Darling, W. G.; Griesshaber, Erika; Andrews, J. N.; Armannsson, H.; O’Nions, R. K.
The origin of hydrothermal and other gases in the Kenya Rift Valley.
In: Geochimica et Cosmochimica Acta, Vol. 59, No. 12: pp. 2501-2512
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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. N2 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 (CH4 and homologues) appears to be exclusively derived from the shallow crust, with thermogenic δ13C values averaging -25‰ PDB for CH4. H2 is likely also to be crustally formed. CO2, generally a dominant constituent, has a narrow δ13C 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/RA 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 (C1/ΣC2-4) 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.