[1] At the Rayleigh number appropriate to Earth's mantle, radial heat transport is dominated by solid state thermal convection. Because of the large number of physical properties required to determine the Rayleigh number, and because these properties are expected to be (perhaps strong) functions of pressure and temperature (P-T), laboratory measurements of them under the high pressure and temperature conditions that occur in the deep Earth are of fundamental importance. Recent experimental data demonstrate that an electronic spin transition in iron that occurs at midmantle depths results in significant changes in the physical properties of the ferropericlase component of mantle mineralogy. Additional recent results suggest that it may also exist in the dominant perovskite component. Using control volume based numerical models we investigate the impacts on mantle mixing of this spin transition through its influence on the most important subset of these physical properties, namely density, thermal expansivity, bulk modulus and heat capacity. Our numerical model results demonstrate that this electronic transition enhances mixing in the lower regions of the lower mantle by enhancing the vigor of rising plumes. The lowermost region of the mantle is slightly warmed and the upper mantle slightly cooled by spin-induced effects. However, the spin crossover in the lower mantle appears not to significantly influence mantle layering. Due to the competition that could exist between the strength of the spin-induced thermodynamic properties of ferropericlase and perovskite, cold descending thermal anomalies could stagnate at middle-to-lower mantle depths and lead to the occurrence of "mid mantle avalanches."

[1] The issue of the style of the mantle convection process remains important to the understanding of Earth's deep interior and surface processes. While results from structural seismology may be interpreted to support the existence of a whole mantle convection regime, in several geographic regions high-resolution reconstructions of Benioff zone body wave heterogeneity demonstrate that the downgoing slab appears to be "trapped" in the transition zone rather than continuing to penetrate unimpeded into the lower mantle. The presence of the recently discovered exothermic perovskite-postperovskite (Pv-pPv) phase transition that appears to define the top of the D″ layer adjacent to the core-mantle boundary may be expected to exert considerable impact on the mixing process. On the basis of the use of the most recent mantle parameters derived on the basis of mineral physics and a viscosity model inferred from glacial isostatic adjustment and Earth rotation observables, we reinvestigate the impact of the Pv-pPv transition on mantle mixing. Our analyses are based on a newly constructed axisymmetric control volume model, which is described in detail. Analyses with this model demonstrate that the action of the Pv-pPv transitions slightly decreases the tendency to layered flow due to the endothermic transitions that occur at 660 km depth and enhances the absolute radial mass flux especially in the lower mantle. However, the results also demonstrate that the episodically layered style of mantle mixing persists in models in which the strength of the Pv-pPv transition is significant.

SUMMARY Recent geodynamic research has shown that convective flow in the mantle may have an important role in the development of long‐wavelength surface topography. This flow‐induced ‘dynamic topography’ is usually derived from mantle convection models by computing the vertical component of hydrodynamic stress at the top of the model and assuming the stress is compensated by deflection of the surface. However, these models have generally ignored the presence of an overlying buoyant crust and its deformational response to the mantle flow. We consider the effects of horizontal convective forcing on the crust and investigate how this crust/lithosphere deformation interacts with the vertical component of mantle flow‐induced subsidence/uplift at the surface. In particular, we test the response of various rheologies of the crust and mantle lithosphere to an episode of mantle downwelling. The evolution of crustal thickness and topography is tracked using thermomechanical numerical models of the crust–mantle system with a free surface upper boundary. A strong crust (ηc= 2.5 × 1025 Pa s) does not experience significant internal deformation and subsides above a mantle downwelling. For a weaker crust (ηc < =1023 Pa s), descending mantle flow initially induces subsidence, but there is an inversion to surface uplift as crustal thickening induced by convergent mantle flow overcomes the dynamic subsidence. The presence of a strong mantle lithosphere, however, may effectively shield a weak crust from deformation imposed by the underlying horizontal mantle convective stresses. With temperature‐dependent rheologies in the model, the interplay of the vertical/horizontal mantle forcings with the thermal evolution of the crust results in a highly time‐dependent signal of topography. Subsequent to crustal thickening and uplift, the system may undergo rapid topographic subsidence and crustal thinning by localized channel flow in the hot and weak lower crust. These results suggest an alternative interpretation for the development of mantle‐flow induced topography in certain tectonic environments.

No abstract