The geochemistry of fluid processes in the eastern branch of the East African Rift System.
The East African Rift System is the world's major continental rift. While much geophysical
and petrological attention has been paid to it, at least in the eastern branch ("Eastern Rift"),
comparatively little research has been carried out into the geochemistry of rift fluids, despite
the potential benefits to theoretical and practical studies. Hydrothermal activity provides the
opportunity to sample a very wide range of waters and gases in the Eastern Rift. This thesis
combines data from the literature with the results of the author's major investigation into
Eastern Rift fluids carried out chiefly in Kenya, though with subsidiary investigations in
Ethiopia and Djibouti. A synthetic approach has been adopted, whereby relationships and
interactions are cross-referenced as far as possible to illuminate problems, thus demonstrating
the usefulness of considering a wide range of fluids in the rift context.
After reviews of physiography, geology, geothermics and hydrology of the Eastern Rift, the
following topics are considered in detail: (i) chemical and isotopic evidence for water origin,
movement and evolution, (ii) chemical and isotopic evidence for gas origin, movement and
evolution, and (iii) fluid geothermometry.
Within this context some specific aims have been pursued. The strong possibility of longdistance
axial flows of groundwater has been shown by use of stable isotopic techniques.
Further insights into alkaline hydrological discharge areas have been obtained by a combination
of chemical and isotopic approaches. While carbon isotope ratios indicate an apparently
homogeneous source of carbon dioxide, the existence of distinct mantle sources for volcanic
rocks beneath different parts of the Eastern Rift has been comprehensively demonstrated by use
of helium isotope ratios. Following the rather unsatisfactory performance of various gas
geothermometers for evaluating geothermal system temperatures, a new relationship based on
hydrocarbon breakdown has been developed. The possible use of oxygen isotopes in
hydrothermal sinters as indicators of palaeoconditions has been investigated.
In conclusion, topics worthy of further research in the Eastern Rift are suggested where the
study of fluids would be of benefit.