Noble gas isotopic compositions and water/gas chemistry of soda springs from the islands of Bioko, Sao Tome and Annobon, along with Cameroon Volcanic Line, West Africa

Chemical and isotopic compositions are reported for water, and CO2 and noble gases in groundwater and soda springs from Bioko. Principe, Sao Tome and Annobon, all islands located in the off-shore part of the Cameroon Volcanic Line in West Africa. The soda spring waters are of Ca-Mg-HCO3 type, with deltaD and delta O-18 values that range from -20 to -8% and -5.4 to -2.7% respectively, indicative of a meteoric origin. CO2 is the main gas species in the springs. delta C-13-CO2 values vary from -2.8 to -5.0%, overlapping the observed mantle C range (-3 to -8%). CO2/He-3 ratios (3-9 x 10(9)) suggest that most C (similar to 90%) in the samples is derived from the mantle. Neon has atmospheric isotopic compositions, while Ar is slightly enriched in radiogenic Ar-40. He-3/He-4 ratios (3.0 to 10.1 x 10(-6) or 2.1 to 7,2R(a), where R-a is the atmospheric ratio of 1.4 x 10(-6)) are much higher than those for typical crustal fluids (similar to 10(-8)) but lower than those expected for fluids derived from 'high-He-3/He-4' hotspots like Loihi and Iceland. This precludes significant contributions of such fluids in the source regions of the gases, and by inference, in the magmatism of these oceanic islands. Alternatively, approximately 90% of the He in Sao Tome gases is inferred to be derived from a source similar to the MORB source. The He-3/He-4 ratio for the Bioko gas (6.6 x 10(-6)) may be derived from a source with a higher time integrated (U + Th)/He-3 ratio than the MORB source. (C) 2000 Elsevier Science Ltd. All rights reserved.