Thermo-chemical structure in the mantle arising from a three-component convective system and implications for geochemistry

Numerical simulations are used to characterize structures and observational signatures of thermo-chemical convection with three component of differing densities nominally corresponding to regular mantle (the depleted Mid Oceanic Ridge Basalt (MORB) source), primitive mantle and recycled crust. Strong heterogeneity, similar to that observed in seismic tomography models, is generated near the CMB by sweeping of the highest density ‘recycled’ component into piles by convective downwellings. Meanwhile, the medium density ‘primitive’ material is widely dispersed by the convection and forms ‘blobs’. Two types of upwelling plumes are observed: large plumes arising from the top of dense piles and small-scale plumes arising from the base of the transition zone resembling seismic tomography images of upwelling plumes [Zhao, Earth Planet. Sci. Lett. 192 (2001) 251], although the origin of these plumes is unclear. The horizontal spectra of the seismic velocity had anomalies at different depths are most similar to tomographic models when dense basal piles are present. Comparing to geochemically-required residence times, the source of HIMU (this type of rock has high ratio of helium isotope anomaly) Ocean Island Basalt (OIB) is more likely to be dense ‘piles’, while lower mantle blobs have been proposed as a location of primitive material. Thus the combination of blob structures and deep mantle piles may be plausible way of simultaneously satisfying geochemical and seismological constraints on mantle structure.