The world system of underwater rift-type spreading zones is a long-lived complex evolving system. It includes various provinces differing in age and geodynamics. Igneous activity, which accompanied formation of the oceanic lithosphere, can be used as an indicator of processes that controlled specific characters of different regions. The extreme western termination (Spiess Ridge) of the extensive South-West Indian Ridge in the Atlantic Ocean is referred to the African-Antarctic Ridge (called in the Russian literature) of relatively young age. Proceeding from results of magnetic surveys, the onset of spreading was dated less than 2 Ma. Of particular interest in this connection was to study conditions of formation and character of igneous activity under the newly formed and developing spreading zone. The Spiess Ridge was formed under specific conditions, where the new rift zone originated in the oceanic crust that had been formed earlier in the MAR region. Complex dynamics of the Spiess Ridge area evolution resulted in formation of a new central-type volcanic rise with relative height of about 2 km and minimal depth of 320 m, and a caldera up to 4 km in diameter. The eastern and western slopes of the Spiess Ridge are marked by a series of small secondary volcanic cones. The preliminary study of igneous rocks from this ridge ranked them as tholeiites enriched in lithophile elements. The rise of an enriched plume under the area of the Bouvet Island was a source of this enrichment. The aim of this study was to reconstruct conditions of magma formation under the ridge, to verify potential existence of an intermediate-depth magma chamber, and to estimate its potential size. This information is important for deriving a model for evolution of the oceanic lithosphere in the region of the triple junction.

Supplement to: Sushchevskaya, Nadezhda M; Koptev-Dvornikov, E V; Migdisova, N A; Khvorov, D M; Peyve, Alexander A; Skolotnev, Sergey G; Belyatsky, Boris V; Kamenetsky, V S (1999): Features of crystallization process and of geochemistry of tholeiite magma in the western end of the African-Antarctic Ridge (Spiess Ridge) in the area of the Bouvet triple junction. Russian Journal of Earth Sciences, 1(3), 221-250