Fat Plumes May Reflect the Complex Rheology of the Lower Mantle (original) (raw)
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Structure and dynamics of sheared mantle plumes
[1] An extensive series of laboratory experiments is used to investigate the behavior of sheared thermal plumes. The plumes are generated by heating a small circular plate on the base of a cylindrical tank filled with viscous fluid and then sheared by rotating a horizontal lid at the fluid surface. The motion of passive tracers in the plumes is visualized by the release of several dye streams on the hot plate. We systematically examine the dependence of the convective flow on four dimensionless numbers: a velocity ratio, a Rayleigh number, the viscosity ratio, and an aspect ratio. We identify and delineate two transitions in the convective behavior: from a regime where the plume can spread upstream against the shear to a regime where the entire plume is advected downstream, and from a regime of negligible cross-stream circulation to a regime with significant cross-stream circulation and thermal entrainment. Our analysis of the steady profiles of the plumes shows that they initially rise with a constant vertical rise velocity. This rise velocity depends on the buoyancy flux and ambient viscosity but is almost independent of the centerline plume viscosity, which suggests that most of the thermal plume has a viscosity that is much closer to the ambient viscosity than the centerline viscosity. As the plumes approach the lid, they decelerate as the viscous drag on them steadily increases. The lateral spreading of the plumes under the lid is found to be well described by similarity solutions derived for the spreading of compositional plumes on a rigid surface, if the effective viscosity of the thermal plumes is taken to be the ambient viscosity rather than the centerline viscosity. A similar theoretical model is found to roughly predict the upstream spreading of thermal plumes at low shear, but it breaks down at moderate to high shear, where the entire plumes are advected downstream. When our results are applied to the Earth, we find that mantle plumes are mostly divided into only two flow regimes in the upper mantle: plumes under slow moving plates experience upstream flow and negligible cross-stream circulation, while plumes under faster moving plates (including all Pacific plumes) experience significant cross-stream circulation and are advected downstream. We also demonstrate that geochemical heterogeneities in a plume's source region will result in an azimuthally zoned plume and in an asymmetric geographical distribution of geochemical heterogeneities in the erupted hot spot basalts, as is seen in the Hawaiian, Galápagos, Marquesas, and Tahiti/Society island chains. For individual mantle plumes, we determine their diameter and vertical rise velocity as well as the extent of upstream spreading and the rate of lateral spreading under the lithosphere. Components: 16,471 words, 30 figures, 14 tables.
Viscosity profile of the lower mantle
Geophysical Journal International, 1985
We determine the variation of effective viscosity q across the lower mantle from models of the Gibb's free energy of activation G* and the adiabatic temperature profile. The variation of G* with depth is calculated using both an elastic strain energy model, in which G* is related to the seismic velocities, and a model which assumes G* is proportional to the melting temperature. The melting temperature is assumed t o follow Lindemann's equation. The adiabatic temperature profile is calculated from a model for the density dependence of the Gnineisen parameter. Estimates of q depend on whether the lower mantle is a Newtonian or power law fluid. In the latter case separate estimates of q are obtained for flow with constant stress, constant strain rate, and constant strain energy dissipation rate. For G* based on the melting temperature, increases in q with depth range from a factor of about 100 for Newtonian deformation or power-law flow with constant stress to about 5 for non-Newtonian deformation with constant strain rate. For G* based on elastic defect energy, increases in q with depth range from a factor of about 1500 for Newtonian deformation or power-law flow with constant stress to about 10 for non-Newtonian deformation with constant strain rate. Among these models, only a non-Newtonian lower mantle convecting with constant strain rate or constant strain energy dissipation rate is consistent with recent estimates of mantle viscosity from post-glacial rebound and true polar wander data.
Generation of plate-tectonic behavior and a new viscosity profile of the Earth's mantle
2003
This paper reports a series of compressible spherical-shell convection calculations with a new viscosity profile, called eta3, that is derived from PREM and mineral physics. The viscosity profile displays not only a high-viscosity lithosphere and a viscosity hill in the central region of the lower mantle of the Earth but also a prominent high-viscosity transition layer inferred to arise from a high garnet content. Moreover, there is not only the usual asthenosphere but also a second low-viscosity zone just below the 660-km discontinuity. We introduced a viscoplastic yield stress and obtained plate-like movements near the surface. A variation of the parameters revealed a Rayleigh-number-yield-stress area where the plate-tectonic character of the solution is stable and pronounced. Runs with eta3 but without any yield stress show networks of reticularly connected very thin sheet-like downwellings but, of course, no plates. For calculations with eta3 plus yield stress, the distributions of the downwellings are more Earth-like.
Special Paper 430: Plates, Plumes and Planetary Processes, 2007
Recent seismological studies demonstrate the presence of strong deep-mantle elastic heterogeneity and anisotropy, consistent with a dynamic environment having chemical anomalies, phase changes, and partially molten material. The implications for deep-mantle plume genesis are discussed in the light of the seismological findings. Nearly antipodal large low-shear velocity provinces (LLSVPs) in the lowermost mantle beneath the Pacific Ocean and Africa are circumscribed by high-velocity regions that tend to underlie upper-mantle downwellings. The LLSVPs have sharp boundaries, low V S /V P ratios, and high densities; thus, they appear to be chemically distinct structures. Elevated temperature in LLSVPs may result in partial melting, possibly accounting for the presence of ultra-low-velocity zones detected at the base of some regions of LLSVPs. Patterns in deep-mantle fast shear wave polarization directions within the LLSVP beneath the Pacific are consistent with strong lateral gradients in the flow direction. The thermal boundary layer at the base of the mantle is a likely location for thermal instabilities that form plumes, but geodynamical studies show that the distribution of upwellings is affected when piles of dense chemical heterogeneities are present. The location of lowermost mantle plume upwellings is predicted to be near the boundaries of the large thermochemical complexes comprising LLSVPs. These observations suggest that any large mantle plumes rising from the deep mantle that reach the surface are likely to be preferentially generated in regions of distinct mantle chemistry, with nonuniform spatial distribution. This hypothesis plausibly accounts for some attributes of major hotspot volcanism.
Geophysical Journal International, 2004
We have developed a channel flow model that dynamically couples plate motion and mantle stress with a composite rheology (diffusion creep and dislocation creep) to study the rheological and anisotropic structures of the oceanic upper mantle. A semi-analytic approach is used to solve for mantle stress and viscosity, allowing fast calculations and exploration of a wide range of rheological parameters. Mantle stress in our model is due to shearing by a moving plate. By comparing mantle stress with a transition stress for dislocation creep, we identify regions where either diffusion creep or dislocation creep is active. Deformation by dislocation creep results in a mineral fabric that may be responsible for observed seismic anisotropy. Our study suggests that there is an important relation between plate motion, seismic anisotropy, mantle viscosity and transition stress. Using laboratory results for rheological parameters, we find that dislocation creep exists only in a layer at certain depths in the upper mantle. For a plate velocity of 10 cm yr-1, an asthenospheric viscosity of 1019 Pa s and an asthenospheric transition stress of 0.1 MPa, our model predicts a ~200 km thick dislocation creep layer, which is broadly consistent with the observations of seismic anisotropy. For a plate velocity of 10 cm yr-1 and an asthenospheric transition stress of 0.1 MPa, the asthenospheric viscosity needs to be greater than 5×1018 Pa s to produce any dislocation creep deformation, and the asthenospheric viscosity needs to be larger for slower plate motion or larger transition stress. Slower plate motion leads to a thinner dislocation creep layer, which may partially explain the observed asymmetry in anisotropic structure in the East Pacific Rise.
Synthetic seismic signature of thermal mantle plumes
Earth and Planetary Science Letters, 2004
The first seismic images of mantle plumes have been a source of significant debate. To interpret these images, it is useful to have an idea of a plume's expected seismic signature. We determined a set of dynamic thermal whole-mantle plumes, with parameters appropriate for the Earth's mantle and shallow-mantle temperature contrasts compatible with surface observations. We explore the sensitivity of amplitude and width of thermal plume anomalies to model parameters. The conversion of thermal to seismic structure accounts for effects of temperature, pressure, an average mantle composition including phase transitions, and anelasticity. With depth-dependent expansivity and temperatureand depth-dependent viscosity, these relatively weak plumes have lower-mantle diameters of 300^600 km at one half of the maximum temperature anomaly. To attain the narrow upper-mantle plumes inferred from surface observations and tomography, viscosity reduction by a factor 30^100 is necessary, either as a jump or as a strong gradient. All model plumes had buoyancy fluxes v 4 Mg/s and it seems difficult to generate whole-mantle thermal plumes with fluxes much lower. Due to changing seismic sensitivity to temperature with depth and mineralogy, variations in the plumes' seismic amplitude and width do not coincide with those in their thermal structure. Velocity anomalies of 24 % are predicted in the uppermost mantle. Reduced sensitivity in the transition zone as well as complex velocity anomalies due to phase boundary topography may hamper imaging continuous whole-mantle plumes. In the lower mantle, our plumes have seismic amplitudes of only 0.5^1%. Unlike seismic velocities, anelasticity reflects thermal structure closely, and yields plume anomalies of 50^100% in dln(1/Q S ). ß
Viscoelastic Mantle Density Heterogeneity and Core-Mantle Topograph
Geophysical Journal International, 1996
Starting with the model of mantle density heterogeneity derived by , which uses plate-motion reconstructions under the assumption that subducted slabs sink vertically into the mantle, we model the temporal evolution of these internal loads. We investigate the viscoelastic deformation of the Earth due to this time-dependent excitation source using a Love-number formalism for a linear viscoelastic mantle with a Maxwell model of rheology. We are especially interested in the deep-Earth viscoelastic deformation, such as the radial displacement that appears at the core-mantle boundary, and we compare the calculated topography with that proposed by seismologists.
Geophysical Journal International, 1998
Because of their slow relative motion, hotspots, mainly in the Paci¢c, are often used as a reference frame for de¢ning plate motions. A coherent motion of all Paci¢c hotspots relative to the deep mantle may, however, bias the hotspot reference frame. Numerical results on the advection of plumes, which are thought to cause the hotspots on the Earth's surface, in a large-scale mantle £ow ¢eld are therefore presented. Bringing the results into agreement with observations also leads to conclusions regarding the viscosity structure of the Earth's mantle, as well as the sources and distribution of plumes.