Abstract

Co-authors: Gardés Emmanuel (CNRS – Université Caen), Sifré David (CNRS – Université d'Orléans), Morales Luiz F.G. (GFZ Potsdam), Précigout Jacques (CNRS – Université d'Orléans), Gaillard Fabrice (CNRS – Université d'Orléans)

Grain size is a critical parameter for the understanding of our planet’s mantle since it has considerable impact on seismological properties, on the permeability of mantle rocks and therefore on melt migration characteristics of Earth’s interior. Grain growth therefore becomes an important process that should be meticulously determined in order to better understand the dynamics of our planet’s interior.

Olivine grain growth has therefore been experimentally determined by a wide number of grain growth studies where different effects have been considered (water, fO2, melt, secondary phases). However, no clear consensus on the values of the different material-dependent parameters in the empirical law has been reached. To increase the existing database on olivine grain growth, we experimentally investigated the effect of melt (from nominally melt-free to 12 wt. melt) and water on San Carlos olivine under different pressure-temperature-duration conditions (0.3 GPa < P < 3.0 GPa, 1200°C < T < 1350°C, 1h < t < 360h).

By combining the existing database on olivine grain growth and our experimental data, we have succeeded in modeling (i) genuinely dry olivine grain growth aggregates, through grain boundary diffusion-controlled processes, (ii) H2O -oversaturated olivine aggregates and (iii) melt-bearing olivine aggregates, from nominally melt-free to ∼ 50 wt. melt. Different important parameters have been constrained by using our model, namely the dry effective grain boundary thickness (δ ∼ 6 nm), melt contents in nominally melt-free samples (Φ ≤ 0.5 wt.%) as well as the wetting properties (Ψ) of melt as a function of melt content, pressure and temperature. We expect that our results will not only have considerable implications on the grain size-dependent deformation mechanisms of mantle rocks but also reconcile macroscopic observations to microscopic-scale key processes governing the mantle behavior, particularly in intergranular zones impregnated by low melt content s.