GEODYNAMIC PROCESSES DURING ASCENT OF A PLUME WITH INTERMEDIATE THERMAL POWER THROUGH THE CONTINENTAL LITHOSPHERE AND DURING ITS ERUPTION ON THE SURFACE

Abstract

The study is focused on thermochemical mantle plumes with intermediate thermal power (1.15 < Ka < 1.9). Previously we have shown that these plumes are diamondiferous. Based on the laboratory modeling data, the flow structure of a melt in a plume conduit is represented. A plume melts out and ascends from the core – mantle boundary to the bottom of the continental lithosphere. The plume roof moves upwards in the lithosphere because of melting of the lithospheric matter at the plume roof and due to the effect of superlithostatic pressure on the roof, which causes motion in the lithosphere block above the plume roof. The latter manifests itself by uplifting of the ground surface above the plume. As the plume ascends through the lithosphere, the elevation of the surface increases until the plume ascends to critical level xкр, where an eruption conduit is formed. In our model, plume ascent velocity uпл is the rate of melting at the plume roof. Values of uпл and the ascent velocity of a spherical plume roof due to superlithostatic pressure U are calculated. Relationships are found between these velocities and the plume roof depth. The dependence of the velocity of the surface’s rise on the dynamic viscosity of the lithosphere block above the plume is obtained. A relationship is determined between the maximum surface elevation and the lithosphere viscosity. The elevation values are determined for different times and different lithosphere viscosities.The results of laboratory modeling of flow structure at the plume conduit/eruption conduit interface are presented. The flow was photographed (1) in the plane passing through the axes of the plume conduit and the eruption conduit; and (2) in case of the line-focus beam perpendicular to the axial plane. The photographs were used for measuring the flow velocities in the plume conduit and the eruption conduit. Corresponding Reynolds numbers and flow regimes are determined. The relation of dynamic pressure in the eruption conduit to that in the plume conduit is found for intermediate-power plumes. The melt flow velocity in the eruption conduit depends on superlithostatic pressure on the plume roof, plume diameter and kinematic viscosity of the melt. Its values are determined for different kinematic viscosities of melt.