Arnab Roy , Dip Ghosh, Nibir Mandal

It is a well-accepted hypothesis that deep-mantle primary plumes originate from a buoyant source layer at the core-mantle boundary (CMB), where Rayleigh–Taylor (RT) instabilities play a key role in the plume initiation process. Previous studies have characterized their growth rates mainly in terms of the density, viscosity and layer-thickness ratios between the denser overburden and the source layer. The RT instabilities, however, develop in the presence of global flows in the overlying mantle, which can act as an additional factor in the plume mechanics. Combining 2D computational fluid dynamic (CFD) model simulations and a linear stability analysis, this article explores the influence of a horizontal global mantle flow in the instability dynamics. Both the CFD simulation results and analytical solutions reveal that the global flow is a dampening factor in reducing the instability growth rate. At a threshold value of the normalized global flow velocity, short as well as long wavelength instabilities are completely suppressed, allowing the entire system to advect in the horizontal direction. Using a series of real-scale numerical simulations this article also investigates the growth rate as a function of the density contrast, expressed in Atwood number A_T = (ρ_1-ρ_2)/ (ρ_1+ρ_2), and the viscosity ratio μ^*= μ_1/μ_2, where ρ_1, μ_1 and ρ_2, μ_2 are densities and viscosities of the overburden mantle and source-layer, respectively. It is found that increase in either A_T or μ^* promotes the growth rate of a plume. In addition, the stability analysis predicts a nonlinearly increasing RT instability wavelength with increasing global flow velocity, implying that the resulting plumes widen their spacing preferentially in the flow direction of kinematically active mantle regions. The theory accounts for additional physical parameters: source-layer viscosity and thickness in the analysis of the dominant wavelengths and their corresponding growth rates. The article finally discusses the problem of unusually large inter-hotspot spacing, providing a new conceptual framework for the origin of sporadically distributed hotspots of deep-mantle sources.