Gas isotopic signatures (He, C, and Ar) in the Lake Kivu region western branch of the East African rift system):Geodynamic and volcanological implications

摘要

On 17 January 2002, the city of Goma was partly destroyed by two of the several lavaflows erupted from a roughly N-S oriented fracture system opened along the southern flankof Mount Nyiragongo (Democratic Republic of Congo), in the western branch of the EastAfrican rift system. A humanitarian and scientific response was promptly organized by international, governmental, and nongovernmental agencies coordinated by the UnitedNations and the European Union. Among the different scientific projects undertaken tostudy the mechanisms triggering this and possible future eruptions, we focused on theisotopic (He, C, and Ar) analysis of the magmatic-hydrothermal and cold gas dischargesrelated to the Nyiragongo volcanic system, the Kivu and Virunga region. The studied areaincludes the Nyiragongo volcano, its surroundings, and peripheral areas inside and outsidethe rift. They have been subdivided into seven regions characterized by distinct ~3He/~4He(expressed as R/Rair) ratios and/or δ~(13)C-CO_2values. The Nyiragongo summit craterfumaroles, whose R/Rair and δ~(13)C-CO_2 values are up to 8.73 and from -3.5‰ to -4.0‰ VPDB, respectively, show a clear mantle, mid-ocean ridge basalt (MORB)-likecontribution. Similar mantle-like He isotopic values (6.5-8.3 R/Rair) are also found inCO_2-rich gas emanations(mazukus)along the northern shoreline of Lake Kivu mainbasin, whereas the ~(13)δC-CO_2 values range from -5.3‰ to -6.8‰ VPDB. The mantleinfluence progressively decreases in (1) dissolved gases of Lake Kivu (2.6-5.5 R/Rair)and (2) the distal gas discharges within and outside the two sides of the rift (from 0.1to 1.7 R/Rair). Similarly, δ~(13)C-CO_2ratios of the peripheral gas emissions are lighter(from -5.9‰ to -11.6‰ VPDB) than those of the crater fumaroles. Therefore, thespatial distribution of He and C signatures in the Lake Kivu region is mainly producedby mixing of mantle-related (e.g., Nyiragongo crater fumaroles and/or mazukus gases)and crustal-related (e.g., gas discharges in the Archean craton) fluids. The CO2/3Heratio (up to 10 x 10~(10)) is 1 order of magnitude higher than those found in MORB,and it is due to the increasing solubility of CO_2in the foiditic magma feeding theNyiragongo volcano. However, the exceptionally high ~(40)Ar~*/~4He ratio (up to 8.7) ofthe Nyiragongo crater fumaroles may be related to the difference between He and Arsolubility in the magmatic source. The results of the present investigation suggest thatin this area the uprising of mantle-originated f luids seems strongly controlled byregional tectonics in relation to the geodynamic assessment of the rift. These fluids aremainly localized in a relatively small zone between Lake Kivu and Nyiragongovolcano, with important implications in terms of volcanic activity.