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

Abstract

Helium, argon and lead isotopic ratios have been determined for volcanics from two of the youngest Cape Verde Islands, Santo Antão and Fogo. Helium isotopic ratios range from radiogenic 4He/ 3He 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. 40Ar/ 36Ar isotopic ratios range from slightly higher values than atmospheric up to around 1250. Pb isotopic ratios are relatively radiogenic: 206Pb/ 204Pb=18.90–19.63, 207Pb/ 204Pb=15.526–15.621 and 208Pb/ 204Pb=38.694–39.272, with volcanics from Santo Antão being the most radiogenic. Several factors like high 4He/ 3He ratios, relatively radiogenic Pb isotopes, high Ce/Pb, Zr/Hf, low Ba/La, LILE/Nb and low 40Ar/ 36Ar indicate a significant contribution from a HIMU-type source. A lower mantle contribution is indicated from the low 4He/ 3He ratios in some samples. Moreover, relatively low 40Ar/ 36Ar ratios, low 4He/ 3He ratios, negative Δ7/4 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.