Helium Isotope Evidence for the Mantle Connection of the Southwestern Rift; a Developing Plate Boundary in Africa.

Abstract:

The southwestern extension of the African Rift System has been considered a broad zone of distributed deformation characterised by weak and infrequent seismicity, a lack of volcanoes and no significant Quaternary sedimentation. However, recent multi-disciplinary tectonic analysis has argued that, within the widespread distributed deformation, there is a localised, active, lithosphere scale rift that breaks the Nubian Plate. The isotopic composition of helium from hot springs in the centre of this rift system adds new and compelling data to this hypothesis. Gas samples were collected from the Bwenga Hot Springs on the southern, faulted margin of the Kafue Rift Basin of Zambia, an integral component of the Southwestern Rift. The noble gas concentration and isotopic composition was determined at Oxford University’s Department of Earth Sciences. Samples from distinct but adjacent surface vents recorded 1.5% and 2.3% helium gas with ³He/⁴He ratios of 0.15 R/R_A (relative to the value of air, R_A = 1.4 x 10^-6). The ³He/⁴He ratio lies between the 0.02 R/R_A expected of radiogenic He produced by U and Th decay in Proterozoic and Archean aged crystalline rocks, and the 6-8 R/R_A characteristic of the upper mantle. The observed ⁴He/²⁰Ne ratios (2261 – 3229) are high relative to air (0.29) and indicate that any admixtures of air helium are negligible. We therefore interpret the intermediate ³He/⁴He ratios as a result of mixing between crustal and mantle derived helium. The crustal derived gas coming from the Mesoproterozoic basement rocks in the footwall of the southern faulted margin, mixing with mantle derived gas associated with an active fault zone connected to the mantle. These data and their interpretation support the thesis that the Southwestern Rift penetrates at least the African crust of the Nubian Plate and defines a potentially discrete San Plate to the south. The He gas association may also document the very early stage processes of lithospheric heating and weakening associated with the propagation of continental rifting. Similar mantle linked conclusions have been drawn from the Lake Rukwa area of the Western Rift where He gas is present. A regional sampling programme of associated hot springs along the length of the Southwestern Rift will follow up on these early results.