Chemistry of Tholeiites from the Galapagos Islands and Adjacent Ridges

Abstract

Petrological evidence suggesting a mantle plume beneath Iceland has been presented1,2. Schilling1 gives geochemical data which indicate that the incompatible elements TiO2, P2O5, K2O and La decrease regularly southwards from Iceland along the Reykjanes Ridge axis to about 61° N. The rare earth element pattern for Icelandic tholeiites shows a high normalised La/Sm ratio which steadily decreases to less than 1 along the ridge. The crystallisation index (FeO*/ MgO) and the ratio of pyroxene/plagioclase among the phenocrysts also follow this trend. Campsie et al.2 noted that tholeiites all the way from Iceland to the Charlie Gibbs Fracture Zone retain chemical coherence in their major elements (except SiO2) and belong to a single pyroxene-tholeiite magma type. Here we summarise petrological evidence for a Galapagos mantle plume, present new data on basalts from the rift valley of the Galapagos Ridge and discuss the origin and composition of the Cocos and Carnegie Ridges.