Models of thermo-chemical convection: which ingredients are needed to fit probabilistic tomography?

Category Geophysic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Deschamps, Frederic; Tackley, Paul
Holding Date 21 September 2008

Recent seismological observations suggest that strong lateral density anomalies, likely due to compositional anomalies, are present in the deep mantle. We aim to identify models of thermo-chemical convection that can generate strong thermo-chemical density anomalies. For this, we explore the model space of thermo-chemical convection, determine the thermal and chemical density distributions predicted by these models, and compare their power spectra against those from probabilistic tomography. Using STAG3D, we conducted 3D-Cartesian numerical experiments, in which we varied compositional (buoyancy ratio, fraction of dense material), physical (Clapeyron slope of the phase change at 660 km, internal heating), and viscosity (thermal, radial, and compositional viscosity contrasts) parameters. We identified five important ingredients for a successful (in the sense that it fits seismological observations well) model of thermo-chemical convection. (1) A reasonable buoyancy ratio, between 0.15 and 0.25 (corresponding to chemical density contrasts in the range 60-100 kg/m3). Larger density contrasts induce stable layering for long period of time, instead of the strong topography required by seismic observations. (2) A moderate chemical viscosity contrast, around 0.1-10. Smaller viscosity contrasts induce rapid mixing, whereas larger viscosity contrasts lead to stable layering. (3) A large (≥ 104) thermal viscosity contrast. Temperarure-dependent viscosity creates and maintains pools of dense material with large topography at the bottom of the mantle. (4) A 660-km viscosity contrast around 30. (5) And a Clapeyron slope of the phase transition at 660-km around 1.5-3.0 MPa/K. These two last ingredients help to maintain dense material in the lower mantle. Interestingly, they strongly inhibit thermal plumes, but still allow the penetration of downwellings in the lower mantle. Interestingly, additional independent geophysical constraints support the parameter values suggested by our numerical experiments. Finally, we test models that include various combinations of the previous ingredients.