Cosmogenic helium and neon in mantle xenoliths from the Cameroon Volcanic Line (West Africa): Preliminary observations

Step heating and in-vacuo crushing helium and neon data of alkali basalt-hosted ultramafic xenolith whole rock and mineral separates from the Cameroon Volcanic Line centers of Annobon, Bioko, Oku and Ngaoundere are reported for the first time. Step heating data reveals large 3He/4He (up to 118 ± 8Ra, where Ra is the atmospheric ratio of 1.4 × 10−6) and 21Ne/22Ne (up to 0.20 ± 0.03) isotopic anomalies resulting from the presence of cosmic ray-derived 3He (3Hec) and 21Ne (21Nec). Crushing of the same phases releases fluids with morb-like Ne for all the centers considered, but morb-like (Annobon and Ngaoundere) and high-μ-like (Bioko and Oku) 3He/4He ratios. 3Hec and 21Nec make up ∼77% and ∼97% respectively of the 3He and excess 21Ne observed in the xenoliths. With 2.44 ± 0.37, the cosmogenic 3He/21Ne ratio in the Cameroon Line xenolith olivines is similar to that previously observed. Preliminary3Hec and 21Nec production rates were calculated for the xenolith localities and used to estimate the preliminary3Hec and 21Nec xenolith exposure ages. Resulting 3Hec and 21Nec ages are similar and suggest that the Annobon, Oku and Ngaoundere xenoliths erupted between 14 and 195 kyr BP. This is the first cosmonuclide age report of Upper Pleistocene volcanic activity in these centers. The cosmogenic age of the Bioko sample is only ∼25% of its K–Ar age, and suggests (assuming the K–Ar age reflects the true eruption age) erosion of the Bioko lava surface. Rigorous evaluation of the erosion rate and the potentially viable general use of cosmonuclides in rocks of the Cameroon Line for studies of the geomorphic evolution of its volcanic centers need a more cosmogenic nuclide-oriented investigation.