On the link between subduction and lithospheric texture : insights from convection in colloidal dispersions

Strain-localization along plate boundaries and subduction of the lithosphere are key-features of Earth’s mantle plate tectonics and convective dynamics. Numerous mechanisms, operating on various time and length-scales, are able to produce strain localization. But the exact process(es) that control lithospheric subduction from convection in a viscous mantle remain debated. Laboratory experiments, where one can 1) characterize and control the fluid rheology, and 2) determine the structure/texture of the resulting lithosphere, can give valuable constraints to the ongoing discussion. To date, we have found only one fluid able to produce in the laboratory self-consistent subduction during convection: colloidal dispersions of silica nanoparticles (‘NP’). These fluids encompass a large diversity of rheological behavior, from viscous to elasto-visco-plastic to brittle, depending on the nanoparticles volume fraction. But we can now go one step further, and investigate the links between the rheological properties and the nanoparticles’organization.   So we studied Ludox® dispersions with NP of two different diameters (16 and 28 nm) and vary the water concentration, while keeping the ionic content constant. The rheology is characterized using shear rate and oscillatory tests. The organization of the nanoparticles is probed with Small-Angle Neutron Scattering (SANS) and Small-Angle X-ray Scattering (SAXS) measurements, and the solvent (water) state thanks to thermal analysis (ThermoGravimetric Analysis, TGA, and Differential Scanning Calorimetry, DSC). These measurements allow us to identify three types of water, (i) water adsorbed at the surface of the particles, (ii) water confined in nano-cavities and (iii) free water, and to determine the evolution of their amounts with increasing particle volume fraction. Rheology seems mainly controlled by the amount of free water. As the fraction of the latter decreases, the material becomes more heterogeneous at the meso-scale, with the formation of particle aggregates and free water channels. The rheology becomes more non-newtonian with the apparition of a yield stress, which value increases with decreasing free water. In convection-evaporation experiments, this results in the formation of a lithospheric « skin » floating on a less concentrated solution, and strong localization of deformation on this heterogeneous skin that can induce its break-up and subduction. Interestingly enough, subduction is observed for skins still containing a little amount of free water (0.2-1%). On a rocky planet, this suggests that the existence of partial melt in the asthenosphere and the lithosphere could be important to allow subduction. This could explain why localized subduction may exist on present-day Venus: eventhough there is no liquid water ocean on the surface of Venus today, there is plenty of evidence of active volcanism, which indicate the existence of a sizable amount of partial melt.