Symmetrical Helium isotope distribution on the Cameroon Volcanic Line, West Africa Festus T. Aka a, * , Keisuke Nagao b , Minoru Kusakabe c , Hirochika Sumino b , Greg Tanyileke d , Bekoa Ateba a , Joseph Hell d a Institute of Mining and Geological Research (IRGM), Center for Volcanological and Geophysical Research (ARGV), P.O. Box 370 Buea, Cameroon b Laboratory for Earthquake Chemistry, Graduate School of Science, University of Tokyo, Tokyo 113-003, Japan c Institute for Study of the Earth’s Interior, Okayama University at Misasa Tottori-ken 682-0193, Japan d Institute of Mining and Geological Research (IRGM), P.O. Box 4110 Yaounde, Cameroon Received 17 February 2003; accepted 2 October 2003 Abstract We report 38 new 3 He/ 4 He 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 f 6 Ma to the present. This constitutes yet the largest helium data set for the 1600-km long mainly alkaline volcanic chain. 3 He/ 4 He ratios are all lower than expected for ‘high 3 He/ 4 He’ 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 3 He/ 4 He ratios that decrease from MORB-like values in the oceanic (8.31R a , where R a is the atmospheric ratio of 1.4  10  6 ) and continental (7.85R a ) terminals of the line to high-A OIB-like values (4-6R a ) 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-A 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 238 U/ 3 He 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 ( f 130 – 128 Ma ago). Areas of the mantle like the ocean/continent boundary zone that were most affected by the contamination had their 3 He/ 4 He ratios reduced by radiogenic 4 He in-growth while those that were least affected like the ones now generating melts for the more distant volcanoes still inherit their initial 3 He/ 4 He characteristics. Such a model is consistent with geologic evidence and also with the co-variation of 3 He/ 4 He with lithophile element ( 206,208 Pb/ 204 Pb) isotopic ratios. The results suggest that the whole Cameroon Line mantle may not be as homogenous in its 3 He/ 4 He ratios as was previously thought. D 2003 Elsevier B.V. All rights reserved. Keywords: Cameroon Volcanic Line; 3 He/ 4 He ratios; MORB source isolation; High-A; Primitive plume-derived melts; Metasomatism 0009-2541/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2003.10.003 * Corresponding author. E-mail address: akatongwa@yahoo.com (F.T. Aka). www.elsevier.com/locate/chemgeo Chemical Geology 203 (2004) 205 – 223