The core‐mantle boundary region

Loper, David E.; Lay, Thorne

doi:10.1029/94jb02048

Cited by 159 publications

(82 citation statements)

References 348 publications

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“…Seismic body waves have been interpreted in terms of a 2-3% discontinuous velocity increase 150 to 350 km above the core-mantle boundary with a median height of 250 km in many regions of the world [Loper and Lay, 1995]. There are, however, reports of the height of the discontinuity as low as 130 km [ Vidale and Benz, 1993] and as high as 450 km [Kendall and Shearer, 1994].…”

Section: Discussionmentioning

confidence: 99%

“…There are, however, reports of the height of the discontinuity as low as 130 km [ Vidale and Benz, 1993] and as high as 450 km [Kendall and Shearer, 1994]. One of the controversial issues about the discontinuity is the problem of its intermittent nature (see discussion by Loper and Lay (1995]). It is still not clear if the discontinuity is a global feature [Nata!…”

Section: Discussionmentioning

confidence: 99%

“…The D" region at the base of the mantle has been the focus of considerable seismological, geochemical and geodynamical study (see the review by Loper and Lay, [1995]). Although our knowledge of the region has dramatically improved, we still lack a comprehensive understanding of the processes responsible for the structures at the bottom of the mantle.…”

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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“…There have been numerous studies of the seismic discontinuity that lies a few hundred kilometers above the core-mantle boundary (see reviews by Lay et al [1998a] and Loper and Lay [1995]). At the discontinuity, seismic velocities increase [Lay and Helmberger, 1983;Wright et al, 1985], and the increase is taken to be the top of the D" region.…”

Section: Seismological Observations 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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“…[2] Seismic studies show that the base of the Earth's mantle (the D 00 layer) has a complex structure [Young and Lay, 1987;Loper and Lay, 1995;Lay et al, 1998a;Wysession et al, 1998;Garnero, 2000]. The upper boundary of the D 00 layer is usually modeled as a discontinuity characterized by 1.5 -3% increase in shear velocities.…”

Section: Introductionmentioning

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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The core‐mantle boundary region

Cited by 159 publications

References 348 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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