Seismic structure of the deep mantle arising from thermal, chemical and phase variations in spherical convection simulations with self-consistent mineral physics
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Our modeling work in spherical geometry shows that a wide variety of upwelling behaviour can be produced in models that have NO compositional variations and are only driven by thermal anomalies. The critical component of this family of models is a high, Earth-like Rayleigh number. Our models have also reproduced time varying magma production on a long time-scale in thermal convection models. The critical element is again a very high Rayleigh number, but this time combined with a realistic Clapeyron slope at the 660km discontinuity. Schuberth et al., 2009, have also shown that composition is not required to explain some of the seismic signatures of mantle convection models either, including at the base of the mantle.
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3-dimensional spherical geometry models at Earth-like Rayleigh numbers suggest that the transition from layered mantle convection to whole mantle convection is likely to result in an extended period of partial layering. During partial layering, the modelled mantle undergoes periodic avalanche and plume events, which result in significant peaks in global surface heat flux. Such processes have been suggested for a partially layered mantle using parameterized models (Davies 1995) and inferred from observational work (Condie 1998).
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Phase assemblages of mantle rocks calculated from the ratios of five oxides (CaO‐FeO‐MgO‐Al 2 O 3 ‐SiO 2 ) by free energy minimization were used to calculate the material properties density, thermal expansivity, specific heat capacity, and seismic velocity as a function of temperature, pressure, and composition, which were incorporated into a numerical thermochemical mantle convection model in a 3‐D spherical shell. The advantage of using such an approach is that thermodynamic parameters are included implicitly and self‐consistently, obviating the need for ad hoc parameterizations of phase transitions which can be complex in regions such as the transition zone particularly if compositional variations are taken into account. Convective planforms for isochemical and thermochemical cases are, however, not much different from those computed using our previous, simple parameterized reference state, which means that our previous results are robust in this respect. The spectrum and amplitude of seismic velocity anomalies obtained using the self‐consistently calculated material properties are more “realistic” than those obtained when seismic velocity is linearly dependent on temperature and composition because elastic properties are dependent on phase relationship of mantle minerals, in other words, pressure and temperature. In all cases, the spectra are dominated by long wavelengths (spherical harmonic degree 1 to 2), similar or even longer wavelength than seismic tomographic models of Earth, which is probably due to self‐consistent plate tectonics and depth‐dependent viscosity. In conclusion, this combined approach of mantle convection and self‐consistently calculated mineral physics is a powerful and useful technique for predicting thermal‐chemical‐phase structures in Earth's mantle. However, because of uncertainties in various parameters, there are still some shortcomings in the treatment of the postperovskite phase transition. Additionally, transport properties such as thermal conductivity and viscosity are not calculated by this treatment and are thus subject to the usual uncertainties.
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Two‐dimensional, time‐dependent calculations show that the thermal and flow structures can be influenced by the lower‐mantle thermal‐chemical instabilities. The style of the core‐mantle boundary (CMB) deformation in thermal‐chemical convection is different from that obtained in thermal convection. The amplitude of CMB hills is reduced greatly from the depth‐dependence of thermal expansivity, found in recent high‐pressure experiments.
Core–mantle boundary
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In this paper we elaborate on suggestions in the recent literature that the mantle convects uniformly through its entire depth. The main novel feature introduced here, which leads to a satisfactory account of earth temperatures, it the assumption of a thermal boundary layer at the mantle. Such a layer is produced by the contrast of thermal conductivities of core and mantle if, as is now generally believed, the effects of radiative heat transfer in the mantle are small. On assuming in agreement with much current geochemical thinking that the core is mainly a solution of FeS in Fe, it is possible to estimate crudely the temperature of the core‐mantle boundary as 4000±500°K. The temperature curve in the mantle can be approximated by adding two boundary layer temperature drops that can be calculated to the adiabat in between; this is also calculable, and the sum of these three agrees roughly with the numerical value just given. It has long been known that the Rayleigh number of the mantle is so large as to make convection likely. Lately, Golitsyn has introduced a scaling analysis that allows one to express the depth of the convective zone as a function of known parameters; this yields a depth in rough agreement with the depth of the entire mantle. Finally, we discuss the likelihood that mantle convection has been going on through the entire life of the earth, beginning with the early formation of the core; this has obvious geological implications.
Core–mantle boundary
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Post-perovskite
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Mantle flow induces dynamic topography at the core–mantle boundary (CMB), with distribution and amplitude that depend on details of the flow. To assess whether observations of CMB topography can give constraints on deep mantle structure, we determine CMB dynamic topography associated with different models of mantle convection, including thermochemical and purely thermal models. We investigate the influence of key controlling parameters, specifically the thermal viscosity ratio (ΔηT) and, for thermochemical models, the density contrast (ΔρC) and viscosity ratio (ΔηC) between primordial and regular materials. In purely thermal models, plume clusters induce positive topography with an amplitude that decreases with increasing ΔηT. In thermochemical models with moderate density contrasts, around 100–200 kg m−3, reservoirs of dense material induce depressions in CMB topography, surrounded by a ridge of positive topography. The average depression depth and ridge height increase with increasing ΔρC and ΔηC, but decrease with increasing ΔηT. We find that for purely thermal models or thermochemical models with ΔρC ∼ 90 kg m−3 and less, the long-wavelength (spherical harmonic degrees up to l = 4) dynamic topography and shear wave velocity anomalies predicted by thermochemical distributions anticorrelate. By contrast, for models with ΔρC ≥ 100 kg m−3 and ΔηC > 1, long-wavelength dynamic topography and shear wave velocity anomalies correlate well. This potentially provides a test to infer the nature, that is, either purely or mostly thermal (ΔρC ≤ 100 kg m−3 m−3) or strongly thermochemical (ΔρC ≥ 100 kg m−3), of the low shear wave velocity provinces observed by global tomographic images. The presence of post-perovskite, provided that its viscosity is similar to that of bridgmanite, does not alter these conclusions.
Core–mantle boundary
Mantle plume
Ocean surface topography
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