| Abstract |
Tanzania is characterized by unique volcanoes. Most of those volcanoes are located within the eastern and western branches of the Cenozoic East African Rift. Latest helium and neon isotope investigations of lavas and xenoliths are indicating a super-plume beneath East Africa (Halldórsson et al. 2014). Additionally to the distinction between plume and non-plume related volcanism, helium isotope analyses are a versatile tool for relating geothermal resources to any type of volcanic heat source. In Tanzania there are several distinct areas with significant geothermal surface manifestations: (i) Mwanza region in the N, (ii) Arusha region in the NE, (iii) Dodoma region in central Tanzania, (iv) Dar es Salaam region in the E, (v) Kigoma region in the W and (vi) Mbeya region in the SW. Helium isotope data of Ruhoi hot spring in E Tanzania does not show any indications of a volcanic heat source in contrast to Ngozi-Songwe geothermal system in SW Tanzania (Kraml et al. 2014a,b). In N Tanzania (data this study) the normalised 3He/4He ratio (R/Ra) of the Kogaja mineralized spring indicates crustal helium. Also the Maji Moto hot spring is characterized by non-volcanic gas as indicated by the carbon isotopic composition of CO2. In NE Tanzania the R/Ra values of Ol Doinyo Lengai fumaroles indicates a mantle composition (Fisher et al. 2009, Barry et al. 2013). The Isotopic composition of Lake Natron hot springs (Barry et al. 2013) is dominated by crustal helium but show a small but significant mantle component due to the proximity to Ol Doinyo Lengai, whereas R/Ra of Lake Manyara hot spring (Pik et al. 2006) indicates crustal helium. In W Tanzania noble gas isotope data is only available for Lake Rukwa area (Pik et al. 2006; Danabalan et al. 2016) indicating crustal compositions. Hydrothermal emanations at the floor of Lake Tanganyika (D.R.C.) are characterized by a magmatic carbon isotopic composition of CO2 (Botz & Stoffers 1993).The implications of a crustal or magmatic helium and/or carbon isotopic composition (and supporting evidence) are discussed concerning the existence of viable geothermal heat sources at individual sites of major Tanzanian geothermal areas. |