The origin of hydrothermal and other gases in the Kenya Rift Valley

• Published in: Geochimica et cosmochimica acta 1995-06, Vol.59 (12), p.2501-2512
• Main Authors: Darling, W.G., Griesshaber, E., Andrews, J.N., Armannsson, H., O'Nions, R.K.
• Format: Article
• Language: English
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/0016-7037(95)00145-X

The Kenya Rift Valley (KRV) is part of a major continental rift system from which much outgassing is presently occurring. Previous research on gases in the KRV has tended to concentrate on their geothermal implications; the present paper is an attempt to broaden the interpretation by consideration of new data including helium and carbon isotope analyses from a wide cross-section of sites. In order to do this, gases have been divided into categories dependent on origin. N 2 and noble gases are for the most part atmospherically derived, although their relative concentrations may be altered from ASW ratios by various physical processes. Reduced carbon (CH 4 and homologues) appears to be exclusively derived from the shallow crust, with thermogenic δ 13C values averaging −25‰ PDB for CH 4. H 2 is likely also to be crustally formed. CO 2, generally a dominant constituent, has a narrow δ 13C range averaging −3.7‰ PDB, and is likely to be derived with little modification from the upper mantle. Consideration of the ratio C/ 3He supports this view in most cases. Sulphur probably also originates there. Ratios of 3He/ 4He reach a MORB-like maximum of 8.0 R/R A and provide the best indication of an upper mantle source of gases beneath the KRV. A correlation between 3He/ 4He and the hydrocarbon parameter log (C 1/ΣC 2–4) appears to be primarily temperature related. The highest 3He/ 4He ratios in spring waters are associated with basalts, perhaps because of the leaching of basalt glasses. There may be a structural control on 3He/ 4He ratios in the KRV as a whole.