RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 3, NO. 1, PAGES 33–57, doi:10.2205/2001ES000052, 2001

Volcanic and geodynamic evolution of the Bouvet triple junction: Evidence from basalt chemistry

Geological Institute (GIN), Russian Academy of Sciences, Moscow, Russia

Abstract

[1]  This study focuses on mafic volcanic rocks from the Bouvet triple junction, which fall into six geochemically distinct groups:
(1) N-MORB, the most widespread type, encountered throughout the study area.
(2) Subalkaline volcanics, hawaiites and mugearites, strongly enriched in lithophile elements and radiogenic isotopes and composing the Bouvet volcanic rise, and compositionally similar basalts and basaltic andesites from the Spiess Ridge, generated in a deeper, fertile mantle region.
(3) Relatively weakly enriched basalts, T-MORB, derived by the mixing of Type 1 and 2 melts and exposed near the axes of the Mid-Atlantic, Southwest Indian, and America-Antarctic ridges.
(4) Basalts with a degree of trace lithophile element enrichment similar to the Spiess Ridge and Bouvet I. rocks, but higher in K, P, Ti, and Cr. These occur within extensional structures: the rift valley of the Southwest Indian Ridge, grabens of the East Dislocation Zone, and the linear rise between the Spiess Ridge and Bouvet volcano. Their parental melts presumably separated from a plume material that spread from the main channels and underwent fluid-involving differentiation in the mantle.
(5) A volcanic suite ranging from basalt to rhyolite, characterized by low concentrations of lithophile elements, particularly TiO₂, and occurring on the Shona Seamount and other compressional features within the Antarctic and South American plates near the Bouvet triple junction. Unlike Types 1 to 4, which display tholeiitic differentiation trends, this suite is calc-alkaline. Its parental melts were presumably related to a plume material as well but, subsequently, they underwent a profound differentiation involving fluids and assimilated surrounding rocks in closed magma chambers in the upper mantle. Alternatively, the Shona Smt. might be a fragment of an ancient oceanic island arc.
(6) Enriched basalts, distinguished from the other enriched rock types in having very high P and radiogenic isotope abundances and composing a tectonic uplift near the junction of the three rifts. It thus follows that the main factors responsible for the compositional diversity of volcanic rocks in this region include (i) mantle source heterogeneity, (ii) plume activity, (iii) an intricate geodynamic setup at the triple junction giving rise to stresses in adjacent plate areas, and (iv) the geological prehistory. The slow spreading rate and ensuing inefficient mixing of the heterogeneous mantle material result in strong spatial variations in basaltic compositions.

Received 19 July 2001; published 31 July 2001.

Keywords: geodynamic evolution, Bouvet triple junction, basalt chemistry, mafic volcanic rocks.

Version of this paper in Russian