has been the main approach to construct a picture of the mantle structure and identify regions of partial melting.
Magnetotelluric (MT) surveys reveal the electrical properties of the deep Earth and show surprising highly conductive regions within the mantle, most likely related to the presence of volatiles and melts. However, melting zones disclosed by electrical conductivity do not always corroborate usual scenarios deduced from experimental petrology. In 2008, I proposed that small amount of melts, extremely rich in volatiles species and with unusual physical properties, could reconcile petrological and geophysical observations. The broadening of this idea is however limited by (i) the incomplete knowledge of both petrological and electrical properties of those melts and (ii) the lack of petrologically based models to fit MT data. ELECTROLITH will fill this gap by treating the following points :
1- How do volatiles of the H-C-S-Cl-F system affect the beginning of melting ? And how do such low melt fractions affect mantle conductivity ? Volatiles deepen the initiation of melting, but appropriate experiments do not exist and their effect on electrical conductivity is mostly unknown.
2- Which are the physical properties of those H-C-S-Cl-F-rich melts ? Spectroscopy and molecular dynamics will define the ionic structures and physical properties of these melts.
3- How can produced melts migrate in the mantle ? Lab measurements on rates and mechanisms of low melt fraction migrations in olivine aggregates will be conducted using in situ conductivity measurements. Surface tension, wetting properties, dissolution-precipitation, and deformation effects will be tackled.
4- How melting and melt migration in mantle can be recognized in geophysical measurements ? The scale transfers from the microscopic and hand-scale physical properties to the MT scale will be modelled in a petrological scheme in order to decipher MT data in terms of melt percolation models, strain distributions and chemical redistributions within the mantle.
ELECTROLITH milestone is therefore a reconciled perspective of geophysics and petrology that addresses the chemical complexity of volatiles and their impact on the mantle at an atomic to mantle scale. Such achievement will profoundly enrich our vision of the mantle geodynamics.