Mass transport mechanism between the upper and lower mantle in numerical simulations of thermochemical mantle convection with multicomponent phase changes

Numerical simulations of thermochemical mantle convection are used to investigate the influence of multicomponent phase changes on mass transport between the upper and lower mantle. The model includes both olivine and pyroxene-garnet components based on a pyrolite composition. The simulations reveal large and focused upwellings through the 660-km phase boundary in the vicinity of subducting slabs. The upwellings are composed of depleted harzbergitic material, which contributes to the depletion of the upper mantle and may generate distinct reservoirs of trace elements in the upper and lower mantle as well. The position and composition of the upwelling is attributed to the development of compositional stratification at the top of the lower mantle, where dense harzbergitic material underlies enriched basaltic material due to a density inversion in the combine phase system. Subducting slabs disrupt the stratification in the vicinity of subduction, permitting a counter flow of harzbergitic material into the upper mantle. This behavior is not observed in a pure olivine phase system, suggesting that realistic mantle compositions are needed to assess the generation of distinct reservoirs in mantle convection simulations. Phase transitions in a multicomponent system provide a plausible mechanism for explaining the formation of a depleted reservoir in the upper mantle without implementing dehydration-induced melting and differentiation in the transition zone.