The Southwestern Rift of Africa: isotopic evidence of early-stage continental rifting

Helium and carbon isotope data ( 3 He/ 4 He = 0.14–0.17 R/R a ; δ 13 C(CO 2 ) = −3.9‰) from hydrothermal springs within the Kafue Rift of Zambia provide the first geochemical characterization of thermal springs along a broad extensional zone connecting the African Rift System through central Africa to Namibia. These results reveal mantle-derived fluids at the surface, and associated mobilization of crustal N 2 (84.4%–97.6%) with elevated 4 He concentrations (0.4%–2.3%). Active hydrothermal groundwaters from outside of the Kafue Rift boundary faults show no isotopic evidence of mantle-derived helium or carbon dioxide. These geochemical compositions and spatial trends resemble those observed in other early rifts within the more thermally developed East African Rift System. The data is consistent with early stages of active lithospheric rifting, supported by previous geophysical observations globally. In addition to the regional tectonic importance of these data, these findings highlight the resource potential along central African active fault boundaries. The combination of a mantle fluid source, advective flow along crustal scale fault zones with low level seismicity, and groundwater serving as a sink for mantle CO 2 with minimal crustal fluid dilution, indicate potentially favorable conditions for both geothermal energy development and the exploration of economically significant gases in crustal fluids, particularly helium and hydrogen.