The Institute of Earth Sciences meeting room
Master's student: Tobias Alexander Soutar
Title: Tracing mantle degassing along the East African Rift using He and C isotopes: A geospatial approach
Faculty: Faculty of Earth Sciences
Advisors: Sæmundur Ari Halldórsson, Research Scholar and Andri Stefánsson, Professor, both at the Faculty of Earth Sciences
Examiner: Halldór Ármannsson, Chief Geochemist at ISOR
Helium (R/Ra) and carbon (δ13C-CO2) isotopes in fluids and lavas were used as tracers to detect mantle-degassing across the East African Rift. From the literature, a database of R/Ra and δ13C-CO2 values was produced with accompanying isotopic and geochemical data. The data was filtered, removing samples of ambiguous origin, and samples heavily altered by secondary processes. The remaining data was interpolated using ‘Natural Neighbour’, creating maps of R/Ra and δ13C-CO2. The helium isotope data reinforces the findings of past studies, though with a more extensive data set. In agreement with those studies, a high (up to 16 R/Ra) helium signature is seen along the Afar and Main Ethiopian Rift, indicative of the underlying plume. DMM (Depleted MORB Mantle)-like helium is seen along the Kenyan and Kivu rifts, due to either diluted plume influence and/or input from DMM or SCLM (Sub-Continental Lithospheric Mantle). Plume-like helium is seen in Rungwe, indicating deep-mantle input. This study introduced carbon as a concurrent tracer, which showed little variation (-6 < δ13C < -2) in the underlying mantle. Both isotopes demonstrated mantle degassing is concentrated along the central axes of the mature rifts (Main Ethiopian, Kivu and Kenyan rifts). In contrast, the younger rift segments (Rungwe and Northern Tanzania) showed dispersed mantle volatile signatures. Due to insufficient data, the younger rifts could not provide a meaningful contrast to the mature rifts. Lack of data was the primary limitation of this study, with poorly sampled rift sections or samples altered by secondary processes. More data is needed to provide a better comparison between these isotope systems.