FEATURES OF GEODYNAMIC AND MAGMATIC SYSTEMS AND MANTLE STRUCTURE IN THE AREA OF THE BOUVET TRIPLE JUNCTION (SOUTH ATLANTIC)
A.A. Kirdyashkin1, V.A. Simonov1, A.V. Kotlyarov1, A.V. Jakovlev2
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
2Alfred-Wegener-Institut, Helmholtz Zentrum für Polar- und Meeresforschung, Bremerhaven, Germany
Keywords: Mantle plume, hotspot, asthenospheric free-convective flows, triple junction, mid-oceanic ridge, transform fault, content of rare and rare earth elements, volatile components, melt inclusions, P-wave velocity anomalies
Abstract
Three mid-oceanic ridges meet in the area of the Bouvet triple junction the Mid-Atlantic, Southwest Indian and American-Antarctic one. At the same time, the triple junction area is characterized by manifestations of magmatism of the Bouvet hotspot. Using available laboratory modeling data, a scheme is presented for the conduit of a thermochemical mantle plume melting from the core-mantle boundary and breaking through to the surface. Using morphobatimetric data for the area of the Bouvet volcanic island, a mass flow rate of magmatic melt for the plume of the Bouvet hotspot was found. Taking into account the melt flow rate, the thermal power of the Bouvet plume source is NB = (1.7–2.0) · 1010 W, and the plume conduit diameter is 9 to16 km. Based on the consideration of the geodynamic regime, the possible evolution of the Bouvet plume is presented. The influence of the geodynamic system of asthenospheric convective flows on the structure of the ocean floor in the Bouvet area is shown. The Bouvet Island was formed under the influence of mantle plume. The Bouvet plume is located in the area of the ascending flow of the asthenospheric rolls and contributes to the roll flow intensification. Transform faults in the Bouvet area were formed under the influence of descending flows of asthenospheric rolls. The width of the trough and the depth of the Bouvet transform fault are determined based on the analysis of the flow structure and heat transfer in the asthenosphere in the Bouvet area. The contribution of the Bouvet plume to the asthenospheric convection roll intensification is also taken into account in our calculations. The conducted geochemical and thermobarogeochemical studies indicate the decisive role of fluid components in magmatic systems of the Bouvet hotspot, which are characterized by enrichment with volatiles (H2, H2O, CO2), alkali (primarily potassium) and lithophilic rare and rare earth elements (La, Ce, Th, Nb, Rb). Taking into account the seismotomographic results, the features of the mantle structure in the triple junction are considered. A high-velocity anomaly is detected along the axial zone of the Bouvet transform fault. The roots of this upper-mantle anomaly can be traced to a depth of 250 km. A low-velocity anomaly has been detected under Bouvet Island. This anomaly can be traced to depths of about 500 km.
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