Slabs in the lower mantle and their modulation of plume formation

Abstract

[1] Numerical mantle convection models indicate that subducting slabs can reach the core-mantle boundary (CMB) for a wide range of assumed material properties and plate tectonic histories. An increase in lower mantle viscosity, a phase transition at 660 km depth, depth-dependent thermal expansivity, and depth-dependent thermal diffusivity do not preclude model slabs from reaching the CMB. We find that ancient slabs could be associated with lateral temperature anomalies $500°C cooler than ambient mantle. Plausible increases of thermal conductivity with depth will not cause slabs to diffuse away. Regional spherical models with actual plate evolutionary models show that slabs are unlikely to be continuous from the upper mantle to the CMB, even for radially simple mantle structures. The observation from tomography showing only a few continuous slab-like features from the surface to the CMB may be a result of complex plate kinematics, not mantle layering. There are important consequences of deeply penetrating slabs. Our models show that plumes preferentially develop on the edge of slabs. In areas on the CMB free of slabs, plume formation and eruption are expected to be frequent while the basal thermal boundary layer would be thin. However, in areas beneath slabs, the basal thermal boundary layer would be thicker and plume formation infrequent. Beneath slabs, a substantial amount of hot mantle can be trapped over long periods of time, leading to ''mega-plume'' formation. We predict that patches of low seismic velocity may be found beneath large-scale high seismic velocity structures at the core-mantle boundary. We find that the location, buoyancy, and geochemistry of mega-plumes will differ from those plumes forming at the edge of slabs. Various geophysical and geochemical implications of this finding are discussed.

Development of finite strain in the convecting lower mantle and its implications for seismic anisotropy

“…Perhaps some type of SPO such as oriented melt inclusions is required to explain the complex anisotropy observed in the central Pacific. Another explanation is based on the hypothesis that slabs induce upwellings at the base of the mantle [Tan et al, 2002]. Because of the sluggish decay of preexisting fabric the laterally oriented strain may turn to vertical in the upwellings.…”

The odyssey of the Cache Creek terrane, Canadian Cordillera: Implications for accretionary orogens, tectonic setting of Panthalassa, the Pacific superwell, and break-up of Pangea

“…The heat transferred to the lowermost mantle is then trapped beneath the cold slabs which, because of their high viscosity must be conductively heated. The consequence is a long term build-up of heat beneath the slabs that eventually gives rise to a superplume event [68]. If all the oceanic lithosphere subducted during the transit of the Cache Creek seamounts sank to the Core Mantle Boundary, the result would have been the development, between 280 Ma and 150 Ma, of a substantial slab blanket on the sub-Pacific portion of the core (Fig.…”

“…7). Thermal modeling of subducted slabs [68] suggests that they can penetrate intact to the core-mantle boundary where they would give rise to "normal" hotspots along the edges of the slabs. Blanketing of the core-mantle boundary by subducted slabs is interpreted to result in an increased temperature gradient at the core-mantle boundary, resulting in increased heat transfer from the core to the mantle.…”

On the location of plumes and lateral movement of thermochemical structures with high bulk modulus in the 3-D compressible mantle

“…In isochemical models, plume roots commonly locate near the edges of slabs [Lenardic and Kaula, 1994;Lowman and Jarvis, 1996;Tan et al, 2002], which have strong lateral temperature gradients at the CMB. However, in thermochemical models, plumes are more likely to root on crests of the chemical anomalies McNamara and Zhong, 2005;Deschamps and Tackley, 2009].…”