EXPERIMENTAL AND THEORETICAL MODELING OF DIAMONDIFEROUS PLUMES

Abstract

We consider thermochemical mantle plumes with thermal power 1.6·1010 W<N<2.7·1010 W (relative thermal power 1.15<Ka<1.9) as plumes with an intermediate thermal power. Such plumes are formed at the core–mantle boundary beneath cratons in the absence of horizontal free‐convection mantle flows beneath them, or in the presence of weak horizontal mantle flows. A proposed scheme of convection flows in the conduit of a plume with an intermediate thermal power is based on laboratory and theoretical modeling data. A plume ascends (melts out) from the coremantle boundary to critical depth xкр from which magma erupts on the Earth’s surface. The magmatic melt erupts from the plume conduit onto the surface through the eruption conduit. The latter forms under the effect of superlithostatic pressure on the plume roof. While the thickness of the block above the plume roof decreases to a critical value xкр, the shear stress on its cylindrical surface reaches a critical value (strength limit) τкр.Rock fails in the vicinity of the cylindrical block and, as a consequence, the eruption conduit is formed. We estimate the height of the eruption conduit and the time for the plume to ascent to the critical depth xкр. The volume of erupted melt is estimated for kinematic viscosity of melt =0.5–2 м2/с. The depth Δx from which the melt is transported to the surface is determined. Using the eruption volume, we obtain a relationship between the depth Δx and the plume conduit diameter for the above‐mentioned kinematic viscosities. In the case that the depth Δx is larger than 150 km, the melt from the plume conduit can transport diamonds to the Earth’s surface. Thus, the plumes with an intermediate thermal power are diamondiferous. The melt flow structure at the plume conduit/eruption conduit interface is determined on the basis of the laboratory modeling data. The photographs of the simulated flow were obtained. The flow line velocities were measured in the main cylindrical conduit (plume conduit) and at the main conduit/eruption conduit interface. A stagnant area is detected in the 'conduit wall/plume roof’ interface zone. The melt flow in the eruption conduit was analyzed as a turbulent flow in the straight cylindrical conduit with diameter dк. According to the experimental modeling and theoretical data, the superlithostatic pressure in the plume conduit is the sum of the frictional pressure drop and the increasing dynamic pressure in the eruption conduit. A relationship between the melt flow velocity in the eruption conduit and superlithostatic pressure has been derived.