Fabien Dubuffet

Thermo-mechanical adjustment after impacts during planetary growth

The thermal evolution of planets during their growth is strongly influenced by impact heating. The temperature increase after a collision is mostly located next to the shock. For Moon to Mars size planets where impact melting is limited, the long term thermo-mechanical readjustment is driven by spreading and cooling of the heated zone. To determine the time and length scales of the adjustment, we developed a numerical model in axisymmetric cylindrical geometry with variable viscosity. We show that if the impactor is larger than a critical size, the spherical heated zone isothermally flattens until its thickness reaches a value for which motionless thermal diffusion becomes more effective. The thickness at the end of advection depends only on the physical properties of the impacted body. The obtained timescales for the adjustment are comparable to the duration of planetary accretion and depend mostly on the physical properties of the impacted body.

An automatically updated S‐wave model of the upper mantle and the depth extent of azimuthal anisotropy

We present 3D2015_07Sv, an S wave model of the upper mantle based on the waveform modeling of 1,359,470 Rayleigh waves recorded since 1976. The use of approximate forward theory and modeling allows updating the model with new data on a regular basis. 3D2015_07Sv contains azimuthal anisotropy, achieves a lateral resolution of ∼600 km, and is consistent with other recent models up to degree 60 in the uppermost 200 km and degree 15 in the transition zone. Although radial anisotropy has been found to extend deeper beneath continents than beneath oceans, we find no such difference for azimuthal anisotropy, suggesting that beneath most continents, the alignment of olivine crystal is preferentially horizontal and azimuthally random at large scale. As most continents are located on slow moving plates, this supports the idea that azimuthal anisotropy aligns at large scale with the present plate motion only for plates faster than ∼4 cm yr−1.

Compositional and thermal equilibration of particles, drops, and diapirs in geophysical flows

Core formation, crystal/melt separation, mingling of immiscible magmas, and diapirism are fundamental geological processes that involve differential motions driven by gravity. Diffusion modifies the compo- sition or/and temperature of the considered phases while they travel. Solid particles, liquid drops and viscous diapirs equilibrate while sinking/rising through their surroundings with a time scale that depends on the physics of the flow and the material properties. In particular, the internal circulation within a liquid drop or a diapir favors the diffusive exchange at the interface. To evaluate time scales of chemical/thermal equilibration between a material falling/rising through a deformable medium, we propose analytical laws that can be used at multiple scales. They depend mostly on the non-dimensional Peclet and Reynolds numbers, and are consistent with numerical simulations. We show that equilibration between a particle, drop or diapir and its host needs to be considered in light of the flow structure complexity. It is of fundamental importance to identify the dynamic regime of the flow and take into account the role of the inner circulation within drops and diapirs, as well as inertia that reduces the thickness of boundary layers and enhances exchange through the interface. The scaling laws are applied to predict nickel equilibration between metals and silicates that occurs within 130 m of fall in about 4 minutes during the metal rain stage of the Earths core formation. For a mafic blob (10 cm diameter) sinking into a felsic melt, trace element equilibration would occur over 4500 m and in about 3 years.

Lifting the cover of the cauldron: convection in hot planets

Convection models of planetary mantles do not usually include a specific treatment of near-surface dynamics. In all situations where surface dynamics is faster than internal dynamics, the lateral transport of material at the surface forbids the construction of a topography that could balance the internal convective stresses. This is the case if intense erosion erases the topography highs and fills in the depressions or if magma is transported through the lithosphere and spreads at the surface at large distances. In these cases, the usual boundary condition of numerical simulations, that the vertical velocity cancels at the surface should be replaced by a condition where the vertical flux on top of the convective mantle equilibrates that allowed by the surface dynamics. We show that this new boundary condition leads to the direct transport of heat to the surface and changes the internal convection that evolves toward a heat-pipe pattern. We discuss the transition between this extreme situation where heat is transported to the surface to the usual situation where heat diffuses through the lithosphere. This mechanism is much more efficient to cool a planet and might be the major cooling mechanism of young planets. Even the modest effect of material transport by erosion on Earth is not without effect on mantle convection and should affect the heat flow budget of our planet.