Pb-Sr-He isotope and trace element geochemistry of the Cape Verde Archipelago Academic Article uri icon

abstract

  • New lead, strontium and helium isotopic data, together with trace element concentrations, have been determined for basalts from the Cape Verde archipelago (Central Atlantic). Isotopic and chemical variations are observed at the scale of the archipelago and lead to the definition of two distinct groupings, in keeping with earlier studies. The Northern Islands (Santo Antao, Sao Vicente, Sao Nicolau and Sal) present Pb isotopic compositions below the Northern Hemisphere Reference Line (NHRL) (cf. Hart, 1984), unradiogenic Sr and relatively primitive He-4/He-3 ratios. In contrast, the Southern Islands (Fogo and Santiago) display Pb isotopes above the NHRL, moderately radiogenic Sr and MORB-like helium signatures. We propose that the dichotomy between the Northern and Southern Islands results from the presence of three isotopically distinct components in the source of the Cape Verde basalts: (1) recycled similar to1.6-Ga oceanic crust (high Pb-206/Pb-204, low Sr-87/Sr-86 and high He-4/He-3); (2) lower mantle material (high He-3); and (3) subcontinental lithosphere (low Pb-206/Pb-204, high Sr-87/Sr-86 and moderately radiogenie He-4/He-3 ratios). The signature of the Northern Islands reflects mixing between recycled oceanic crust and lower mantle, to which small proportions of entrained depleted material from the local upper mantle are added. Basalts from the Southern Islands, however, require the addition of an enriched component thought to be subcontinental lithospheric material instead of depleted mantle. The subcontinental lithosphere may stem from delamination and subsequent incorporation into the Cape Verde plume, or may be remnant from delamination just before the opening of the Central Atlantic. Basalts from Sao Nicolau reflect the interaction with an additional component, which is identified as oceanic crustal material. Copyright (C) 2003 Elsevier Ltd.

publication date

  • October 2003