Helium, argon and lead isotopic composition of volcanics from Santo Antao and Fogo, Cape Verde Islands

Helium, argon and lead isotopic ratios have been determined for volcanics from two of the youngest Cape Verde Islands, Santo Antao and Fogo, Helium isotopic ratios range from radiogenic He-4/He-3 values of 224,000 (3.2 R/Ra) to more primitive values of 52,000 (13.8 R/Ra), which suggest a contribution from different reservoirs to the magmatism at the Cape Verde Islands. Ar-40/Ar-36 isotopic ratios range from slightly higher values than atmospheric up to around 1250. Pb isotopic ratios are relatively radiogenic: Pb-206/Pb-204 = 18.90-19.63, (207)pb/(204)pb = 15.526-15.621 and Pb-208/Pb-204 = 38.694-39.272, with volcanics from Santo Antao being the most radiogenic. Several factors like high He-4/He-3 ratios, relatively radiogenic Pb isotopes, high Ce/Pb, Zr/Hf, low Ba/La, LILE/Nb and low Ar-40/Ar-36 indicate a significant contribution from a HIMU-type source, A lower mantle contribution is indicated from the low He-4/He-3 ratios in some samples. Moreover, relatively low Ar-40/Ar-36 ratios, low He-4/He-3 ratios, negative Delta7/4 Pb values and HIMU-characteristic trace element ratios preclude the DMM as a significant contributor to the Cape Verde magmatism. The HIMU component is concluded to represent subducted oceanic crust recycled to a boundary layer separating the lower mantle from the upper degassed mantle, from where new plumes originate. The observed isotopic variation can be explained by a lower mantle contribution to the recycled component either through rising of small plumes, entrainment of lower mantle material due to instabilities in the boundary layer or He migration from the lower mantle. (C) 2001 Elsevier Science B.V. All rights reserved.