The Bouvet triple-junction region (South Atlantic); geodynamic and magmatic systems and mantle structure

Three midocean ridges meet in the Bouvet triple-junction region: Mid-Atlantic, Southwest Indian, and American-Antarctic. The triple-junction region is indicated by the Bouvet hotspot magmatism. Available laboratory modeling data are applied to construct a diagram showing the conduit of a thermochemical mantle plume melting from the core-mantle boundary and erupting onto the surface. Morphobathymetric data for the Bouvet Island region are used to obtain the mass flow rate of magmatic melt for the Bouvet plume. Considering the calculated melt flow rate, the thermal power of the Bouvet plume source is N (sub B) = (1.7-2.0) 10 (super 10) W, and the plume conduit diameter is 9-16 km. Possible evolution of the Bouvet plume is presented on the basis of consideration of its geodynamic regime. The influence of the geodynamic system of asthenospheric convection flows on the ocean floor structure in the Bouvet region is shown. The plume under whose influence Bouvet Island formed is located in the region of the ascending asthenospheric roll flow and locally intensifies it. Transform faults in the Bouvet region were formed under the influence of descending asthenospheric roll flows. The width and the depth of the trough of the Bouvet transform fault are determined by analyzing the flow structure and heat transfer in the asthenosphere in the Bouvet region and with regard to the intensifying effect of the Bouvet plume on the ascending asthenospheric roll flow. The conducted geochemical and thermobarogeochemical studies indicate the decisive role of fluid components in the magmatic systems of the Bouvet hotspot, which are characterized by enrichment in volatiles (H (sub 2) , H (sub 2) O, and CO (sub 2) ) and alkalis (primarily potassium) as well as lithophile rare and rare-earth elements (La, Ce, Th, Nb, and Rb). In view of the seismic tomography results, the features of the mantle structure in the triple-junction region are considered. A high-velocity anomaly is identified along the axial zone of the Bouvet transform fault, and the roots of this anomaly in the upper mantle are traced to a depth of 250 km. A lowvelocity anomaly is revealed under Bouvet Island, which is traced to about 500-km depths.