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

Abstract

Highly alkaline silica undersaturated lavas erupted at Trindade Island over its 5 Ma geologic history and comprise primitive nephelinites-basanites and more evolved nepheline-bearing phonolitic rocks. Nephelinites-basanites and phonolitic rocks are thought to be genetically related via crystal fractionation, as indicated by the very limited range in Sr, Nd and Pb isotope ratios, systematically increasing contents of incompatible trace elements from primitive to evolved rock types, and similar variation in chemical composition 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) and silica contents are low (39.8–42.9 wt.% SiO 2), indicating that the melts were generated at deep mantle depths (∼150 km), within the garnet lherzolite stability field. Non-radiogenic 87Sr/ 86Sr (0.70377–0.70421) and radiogenic 143Nd/ 144Nd (0.512752–0.512837) values show that the Trindade and Martı́n Vaz rocks are derived from moderately depleted sources relative to bulk-earth. The lavas have moderate radiogenic 206Pb/ 204Pb ratios of 19.00–19.33, 207Pb/ 204Pb of 15.56–15.60, and 208Pb/ 204Pb of 38.89–39.34; they plot close to the Northern Hemisphere Reference Lines (NHRL). The narrow range of Sr, Nd, and Pb isotopic compositions in the Trindade and Martı́n Vaz lavas suggests either that the source region was homogeneous (similar to the common mantle components FOZO and “C”), or that melts from a heterogeneous three-component mantle source, involving HIMU, enriched mantle EM I, and depleted N-type MORB, were well mixed before eruption. Late Cretaceous to Present volcanism ranging from interior Brazil towards Trindade is thought to record the passage of the South American plate over the Trindade mantle plume (e.g., O’Connor and Duncan, 1990). Comparison with published data from other mafic rocks along the suggested plume track shows that Trindade isotopic compositions match those of transitional basalts from the Late Cretaceous Poxoreu igneous province (inland Brazil) and the Eocene Abrolhos platform (Brazilian Atlantic margin). Simple batch-melting considerations 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.