Interaction of mantle dregs with convection: Lateral heterogeneity at the core‐mantle boundary

Davies, Geoffrey F.; Gurnis, Michael

doi:10.1029/gl013i013p01517

Cited by 91 publications

(53 citation statements)

References 15 publications

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“…This problem was studied by Davies and Gurnis (1986] in convection models with constant viscosity. It was found that an initially 300 km thick layer which is 2-3% heavier than the surrounding mantle would survive for at least 250 million years.…”

Section: Discussionmentioning

confidence: 99%

“…A chemical layer at the bottom of the mantle has been proposed in several studies [e.g., Davies and Gurnis, 1986;Christensen and Hofmann, 1994;Wysession, 1996] as causing the seismic discontinuity at the top of D". Using both previous and new results, is such an interpretation of the seismology consistent with the dynamics of the mantle and mineralogical and geochemical constraints?…”

Section: Discussionmentioning

confidence: 99%

“…Forte and Peltier (1989] compared seismically inferred CMB topography with that produced by a global flow model using buoyancy constrained by seismic tomography. Several studies attempted to predict temperature or density variations in the mantle from tomographic models [e.g., Hager et al, 1985;Yuen et al, 1993] or seismic velocities from dynamic models [e.g., Davies and Gurnis, 1986]. However, all previous studies generally use simple scalings between temperature and seismic velocity while a comprehensive interrelation between the dynamic modeling and mantle composition and thermoelastic properties is required.…”

Section: Introductionmentioning

confidence: 99%

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Geodynamically consistent seismic velocity predictions at the base of the mantle

Sidorin¹,

Gurnis²

1998

The Core‐Mantle Boundary Region

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A model of thermoelastic properties for a chemically homogeneous adiabatic lower mantle is calculated. Constraints provided by this model are used in convection models to study dynamics of a chemically distinct layer at the bottom of the mantle. We find that the layer must be at least 2% denser than the overlying mantle to survive for a geologically significant period of time. Realistic decrease with depth of the thermal expansivity increases layer stability but is unable to prevent it from entrainment. Seismic velocities are computed for an assumed composition by applying the thermal and compositional perturbations obtained in convection simulations to the adiabatic values. The predicted velocity jump at the top of the chemical layer is closer to the CMB in the cold regions than in the hot. The elevation of the discontinuity above CMB in the cold regions decreases with increasing thermal expansivity and increases with increasing density contrast, while in the hot regions we find that the opposite is true. If the density contrast is small, the layer may vanish under downwellings. However, whenever the layer is present in the downwelling regions, it also exists under the upwellings. For a 4% density contrast and realistic values of expansivity, we find that the layer must be more than 400 km thick on average to be consistent with the seismically observed depth of the discontinuity. A simple chemical layer cannot be used to interpret the D" discontinuity: the required change in composition is large and must be complex, since enrichment in any single mineral probably cannot provide the required impedance contrast. A simple chemical layer cannot explain the spatial intermittance of the discontinuity.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Geodynamically consistent seismic velocity predictions at the base of the mantle

Sidorin¹,

Gurnis²

1998

The Core‐Mantle Boundary Region

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“…Early work at fairly low Rayleigh number (< 10 6) [Christensen, 1984;Davies and Gurnis, 1986;Yuen, 1988, 1989] showed that a dense layer along the lower boundary layer tends to pile up beneath upwellings, is deflected beneath downwellings, and may be entrained if not dense enough. Some of these models of D", especially those of Christensen [1984], who used temperature-dependent viscosity, exhibit small-scale instabilities within the boundary layer, but none have featured complete two-layered convection, possibly because the local Rayleigh number within the layer must be above a critical value in order for internal convection to occur.…”

Section: Previous Models Of D"mentioning

confidence: 99%

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

Montague¹,

Kellogg²

2000

J. Geophys. Res.

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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.

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“…Other factors such as chemical heterogeneities and anisotropy can be more important for seismic velocity variations. Chemical heterogeneities are likely to be present at the bottom of the convective mantle [Christensen, 1984;Davies and Gurnis, 1986;Hansen and Yuen, 1988;Kellogg, 1997;Tackley, 1998]. Since small-scale convection in the D 00 layer is probably not chaotic (Figure 7), lateral mixing is not very efficient [Kellogg, 1992;Schmalzl and Hansen, 1994;Ferrachat and Ricard, 1998].…”

Section: Lateral Temperature Variationsmentioning

confidence: 99%

Small‐scale convection in the D″ layer

Solomatov

Moresi

2002

J. Geophys. Res.

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[1] Small-scale convection has been suggested as a possible explanation for seismic heterogeneities in the D 00 layer of the Earth's mantle. Recently developed scaling laws for convection with realistic viscosities allow quantitative assessment of this hypothesis. Large temperature and viscosity contrasts across the thermal boundary layer at the core-mantle boundary suggest that small-scale convection starts at the bottom of the thermal boundary layer and propagates upward before the thermal boundary layer as a whole becomes unstable. The convection boundary is likely to become a chemical boundary as a result of mixing within the convective layer. This implies that the D 00 discontinuity can represent both convective and chemical boundary and that the presence or absence of small-scale convection can be responsible for the observed intermittent nature of the D 00 discontinuity. Most lateral heterogeneities in the D 00 layer are concentrated near the convection boundary, consistent with seismic data. The length scale of lateral temperature variations, the topography of the core-mantle boundary, and the viscosity of the D 00 layer are in agreement with observational constraints. The thickness of the secondary thermal boundary layer formed at the bottom of the convective layer is similar to the thickness of the ultralow-velocity zone. The magnitude of lateral variations in temperature can only marginally be responsible for lateral variations in seismic velocities. Variations in the topography of the convection boundary and chemical heterogeneities are likely to be more important factors. A large seismic velocity drop in the ultralow-velocity zone cannot be explained by thermal effects alone and must be caused by other factors such as partial melting.

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Interaction of mantle dregs with convection: Lateral heterogeneity at the core‐mantle boundary

Cited by 91 publications

References 15 publications

Geodynamically consistent seismic velocity predictions at the base of the mantle

Geodynamically consistent seismic velocity predictions at the base of the mantle

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

Small‐scale convection in the D″ layer

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