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

Abstract

Abstract. To investigate the dynamics of the mantle' s D" layer, we explore numerical models of mantle convection which feature a dense basal boundary layer. We use a double-diffusive finite element convection scheme and vary thermal and chemical Rayleigh numbers and properties including viscosity, thermal conductivity, and internal heating. For isoviscous models with heating only from below, the thermal Rayleigh number (Ra) is set at either 106 or 107. The negative chemical buoyancy of a dense layer produces a stable boundary layer when the ratio of chemical to thermal buoyancy (the buoyancy number B) is around 1. For B = 0.5 the layer is unstable, while B = 0.6 may produce a layer which remains stable for long periods, depending on other factors (such as Rayleigh number, layer thickness, and thermal diffusivity of the layer). At high enough Ra, small-scale convection can occur within the layer. Using Ra = 107, we look at changes in layer thickness from 100 to 300 km at two values of B (0.6 and 1). Convection within the layer takes place most easily when the layer has a greater initial thickness. For B = 0.6, the layer is not always continuous along the lower boundary, so convection within the layer does not occur unless the initial thickness is fairly high (300 km). Increasing the thermal diffusivity of the layer (to simulate enrichment in metals) enhances heat conduction across the layer and can suppress internal convection within D". Enhanced conduction also leads to higher plume temperatures. Including internal heating mainly increases the overall heat flow through the mantle, leading to higher surface heat flux. Finally, we examine models with temperaturedependent viscosity and pressure-dependent viscosity using Rayleigh numbers in the range of 107.Convection within the dense layer is enhanced by the relative reduction in viscosity. Our models exhibit only a modest decrease in viscosity (an order of magnitude) but illustrate how a dense, low-viscosity layer interacts with cold, viscous downwellings.

Melting in the martian mantle: Shergottite formation and implications for present‐day mantle convection on Mars

“…This layering resembles the models of Kellogg et al (1999) and Coltice and Ricard (1999) for the Earth. The interface between the 2 mantle layers is shown here as occurring at a constant depth, although, if the density contrast between the 2 layers is small, substantial topographic relief can occur on the interface (Tackley 1998;Montague and Kellogg 2000). In assessing the plausibility of such layering, we must address 2 questions: 1) is the layering stable over geologic time?…”

The feedback between surface mobility and mantle compositional heterogeneity: Implications for the Earth and other terrestrial planets

“…The magnitude of the velocity anomalies is believed to be too large for a simple thermal feature (Karato and Karki, 2001;Brodholt et al, 2007) and sharp lateral changes in shear wave velocity, as well as an anticorrelation in the shear wave speed and bulk sound velocity are also not indicative of purely thermal structures (Ritsema et al, 1998;Ni et al, 2002;To et al, 2005;Karato and Karki, 2001;Saltzer et al, 2001;Brodholt et al, 2007). Possible sources of compositionally enriched material include segregation of oceanic slab material (e.g., Brandenburg and van Keken, 2007) (although Li and McNamara, 2013 suggest difficulty in accumulating the observed LLSVP volume in this fashion), direct interaction with the core (e.g., Humayun et al, 2004), or primordial reservoirs (e.g., Labrosse et al, 2007).…”

Effects of a perovskite‐post perovskite phase change near core‐mantle boundary in compressible mantle convection

“…[3] Although there have been many mantle convection modeling studies that have focused on structure in the deep mantle, they have mostly been concerned with the heterogeneous structures that are generated from possible compositional layering in that region [e.g., Davies and Gurnis, 1986;Hansen and Yuen, 1988;Montague and Kellogg, 2000;Tackley, 2002]. A deep phase transition has been considered in only one Earth-related modeling study [Sidorin et al, 1999], which focused on the seismic signature associated with such a phase transition, and concluded that it offered the best explanation of observed seismic structures such as the ''Lay discontinuity'', but did not investigate the dynamical effects of such a transition.…”