High Rayleigh number thermo‐chemical models of a dense boundary layer in D″

Numerical models of a dense layer at the base of the mantle and implications for the geodynamics of D″

“…The structure within D" would be decoupled from the overlying mantle structure but may also be sufficiently complex to generate wide differences in the seismic structure from region to region. This type of layered situation may be difficult to distinguish from a phase transition that takes place at a certain depth, with modulations caused by variations in the large-scale Changes in thermal diffusivity can also act to decrease thermal convection within a dense basal layer and increases the temperature of upwellings [Manga and Jeanloz, 1996;Montague et al, 1998;Farnetani, 1997]. This does not have profound effects on the overall convective pattern but can lead to differences in thermal structure within the dense layer.…”

“…Another result is that when thermal diffusivity is increased, the temperatures of plumes are also increased. We have previously discussed these effects and their implications in greater detail [Montague et al, 1998]. …”

Mantle Geochemical Geodynamics

“…layer In this case the lower layer is embedded within the lower thermal boundary layer of the main convection, and the dense material gets swept by the convection, forming a spoke pattern of ridges on top, with plumes at the junctions of the ridges, and entrainment takes place there, as shown by 3-D numerical experiments (Tackley, 1998) and laboratory experiments (Olson and Kincaid, 1991;Davaille et al, 2002). The numerical experiments show that this dense layer acts like a rigid boundary, reducing the horizontal wavelength of convective planform relative to a free-slip lower boundary (Tackley, 1998), and that if the layer internally convects (Montague et al, 1998;Montague and Kellogg, 2000), as would be the case if its viscosity is low enough (Solomatov and Moresi, 2002;Schott and Yuen, 2004), small-wavelength thermal structure is generated within the layer. These numerical experiments also show that even in 100% internally heated cases, entrainment exists at cusps anticorrelated with where the downwellings are, and larger amounts of internal heating reduce the buoyancy ratio needed for stability.…”

“…If ÁT represents the total thermal contrast available in the system, then it has been found that the layering displays long-term stability with a fairly flat interface if B is greater than some critical value of order 1, and undergoes either large-topography layering or various shorter-lived behaviors that end in mixing and whole-mantle convection, if B < 1 (Richter and Johnson, 1974;Richter and McKenzie, 1981;Olson and Kincaid, 1991;Montague et al, 1998;Davaille, 1999a;Montague and Kellogg, 2000; Le Bars and Davaille, 2004a). If ÁT represents the total thermal contrast available in the system, then it has been found that the layering displays long-term stability with a fairly flat interface if B is greater than some critical value of order 1, and undergoes either large-topography layering or various shorter-lived behaviors that end in mixing and whole-mantle convection, if B < 1 (Richter and Johnson, 1974;Richter and McKenzie, 1981;Olson and Kincaid, 1991;Montague et al, 1998;Davaille, 1999a;Montague and Kellogg, 2000; Le Bars and Davaille, 2004a).…”

Deep Earth Structure: Lower Mantle and D″

“…It remains important to determine whether there is any density increase in D 00 accompanying the seismic velocity increases, as this could help to resolve whether a chemical change or phase change is involved, either of which could strongly affect the dynamics of the boundary layer (e.g., Hansen and Yuen, 1988;Kellogg, 1997;Montague et al, 1998;Sleep, 1988;Tackley, 1998). Unfortunately, wide-angle triplication observations have very limited sensitivity to density contrasts, so this is an exceedingly difficult attribute to resolve, and normal modes have limited resolution of any small, laterally varying density increases in the relatively thin D 00 region (e.g., Koelemeijer et al, 2012).…”