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

Abstract

Chemical and isotopic compositions are reported for water, and CO 2 and noble gases in groundwater and soda springs from Bioko, Principé, São Tomé 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–HCO 3 type, with δD and δ 18O values that range from −20 to −8‰ and −5.4 to −2.7‰ respectively, indicative of a meteoric origin. CO 2 is the main gas species in the springs. δ 13C–CO 2 values vary from −2.8 to −5.0‰, overlapping the observed mantle C range (−3 to −8‰). CO 2/ 3He ratios (3–9×10 9) suggest that most C (∼90%) in the samples is derived from the mantle. Neon has atmospheric isotopic compositions, while Ar is slightly enriched in radiogenic 40Ar. 3He/ 4He ratios (3.0 to 10.1×10 −6 or 2.1 to 7.2 R a , where R a is the atmospheric ratio of 1.4×10 −6) are much higher than those for typical crustal fluids (∼10 −8) but lower than those expected for fluids derived from ‘high- 3He/ 4He’ 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 São Tomé gases is inferred to be derived from a source similar to the MORB source. The 3He/ 4He ratio for the Bioko gas (6.6×10 −6) may be derived from a source with a higher time integrated (U+Th)/ 3He ratio than the MORB source.