| Abstract |
The Lake Kivu bassin, central western branch of the East African rift system, has numerous hot springs expressing heat at depth. The chemistry and isotopic signatures of hydrothermal fluids are known along the northern shore of Lake Kivu, monitoring the Nyiragongo volcano. However, the southern shore of Lake Kivu, down to Tanganyika Lake, a non-volcanic area, has been ignored while tens of hydrothermal manifestations. This study fills the gap by sampling 15 hot springs, two volcanic lakes, and three cold springs on the border of DRC and Burundi. Stable isotopes of water (d2H and d 18O), noble gases He (3He/4He), Ne, and Ar, Sr isotopes (87Sr/86Sr), with major and trace elements were analyzed. Several samples are Na+K- HCO3 and HCO3-Mg+Ca, and are Immature type. The stable isotopes plot shows a meteoric in origin, only the lake waters are fractionationed by evaporation. Helium (3He/4He) ratios are normalized and corrected for the atmospheric component (Rc/Ra) range from 0.056 to 1.987, suggesting a mixing of crustal source and about 13% of mantle helium. The 87Sr/86Sr ratios range from 0.7036 ( Lac Vert) volcanic to 0.77173 in hot springs, radiogenic value likely indicating water-rock interaction with silicate minerals. These results contrast with those on northern shore of Lake Kivu characterized by magmatic helium, thus, indicating that heat controls the volcanoes and the local geothermal gradient facilitated by the extensional faults. Small magmatic helium observed in some samples is possibly related to the leaching of local intrusions rather than addition from magma melts at depth. |