Trindade and Martin Vaz Islands, South Atlantic: Isotopic (Sr, Nd, Pb) andtrace element constraints on plume related magmatism

Highly alkaline silica undersaturated lavas erupted at Trindade Island over its 5 Ma geologic history and comprise primitive nephelinites-basanites an d more evolved nepheline-bearing phonolitic rocks. Nephelinites-basanites a nd phonolitic rocks are thought to be genetically related via crystal fract ionation, as indicated by the very limited range in Sr, Nd and Pb isotope r atios, systematically increasing contents of incompatible trace elements fr om primitive to evolved rock types, and similar variation in chemical compo sition of the major phenocryst phases (clinopyroxene, amphibole, feldspar) in all rock types. Tb/Yb ratios of the primitive lavas are high (2.6 4.1) a nd silica contents are low (39.8-42.9 wt.% SiO2), indicating that the melts were generated at deep mantle depths (similar to 150 km), within the garne t iherzolite stability field. Non-radiogenic Sr-87/Sr-86 (0.70377-0.70421) and radiogenic Nd-143/Nd-144 ( 0.512752-0.512837) values show that the Trindade and Martin Vat rocks are d erived from moderately depicted sources relative to bulk-earth. The lavas h ave moderate radiogenic Pb-206/Pb-204 ratios of 19.00-19.33, Pb-207/Pb-204 of 15.56 15.60, and Pb-208/Pb-204 of 38.89-39.34; they plot close to the No rthern Hemisphere Reference Lines (NHRL). The narrow range of Sr, Nd, and P h isotopic compositions in the Trindade and Martin Vaz lavas suggests eithe r that the source region was homogeneous (similar to the common mantle comp onents FOZO and "C"), or that melts from a heterogeneous three-component ma ntle source, involving HIMU, enriched mantle EMI, and depleted N-type MORE, were well mixed before eruption. Late Cretaceous to Present volcanism ranging from interior Brazil towards T rindade is thought to record the passage of the South American plate over t he Trindade mantle plume (e.g., O'Connor and Duncan, 1990). Comparison with published data from other mafic rocks along the suggested plume track show s that Trindade isotopic compositions match those of transitional basalts f rom the Late Cretaceous Poxorcu igneous province (inland Brazil) and the Eo cene Abrolhos platform (Brazilian Atlantic margin). Simple batch-melting co nsiderations indicate that the degree of melting has decreased from Poxoreu through Abrolhos to Trindade. These estimates might suggest a diminishing thermal input of the Trindade mantle plume through time. (C) 2000 Elsevier Science Ltd. All rights reserved.