THE ORIGIN OF HYDROTHERMAL AND OTHER GASES IN THE KENYA RIFT-VALLEY

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 impl ications; the present paper is an attempt to broaden the interpretatio n by consideration of new data including helium and carbon isotope ana lyses from a wide cross-section of sites. In order to do this, gases h ave been divided into categories dependent on origin. N-2 and noble ga ses are for the most part atmospherically derived, although their rela tive concentrations may be altered from ASW ratios by various physical processes. Reduced carbon (CH4 and homologues) appears to be exclusiv ely derived from the shallow crust, with thermogenic delta(13)C values averaging -25 parts per thousand PDB for CH4. H-2 is likely also to b e crustally formed. CO2, generally a dominant constituent, has a narro w delta(13)C range averaging -3.7 parts per thousand PDB, and is likel y to be derived with little modification from the upper mantle. Consid eration of the ratio C/He-3 supports this view in most cases. Sulphur probably also originates there. Ratios of He-3/He-4 reach a MORB-like maximum of 8.0 R/R(A) and provide the best indication of an upper mant le source of gases beneath the KRV. A correlation between He-3/He-4 an d the hydrocarbon parameter log(C-1/Sigma C-2-4,) appears to be primar ily temperature related. The highest He-3/He-4 ratios in spring waters are associated with basalts, perhaps because of the leaching of basal t glasses. There may be a structural control on He-3/He-4 ratios in th e KRV as a whole.