Abstract

The South Atlantic Ocean islands of Ascension, Bouvet, St. Helena, Gough, and Tristan da Cunha display considerable inter-island (and to a variable extent intra-island) heterogeneity in ratios of highly incompatible trace elements. Basaltic and hawaiitic lavas from Ascension, Bouvet, and St. Helena have consistent trace-lenient ratios (e.g., La/Nb, Ba/Nb, Ba/La, Ba/Th, Rb/Th). In contrast, Tristan da Cunha and Gough (and Walvis Ridge) lavas are depleted in Nb and enriched in Ba relative to other highly incompatible trace elements as compared to the other islands. The trace-element and Pb isotopic geochemistry of these lavas is explicable by contamination of the ocean-island basalt source that gave rise to Ascension, Bouvet, and St. Helena lavas by variable, but small (about 1%), amounts of ancient (1.5–2.0 Ga) pelagic sediment.

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