Geodynamically consistent seismic velocity predictions at the base of the mantle

Abstract

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.

Slabs in the lower mantle and their modulation of plume formation

“…This high pressure behavior has been determined experimentally for perovskite and magnesiowüstite [ Wang et al , 1994; Chopelas , 1996] and is well represented by the nondimensional equation: where α s = 2.93 is the nondimensional thermal expansivity at the surface, a = 10.5 and b = 0.85 are fitting constants. This equation is an empirical fit to the theoretical values of thermal expansivity along a mantle adiabat [ Sidorin and Gurnis , 1998]. The dimensional value of reference thermal expansivity, α 0 , is chosen so that the volume average (geometric mean) of α is equal to 1.…”

Constructing synthetics from deep earth tomographic models

“…A selection of data showing the S cd phase is displayed in Fig. 11, along with synthetics from a 1‐D subducted slab model proposed by Sidorin & Gurnis (1998). Their model contains a double thermal boundary layer, one at the CMB (negative gradient) and the other approaching a 1 per cent velocity jump (positive gradient) a couple of hundred kilometres above the CMB.…”

“…The 2‐D synthetics fit the relative amplitude of S cd to S at some stations, but do not fit the timing separation between S and S c S as well as the 1‐D model. This feature is easily accommodated by adding a low‐velocity boundary layer approaching the CMB, as in Sidorin & Gurnis (1998). Perhaps a more interesting feature displayed by this observed record section is the rapid variation associated with S cd .…”

“…Their waveforms show distinctly different interference patterns for events arriving from South America. It would be particularly useful to explain such variation from tomography‐based models with some fine‐scale adjustments, as proposed by Sidorin & Gurnis (1998). They demonstrated that the above S cd triplication can be produced by a positive velocity gradient induced by subducted material super‐imposed on a small global velocity discontinuity of 1.…”

Mantle convection and evolution with growing continents

“…[10] Other models by Christensen and Hofmann [1994], Sidorin and Gurnis [1998], and Davies [2002] yield considerably thinner layers that are different from the main part of the mantle with respect to major element distribution and correspond to the D 00 layer that develops from subducted oceanic crust. Davies' [2006] results show only a very thin dense layer at the base of the mantle.…”