The role of viscosity contrast on plume structure in laboratory modeling of mantle convection

•Flow visualization of plume structures at varying viscosity ratios (U).•Mantle convection regimes for both hotspots and plate-subduction are captured.•Planforms reveal cellular structures, line plumes and spherical plumes at different U.•Planform plume spacing increases when U>1 and U<1.•Plume mixing is maximum at U=1, and decreases when U>1 and U<1.

We have conducted laboratory experiments to model important aspects of plumes in mantle convection. We focus on the role of the viscosity ratio U (between the ambient fluid and the plume fluid) in determining the plume structure and dynamics. We build on previous studies to highlight the role of viscosity contrast in determining the morphology of mantle plumes and provide detailed visualizations and quantitative information on the convection phenomenon. In our experiments, we are able to capture geophysical convection regimes relevant to mantle convection both for hot spots (when U>1) and plate-subduction (when U<1) regimes. The planar laser induced fluorescence (PLIF) technique is used for flow visualization and characterizing the plume structures. The convection is driven by compositional buoyancy generated by the perfusion of lighter fluid across a permeable mesh and the viscosity ratio U is systematically varied over a range from 1/300 to 2500. The planform, near the bottom boundary for U=1, exhibits a well-known dendritic line plume structure. As the value of U is increased, a progressive morphological transition is observed from the dendritic-plume structure to discrete spherical plumes, accompanied with thickening of the plumes and an increase in the plume spacing. In the vertical section, mushroom-shaped plume heads at U=1 change into intermittent spherical-blob shaped plumes at high U, resembling mantle plume hot spots in mantle convection. In contrast, for low values of U(1/300), the regime corresponds to subduction of plates in the mantle. In this regime, we observe for the first time that plumes arise from a thick boundary with cellular structure and develop into sheet-plumes. We use experimental data to quantify these morphological changes and mixing dynamics of the plumes at different regimes of U. We also compare our observations on plume spacing with various models reported in the literature by varying the viscosity ratio and the buoyancy flux.

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