论文部分内容阅读
Small-scale heterogeneity in the deep mantle is concentrated in the upper-mantle transi-tion zone(TZ),in the depth range 410-660 km and also at the bottom 250 km D“ region.This encour-ages a more detailed investigation of the potential for seismic reflectivity imaging by modelling hetero-geneous structures in mantle convection models including phase transitions of the TZ and D” regions.We applied finite elements with variable spacing near the boundary layers in 2-D cylindrical geometry that allow for sufficient spatial resolution.We investigated several models including an extended Bous-sinesq(EBA) model,focused on the D“ region,and a compressible(ALA) model for the TZ region.The results for the D” region show typical lens-shaped structures of post-perovskite(PPV) embedded in the perovskite(PV) background mantle,where the thickness of the lenses,at 200-400 km,strongly depends on the Clapeyron slope of the PV-PPV transition.A second phase transition(double crossing) occurs in case the core temperature is higher than the in-tercept temperature Ti.Our phase-dependent rheology results in contrasting effective viscosity between PV and PPV.Our model results reveal a distinctly clear mechanical weakening of the PPV lenses with about an order of magnitude lower viscosity.The shear wave-speed distribu-tions computed from our convection results are strongly correlated with the heterogeneous dis-tribution of the mineral phase.Gradients in the seismic wave-speed that are the target of seis-mological reflectivity imaging are clearly revealed.The wave-speed results show a clear resolution of the top and bottom interfaces of the PPV lenses.Our ALA model for the TZ is based on a thermodynamical model for the magnesium end-member of an olivine-pyroxene mantle.The model predicts a much more complex distribution of min-eral phases,compared to our D“ results,in agreement with the greater number of mineral phases in-volved in the olivine-pyroxene phase diagram for the P,T conditions of the transition zone.Near cold downwelling flows representing subducting lithospheric slabs,where the local geotherm can differ by up to 1 000 K from the horizontal average,and small-scale lateral variations in the mineral phases can occur.
Small-scale heterogeneity in the deep mantle is concentrated in the upper-mantle transi- tion zone (TZ), in the depth range 410-660 km and also at the bottom 250 km D ”region.This enc-ages a more detailed investigation of the potential for seismic reflectivity imaging by modeling hetero-geneous structures in mantle convection models including phase transitions of the TZ and D “regions. We applied finite elements with variable spacing near the boundary layers in 2-D cylindrical geometry that allow for sufficient spatial resolution. We investigate several models including an extended Bous-sinesq (EBA) model, focused on the D ”region, and a compressible (ALA) model for the TZ region. The results for the D “ region show typical lens -shaped structures of post-perovskite (PPV) embedded in the perovskite (PV) background mantle, where the thickness of the lenses at at 200-400 km, strongly depends on the Clapeyron slope of the PV-PPV transition. A second phase transition (double crossing) occurs in case t Our core temperature is higher than the in-tercept temperature Ti. Our phase-dependent rheology results in contrasting effective viscosity between PV and PPV. Our model results reveal a distinctly clear mechanical weakening of the PPV lenses with about an order of magnitude lower viscosity. The shear wave-speed distribu-tions computed from our convection results are strongly correlated with the heterogeneous dis-tribution of the mineral phase. Gradients in the seismic wave-speed that are the target of seis-mological reflectivity imaging are clearly revealed. Wave -speed results show a clear resolution of the top and bottom interfaces of the PPV lenses. Our ALA model for the TZ is based on a thermodynamical model for the magnesium end-member of an olivine-pyroxene mantle. The model predicts a much more complex distribution of min-eral phases, compared to our D ”results, in agreement with the greater number of mineral phases in-volved in the olivine-pyroxene phase diagram for the P, T conditions of the transition zone. Near cold downwelling flows representing subducting lithospheric slabs, where the local geotherm can differ from up to 1 000 K from the horizontal average, and small-scale lateral variations in the mineral phases can occur.