Journal of Conference Abstracts

Thermal Convection in an Heterogeneous Mantle

Anne Davaille (davaille@ipgp.jussieu.fr)

IPG, 4 place Jussieu, Paris cedex 05, 75 252, France

The convective and mixing patterns in the mantle remains controversial, in part because the various lines of evidence from geochemistry, seismology and geodynamics are often difficult to reconcile in the same model. I have studied in the laboratory the mixing and thermal pattern resulting from the interaction between thermal convection and a stratification in density and viscosity. Rayleigh numbers between 3. 10² and 10⁸ were obtained for initial density ratios between 0.15 and 5 % and viscosity ratios between 1 and 6. 10⁴.

Motions occur on two scales: (1) small-scale thermal instabilities originating at the outer boundaries are modulated by (2) a large-scale circulation generated at the interface. Linear stability theory shows that (2) is the finite amplitude signature of an overstable interfacial mode. Entrainment occurs only through (2), driven by the horizontal temperature heterogeneities and viscous coupling at the interface. It is asymmetric, 2D cold sheets being entrained in the more viscous layer while hot tubular plumes are entrained in the less viscous layer. This provides a mechanism for hot spot generation. Quantitative measurements of the thermal structure and mass flux were made and scaling laws derived. They predict that the lower reservoir would contribute to between 1 and 15% of each hot spots mass flux.