Symmetrical Helium isotope distribution on the Cameroon Volcanic Line, West Africa

We report 38 new 3He/4He ratios for mafic phases in basalts and ultramafic xenolith whole rock and mineral separates from 10 of the 12 main volcanic centers of the intraplate Cameroon Volcanic Line in West Africa. Samples range in eruption age from ∼6 Ma to the present. This constitutes yet the largest helium data set for the 1600-km long mainly alkaline volcanic chain. 3He/4He ratios are all lower than expected for 'high 3He/4He' hotspots like Loihi and Iceland, demonstrating that none of the volcanic centers is tapping material directly from a mantle source supplying such hotspots. There is a geographic control on the distribution of 3He/4He ratios that decrease from MORB-like values in the oceanic (8.31Ra, where Ra is the atmospheric ratio of 1.4×10-6) and continental (7.85Ra) terminals of the line to high-μ OIB-like values (4-6Ra) in the ocean/continent boundary zone. The variation cannot be due to post-eruptive or magma chamber modification processes and is considered to be an intrinsic characteristic of the Cameroon Volcanic Line system. Previous work has shown that the high-μ helium signature of the Cameroon Line cannot be explained by injection of recycled materials into the mantle. Isolation of a mantle source with MORB-like 238U/3He ratios has the problem that necessary time scales are more than five times higher than required. Alternatively, it is suggested that the mantle underlying the volcanic line spatially acquired high (U+Th)/He ratios from 'spiking or contamination' by primitive plume-derived melts during the Lower Neocomian (∼130-128 Ma ago). Areas of the mantle like the ocean/continent boundary zone that were most affected by the contamination had their 3He/4He ratios reduced by radiogenic 4He in-growth while those that were least affected like the ones now generating melts for the more distant volcanoes still inherit their initial 3He/4He characteristics. Such a model is consistent with geologic evidence and also with the co-variation of 3He/4He with lithophile element (206,208Pb/204Pb) isotopic ratios. The results suggest that the whole Cameroon Line mantle may not be as homogenous in its 3He/4He ratios as was previously thought. © 2003 Elsevier B.V. All rights reserved.