JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, B01205, doi: /2008jb006227, 2010

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1 Click Here for Full Article JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115,, doi: /2008jb006227, 2010 Gas isotopic signatures (He, C, and Ar) in the Lake Kivu region (western branch of the East African rift system): Geodynamic and volcanological implications D. Tedesco, 1,2 F. Tassi, 3,4 O. Vaselli, 1,4 R. J. Poreda, 5 T. Darrah, 5 E. Cuoco, 2 and M. M. Yalire 6 Received 24 November 2008; revised 28 August 2009; accepted 20 October 2009; published 29 January [1] On 17 January 2002, the city of Goma was partly destroyed by two of the several lava flows erupted from a roughly N-S oriented fracture system opened along the southern flank of Mount Nyiragongo (Democratic Republic of Congo), in the western branch of the East African rift system. A humanitarian and scientific response was promptly organized by international, governmental, and nongovernmental agencies coordinated by the United Nations and the European Union. Among the different scientific projects undertaken to study the mechanisms triggering this and possible future eruptions, we focused on the isotopic (He, C, and Ar) analysis of the magmatic-hydrothermal and cold gas discharges related to the Nyiragongo volcanic system, the Kivu and Virunga region. The studied area includes the Nyiragongo volcano, its surroundings, and peripheral areas inside and outside the rift. They have been subdivided into seven regions characterized by distinct 3 He/ 4 He (expressed as R/R air ) ratios and/or d 13 C-CO 2 values. The Nyiragongo summit crater fumaroles, whose R/R air and d 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)-like contribution. Similar mantle-like He isotopic values ( R/R air ) are also found in CO 2 -rich gas emanations (mazukus) along the northern shoreline of Lake Kivu main basin, whereas the 13 dc-co 2 values range from 5.3% to 6.8% VPDB. The mantle influence progressively decreases in (1) dissolved gases of Lake Kivu ( R/R air ) and (2) the distal gas discharges within and outside the two sides of the rift (from 0.1 to 1.7 R/R air ). Similarly, d 13 C-CO 2 ratios of the peripheral gas emissions are lighter (from 5.9% to 11.6% VPDB) than those of the crater fumaroles. Therefore, the spatial distribution of He and C signatures in the Lake Kivu region is mainly produced by 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 CO 2 / 3 He ratio (up to ) is 1 order of magnitude higher than those found in MORB, and it is due to the increasing solubility of CO 2 in the foiditic magma feeding the Nyiragongo volcano. However, the exceptionally high 40 Ar*/ 4 He ratio (up to 8.7) of the Nyiragongo crater fumaroles may be related to the difference between He and Ar solubility in the magmatic source. The results of the present investigation suggest that in this area the uprising of mantle-originated f luids seems strongly controlled by regional tectonics in relation to the geodynamic assessment of the rift. These fluids are mainly localized in a relatively small zone between Lake Kivu and Nyiragongo volcano, with important implications in terms of volcanic activity. Citation: Tedesco, D., F. Tassi, O. Vaselli, R. J. Poreda, T. Darrah, E. Cuoco, and M. M. Yalire (2010), Gas isotopic signatures (He, C, and Ar) in the Lake Kivu region (western branch of the East African rift system): Geodynamic and volcanological implications, J. Geophys. Res., 115,, doi: /2008jb Office for the Coordination of Humanitarian Affairs, United Nations, Geneva, Switzerland. 2 Environmental Sciences Department, Naples 2 University, Caserta, Italy. 3 Istituto di Geologia Ambientale e GeoIngegneria, CNR, Rome, Italy. 4 Earth Sciences Department, University of Florence, Florence, Italy. 5 Environmental and Earth Sciences Department, Rochester University, Rochester, New York, USA. 6 Goma Volcano Observatory, Goma, Democratic Republic of Congo. Copyright 2010 by the American Geophysical Union /10/2008JB006227$ Introduction [2] The western branch (WB) of the East African rift system (EARS), the latter including the Ethiopian and Kenya branches (Figure 1), is an 3000 km long seismically active corridor that borders the mechanically strong [e.g., Petit and Ebinger, 2000] and thick [e.g., Ritsema et al., 1998] Archaean Tanzania craton. The eruptive activity of the WB is substantially lower than that of the volcanic 1of12

2 Figure 1. sites. Map of the Kivu volcanic province with major tectonic lines and locations of gas sampling systems located in the eastern branch of the EARS and can be related to four main volcanic districts: (1) the Toro- Ankole region (western Uganda), (2) the Virunga, (3) the Kivu provinces (at the border between the Democratic Republic of Congo with Uganda, Rwanda, and Burundi), and (4) the Rungwe volcanic field (southwestern Tanzania). [3] The dominant volcanism consists of silica-undersaturated mafic products and includes ultrapotassic, hypersodic, and carbonatitic compositions [Furman, 2007, and references therein]. Presently, the volcanic activity is concentrated in the middle Miocene Virunga Volcanic Province (VVP), whose eight main EW-oriented volcanic edifices [e.g., Mavonga, 2007] can be grouped as follows: (1) Muhavura (4127 m above sea level (asl)), Gahunga (3474 m asl) and Sabinyo (3647 m asl) (eastern group); (2) Visoke (3911 m asl), Karisimbi (4506 m asl), and Mikeno (4437 m asl) (central group); and (3) Nyiragongo (3470 m asl) and Nyamuragira (3056 m asl) (western group). [4] With the exception of the short-lived eruption of Visoke in 1956, the westernmost volcanoes have had frequent historical activity. The Nyamuragira volcano is characterized by flank eruptions, consisting of frequent 2of12

3 K-rich Hawaiian-type lava flows [e.g., Hayashi et al., 1992], although an active lava lake was present from 1921 to 1938 and then disappeared [e.g., Mavonga et al., 2006]. Since 1928, the Nyiragongo volcano has been known for its semipermanent active lava lake, whose level, up to our last visit to the crater (April 2009), has been continuously changing [Tedesco, 2009]. An important rise of the lava lake has triggered the lateral eruption on 10 January 1977 [Tazieff, 1979]. Later, an active lake was present in 1982 (up to 1985) [Durieux, 2002/2003] and again in [Tedesco, 2002/2003]. The Nyiragongo volcano resumed its activity in January 2002 with an approximately 15 km long N-S oriented fissure eruption [Komorowski et al., 2002/ 2003; Tedesco et al., 2007] generating silica-undersaturated (SiO 2 < 40 wt %) products [Santo et al., 2002/2003; Chakrabarti et al., 2009]. Two lava flows originated by this activity entered the city of Goma (500,000 inhabitants in 2002, more than 600,000 in 2008), causing the death of about 170 people and leaving more than 100,000 homeless [Tedesco et al., 2007]. The United Nations and European Union funded several research projects following the 2002 eruptive event, owing to the political and economic importance that the Nyiragongo volcano plays in such an unstable area. This study includes an isotopic (carbon, helium, and argon) survey of gas discharges collected in 2003 and 2005 from the summit crater of the Nyiragongo volcano and the portion of the rift that extends north and south of Lake Kivu, including several areas outside the rift (Figure 1). The main goal of the current research is to constrain the origin of the various fluid sources in order to investigate the complex relationships among rift tectonics, mantle degassing, the existence of different fluid source regions, and the potential future activity of Nyiragongo volcano. 2. Helium, Carbon, and Argon Isotope Ratios in Different Fluid Source Regions [5] The isotopic values of He, C, and Ar of fluid discharges and fluid inclusions can be considered powerful tracers of interactions between mantle- and crust-derived fluids in different geodynamic settings [e.g., Polyak and Tolstikhin, 1985]. In particular, the He isotopic ratio can be used to intimately define the various sources of fumarolic gases and to model hydrothermal fluid circulation patterns related to volcanic and hydrothermal activity [e.g., Giggenbach et al., 1993; Tedesco and Scarsi, 1998], since 3 He is essentially primordial and retained in the Earth s interior at the time of its formation, whereas 4 He is mainly derived from the decay of U and Th series isotopes in both mantle and crust. As a consequence, 3 He/ 4 He ratios higher than the local (crustal) production rate indicate the presence of degassing of mantlederived fluids. [6] Mantle degassing is intimately associated with magmatic activity in ocean basins and continental settings. Mantle degassing mainly occurs in ocean basins, in correspondence with the formation of oceanic lithosphere [Clarke et al., 1969] and less commonly in continental environments [Torgersen, 1989]. Fluids enriched in radiogenic helium, typically found in the crustal environment, are characterized by R/R air values of about 0.01 (where R is the 3 He/ 4 He isotopic ratio measured in the sample and R air is the 3 He/ 4 He ratio of air equal to [Mamyrin and Tolstikhin, 1984]). The R/R air values of the mid-ocean ridge basalt (MORB) and hot spot and plume volcanic fluids are 8 ± 1 and up to 20 or more, respectively [Craig and Lupton, 1976]. Fluids related to continental and arc volcanoes have a wide range of R/R air values (between 3 and 8) depending on the degree of crustal contamination affecting the mantle source [Poreda and Craig, 1989; Hilton et al., 2002]. [7] Carbon isotopic values of CO 2 (hereafter reported as d 13 C-CO 2 VPDB%) in mantle-related fluids usually range between 3% and 7% VPDB, depending on the different geodynamic settings of the volcanic region. Fluids of magmatic systems producing basaltic lava, such as Etna and Hawaii, show d 13 C-CO 2 values approximately 3% VPDB [Gerlach and Taylor, 1990; Poreda et al., 1992; Allard et al., 1997]. More positive d 13 C-CO 2 values (0% to approximately 2% VPDB) in volcanic systems, e.g., Vulcano Island [Tedesco and Nagao, 1996; Capasso et al., 1999], Mt. Vesuvius [Tedesco and Scarsi, 1998, Chiodini et al., 2001], and Nysiros [Brombach et al., 2003], generally imply the presence of limestone interactions with the magmatic fluid or within the magmatic reservoir. Strongly negative d 13 C-CO 2 values ( 20% VPDB) are often related to the degradation of organic matter at relatively shallow depth [O Leary, 1988; Hoefs, 2008], whereas CO 2 produced by thermometamorphic processes in sedimentary realms generally shows d 13 C-CO 2 values 0% VPDB [Tedesco, 1997; Tedesco and Scarsi, 1998; Minissale, 2004]. Intermediate d 13 C-CO 2 values between those characterizing the different fluid source regions can commonly be produced by mixing processes and/or fractionation effects (e.g., prolonged interaction of alkaline waters with a CO 2 -rich gas phase) [Tedesco, 1997; Minissale et al., 1997; Chiodini et al., 1999; Minissale, 2004]. Argon has three stable isotopes, the two of most interest being primitive 36 Ar and radiogenic 40 Ar. There is virtually no primordial 40 Ar in the Earth [Ozima and Kudo, 1972], being entirely produced by 40 K decay. Theoretical calculations, based on the atmospheric 40 Ar budget and the estimate of the total 40 K content of the Earth, indicate that approximately half of all 40 Ar produced within our planet since its formation is retained within the solid Earth [Poreda and Farley, 1992; O Nions and Tolstikhin, 1996]. However, 40 Ar does not seem to be homogeneously distributed within the mantle. MORB lavas are indeed typically characterized by 40 Ar/ 36 Ar ratios up to 28,000 [Farley and Poreda, 1993; Sarda et al., 1985], which are 5 to 10 times higher than those of lower mantle xenoliths, the latter being comprised between 307 and 1870 [Poreda and Farley, 1992; Farley and Poreda, 1993; Nagao and Takahashi, 1993]. 3. Mantle Source(s) in the EARS [8] The Cenozoic volcanism over the eastern and central African plate shows two distinct relationships with the mantle: rift- and hot spot-related volcanism [Pik et al., 2006]. However, the nature of the mantle beneath the EARS is still a matter of debate, likely due to the fact that the geophysical data do not provide complete coverage of the area and uncertainties in the African plate kinematics of the last 45 Myr do not discriminate whether one or two diachronous plumes are interacting with heterogeneous lithosphere [e.g., Paslick et al., 1995; Simonetti and Bell, 1994; 3of12

4 Kalt et al., 1997; Ebinger and Furman, 2002/2003; Furman, 2007]. However, He isotope systematics clearly define the presence of at least one plume at the triple junction of the Gulf of Aden, the Red Sea, and Ethiopian rift [Marty et al., 1993, 1996; Scarsi and Craig, 1996; Pik et al., 2006], whereas the determination of a second plume, based on current Pb and He isotopic data, is ambiguous. Helium isotope systematics, particularly in lava phenocrysts, have recently provided some insights about the interactions between mantle and crustal environments in the EARS [e.g., Marty et al., 1993, 1996; Darling et al., 1995; Scarsi and Craig, 1996; Graham, 2002; Pik et al., 2006]. The He isotope signature (R/R air up to 20) of fluid inclusions and gas discharges collected in Ethiopia (Afar), Yemen, and Djibouti [Scarsi and Craig, 1996; Pik et al., 2006], associated with the Oligocene onward prerift flood basalts, shows the existence of a lower mantle source region [Schilling et al., 1992; Deniel et al., 1994], typical of other volcanic provinces on the Earth, where large volumes of lavas have been emitted, such as the Deccan and the Siberian plateau [Basu et al., 1993]. R/R air values lower than 6 were measured in the same area and are likely due to either (1) accumulation of 4 He in old lavas or (2) crustal contamination that could have affected both old and young lavas [Marty et al., 1996; Pik et al., 2006]. [9] A distinct type of volcanism, presumably related to shallow (<400 km) mantle upwelling, characterizes the WB of the EARS, where R/R air from 5.7 to 8.9 were measured [Graham, 2002; Pik et al., 2006]. Similarly, volcanic products and gas discharges of the Kenya rift [e.g., Darling et al., 1995] have a MORB-like He isotopic signature. [10] The only existing He isotopic data in the VVP are those measured in (1) one sample of dissolved gas collected from the depth of 320 m in the Lake Kivu (R/R air = 3) and (2) one from the several CO 2 -rich gas vents bordering the northern shoreline of the lake (R/R air = 5) (R. Poreda (personal communication, 2003) as cited by Schoell et al. [1988] and Tuttle et al. [1990]). 4. Sampling and Analytical Methods 4.1. Description of the Sampling Sites [11] Fumarolic gas samples were collected within the Nyiragongo summit crater at platform 1 (Goma 1 and Goma 2, with outlet temperature of 82 C and 84 C, respectively) and platform 2 (fractures 1 to 8, with outlet temperature ranging from 118 C to 393 C). The two lava platforms are located at an altitude of 190 and 290 m lower than the crater rim, respectively (Figure 1). Platform 1 corresponds to the lava lake level prior to the 1977 eruption, while platform 2 is related to the lava lake level during the eruptive cycle [Durieux, 2002/2003; Tedesco, 2002/2003]. [12] Most of the gas emissions collected in the area surrounding the Nyiragongo volcano within the rift consist of the so-called mazukus (the local Kinyarwanda word for evil winds ), which are cold (20 C 26 C) low-flux gas emanations seeping out from old fractured and altered lava, or in depressed areas in the Nyamulagira and Nyiragongo lava fields [Vaselli et al., 2002/2003]. The mazukus are prevalently located (1) in the vicinity of northern shoreline of Lake Kivu and (2) close to and within the village of Sake to the west (Figure 1), the latter being built on historical lava flows of Nyamulagira volcano. [13] Gas samples from hydrothermal discharges consisting of bubbling pools were collected at the following sites: (1) Rambo, a group of springs (with a discharge temperature ranging from 50 C to 72 C) located in the Archean crystalline basement outcrops of the eastern border of the rift a few kilometers from the city of Goma on the Rwanda side of lake Kivu, (2) Mai ya Moto, hot water in Swahili (discharge temperature of 96 C), located north of Nyiragongo volcano (Figure 1), (3) Kankule (discharge temperatures of 67 C), and (4) Muganzo (discharge temperatures of 68.5 C), with Kankule and Muganzo both located on the south side of Lake Kivu (Figure 1), in the volcanic realm of the dormant Kahusi volcano. [14] The sampling network also includes three soda springs: (1) Tingi (30 C), located close to the western rift border (Figure 1), in a relatively tiny area characterized by the presence of numerous thermal fluid discharges; (3) Kisuma (39 C), on the sedimentary hills of Masisi, approximately 50 km west of the rift; and (3) Makera (19 C), located outside the rift 90 km NW of Goma (Figure 1). [15] Four samples of dissolved gas were also collected from different locations within Lake Kivu: (1) two samples in the main basin at the depth of 320 m, using the facilities of the local gas methane power plant at Cap Rubona (Figure 1), (2) one at the maximum depth of 475 m, and (3) one at the maximum depth of 148 m of Kabuno Bay, which is a relatively small sub-basin located in the most northwest portion of Lake Kivu (Figure 1). [16] To complete the sampling network, two samples of olivine and clinopyroxene phenocrysts from Rumoka crater (belonging to the Nyamulagira domain) and one sample of clinopyroxene from Mudja lava tunnel, Nyiragongo realm were also collected and analyzed. The Rumoka crater, located half way between Goma and Sake (Figure 1), erupted in This perfectly preserved crater is also the site at its bottom of one of the biggest and most dangerous mazuku (Table 1). The Mudja lava tunnel is located in the outskirt of the Mudja village, between the Nyiragongo crater and the city of Goma (Figure 1) Sampling Procedure [17] Preevacuated ml glass flasks filled with ml of a 4 N NaOH and 0.15 M Cd(OH) 2 suspension [Giggenbach, 1975; Montegrossi et al., 2001] were used on line, with (1) a titanium tube and dewar glass tubes and (2) silicone/tygon tubes connected to a plastic funnel, to sample gases from fumarolic vents and boiling pools, respectively [Tassi et al., 2004; Vaselli et al., 2006]. The mazukus were sampled by using a dewar silicon tube inserted into the vents at a depth between 0.5 and 1 m. The 3 He/ 4 He, 40 Ar/ 36 Ar, and He/Ne ratios were determined in headspace gases of the glass flasks. A further aliquot of gas in 50 ml preevacuated flasks was also collected from each site for determining the d 13 C-CO 2 values. The dissolved gas samples from Lake Kivu main basin and Kabuno Bay basin were collected by using 150 ml preevacuated glass flasks, following the procedure described by Tassi et al. [2008] Analytical Procedure [18] Concentrations of CO 2 were determined in the soda samples by automatic titration with 0.5 N HCl solution, following the procedure of Montegrossi et al. [2001]. The 4of12

5 Table 1. Isotopic Values of R/R air, d 13 C-CO 2, and 40 Ar/ 36 Ar; He/Ne, 40 Ar*/ 4 He, and CO 2 / 3 He Ratios; and CO 2, 36 Ar, and 4 He Concentrations of Gas Samples From the Lake Kivu Region a R/R a (R/R air ) c d 13 C 40 Ar/ 36 Ar He/Ne 4 He 36 Ar CO 2 40 Ar*/ 4 He CO 2 / 3 He (10 9 ) Crater Gas Discharges Goma 1/ , Goma 1/ , Goma 1/ nd , Goma 2/ , Goma 2/ , Fracture , Fracture , Fracture nd nd Fracture , Fracture , Fracture nd , Fracture nd nd nd Eastern Rift Katva , Bulengo Seminaire , Nzulu , Bulengo-Summit nd , Kanyabishoho , Kabutembo , Rumoka crater , Kanyabihomdo , Kanyabihomdo nd , Ruwasimvani , Bikumbo nd , Chalet , Koshokero nd , Esco Himbi , Western Rift Sake Birere , Sake Ecole , Sake Gisimba , Sake Spring , Kabuno Bay 140 m , Tingi Tingi , Eastern Side Rambo 1/ , , Rambo 1/ , , Rambo 1/ , , Rambo 1/ , , Rambo 1/ nd 382 2, , Rambo 1/ nd , , Distal Areas Kankule (South) , , Muganzo (South) , , Mai ya Moto (North) , Kisuma (Northwest) , , Makera (Northwest) , Kivu Lake Kivu (Cap Rubona) 320 m , , Lake Kivu (Cap Rubona) 320 m , Lake Kivu 475 m , Inclusions b cpx Rumoka crater nd Fluid olv Rumoka crater nd cpx Mudja lava tunnel nd a CO 2, 36 Ar, and 4 He concentrations are in units of mmol/mol; nd, not detected. b He and CO 2 are in units of cm 3 STP 10 9 /g. 13 C/ 12 C ratios in CO 2 were determined by mass spectrometry, after a two-step extraction and purification procedures of the gas mixtures by using liquid N 2 and a solid-liquid mixture of liquid N 2 and trichloroethylene. During the period of analysis, internal (San Vincenzo marble, recommended value: +1.58% VPDB) and international (NBS 19 carbonate, recommended value: 1.95% VPDB) standards produced values of 1.59 ± 0.04 and 1.95 ± 0.06, respectively. [19] Gas samples for the isotopic analysis were purified in a high vacuum line constructed of stainless steel and 5of12

6 Corning-1724 glass to minimize helium diffusion. Water vapor and CO 2 were cryogenically trapped at 90 C and 195 C, respectively, whereas N 2 and O 2 were removed by the reaction with a Zr-Al alloy (SAES-ST707). Argon and Ne were adsorbed on activated charcoal at 196 C and at 233 C, respectively. SAES-ST-101 Getters (one in the inlet line and two in the mass spectrometer) reduced the HD + background to 1000 ions/s. [20] Helium isotope ratios and concentrations were analyzed on a VG 5400 Rare Gas Mass Spectrometer fitted with a Faraday cup (resolution of 200) and a Johnston electron multiplier (resolution of 600) for sequential analyses of the 4 He (Faraday cup) and 3 He (multiplier) beams. On the axial collector (resolution of 600), 3 He + was completely separated from HD +, with a baseline separation of <2% of the HD + peak. The contribution of HD + to the 3 He peak was <0.1 ion/s at 1000 ions/s of HD +. For cm 3 STP of He with an air ratio (sensitivity of A/torr), the 3 He signal averaged 2500 ions/s with a background signal of 15 cps, due to either scattered 4 He ions or the formation of 4 He ions at lower voltage potentials within the source of the mass spectrometer. Analytical error for He isotope determination was <0.3%. The 3 He/ 4 He ratios (R) were corrected for the addition of air by employing the standard air correction calculation [Craig and Lupton, 1976]. [21] After completion of the He isotopic analysis, 40 Ar/ 36 Ar ratios were measured with the same VG 5400 mass spectrometer. Sensitivity for Ar concentration was A/ torr on the Faraday cup. Precision for the 40 Ar/ 36 Ar ratios averaged 0.2%. [22] Carbon dioxide concentrations and the ratio of CO 2 / 3 He in olivine phenocrysts were obtained by crushing under high vacuum between two zirconia plates in an allmetal Nupro-valve crusher assembly. CO 2 concentrations were measured by a capacitance manometer in a calibrated volume. Helium concentration was measured by comparison to an aliquot of Yellowstone MM standard. Reproducibility and checks on CO 2 yield (i.e., loss of CO 2 by adsorption) were made by repeated sequential crushing of a high concentration CO 2 olivine Samoan xenolith (total CO 2 = 0.1 cm 3 /g, He = 750 cm 3 /g). Five sequential crushing steps showed a variation in the CO 2 / 3 He ratio of less than 10% (total range) for the five steps and heating of the crusher to 100 did not release measurable CO 2. From this experiment, we concluded that neither the zirconia surfaces nor the olivine powder adsorbed significant amount of CO 2. The blanks were < cm 3 and cm 3 for CO 2 and He, respectively, for 0.55 g of sample. 5. Results 5.1. Helium Isotopes [23] The R/R air values of gases collected at platform 1 and platform 2 of Nyiragongo summit crater are up to 8.73, similar to those (8.13 R/R air ) measured in the fluid inclusions of the Mudja lava tunnel. Gases released from mazukus have relatively high R/R air values (between 6.51 and 8.35), although located tens of kilometers away from the central crater (Figure 1) and affected by significant air contamination, as shown by the relatively low He/Ne ratios (Table 1). The fluid discharge collected at Tingi, in the western peripheral side of the rift (Figure 1), has 1.68 R/R air, a value particularly low if compared to those measured in nearby mazukus of Sake (e.g., Source, Ecole, Birere, and Gisimba) just 0.5 to 1 km away (Figure 1, Table 1). Rambo hot springs, located just outside the eastern border of the rift (Figure 1), show the lowest R/R air values (ranging between 0.09 and 0.21) of the entire data set (Table 1). The R/R air values in dissolved gases of Lake Kivu at depths of 320 (Cap Rubona) and 475 m (lake bottom) are and 3.18, respectively, whereas the gas sample collected from Kabuno Bay basin (Figure 1) has 5.54 R/R air. The thermal springs located within and outside the rift north of Nyiragongo volcano (Mai ya Moto, Kisuma, and Makera springs; Figure 1) have 0.67, 0.76, and 1.09 R/R air, respectively. These isotopic values are similar to those measured at Kankule and Muganzo (0.68 and 0.75 R/R air, respectively) in the southwestern lakeside Carbon Isotopes [24] The d 13 C-CO 2 values of Nyiragongo crater gas discharges are between 4.04% and 3.55% VPDB, a range typical of high-temperature fluids from basaltic systems [Gerlach and Taylor, 1990; Poreda et al., 1992; Allard et al., 1997]. Conversely, the d 13 C-CO 2 values of (1) the mazukus of the northern shoreline of Lake Kivu main basin and (2) those located close to Sake village (Figure 1) vary from 6.76% to 5.29% and from 11.65% to 10.48% VPDB, respectively. It is worth noting that the d 13 C-CO 2 values of the dissolved gas samples collected from Lake Kivu main basin ( 5.43% and 5.60% VPDB at depth of 320 and 475 m, respectively) and from Kabuno Bay basin ( 10.29% VPDB), are similar to those of the geographically analogous mazukus (Figure 1). Tingi spring (Figure 1) has a d 13 C-CO 2 value of 6.58% VPDB. [25] The distal gas discharges located within the rift north of Nyiragongo volcano (Mai ya Moto springs) and south of Lake Kivu (Kankule and Muganzo springs) show d 13 C-CO 2 values between 7.02% and 5.90% VPDB, slightly less negative than those of the thermal fluids discharged outside the rift (Kisuma and Makera springs at west and Rambo soda springs at east; Figure 1), where d 13 C-CO 2 values vary between 8.44% and 7.23% VPDB Argon Isotopes [26] The 40 Ar/ 36 Ar ratios from the majority of the gas discharges of the Nyiragongo volcano and its surroundings are equal to that of air (296), with the exception of (1) the crater emissions of platform 1 collected in 2003 (Goma 1 and Goma 2 samples; Table 1), (2) the samples collected at Rambo springs, and (3) the dissolved gas samples of the deep water of Lake Kivu and Kabuno Bay basins. In these samples, the 40 Ar/ 36 Ar ratios range between 313 and 384, indicating 10% to 30% of the Ar emanates from a radiogenic source. By assuming that 36 Ar is entirely derived from air, the concentration of radiogenic Ar ( 40 Ar*) (Table 1) in the gas samples is commonly calculated, as follows: h 40 Ar* ¼ 36 Ar 40 Ar= 36 Ar measured 295:5 i : 6of12

7 Figure 2. R/R air versus d 13 C-CO 2 binary mixing diagram for gas discharges collected at the Kivu volcanic province. Solid squares indicate Nyiragongo crater fumaroles; solid circles indicate eastern/cenral (rift) mazuku; open triangles indicate western (rift) mazuku, shaded triangle indicates Kabuno Bay; inverted triangle indicates Tingi gas discharge; gray circles indicate Lake Kivu; open squares indicate distal gas discharge inside and outside the rift; shaded diamonds indicate Rambo gas discharges. Obviously, this calculation is meaningful only when the measured 40 Ar/ 36 Ar ratio is more than 5% different from the atmospheric value. 6. Discussion 6.1. Helium Versus Carbon [27] On the basis of coupled (R/R air ) c and the d 13 C-CO 2 values of the sampled gas discharges (Figure 2), seven different areas of the Nyiragongo volcano-hydrothermal system were distinguished: [28] 1. The Nyiragongo summit crater: characterized by the highest (R/R air ) c values and d 13 C ratios typical of mantle-derived CO 2 [e.g., Rollinson, 1993]. The (R/R air ) c values of the crater fumaroles, which are consistent with an upper mantle origin for helium [Graham, 2002], are equal to that measured in the fluid inclusions of the clinopyroxene and olivine phenocrysts from the Mudja lava flow and the Rumoka crater (Table 1). Consequently, they likely represent the most primitive helium of the Nyiragongo volcanic system. As already mentioned, similar MORB-like [Kurz et al., 1982; Mamyrin and Tolstikhin, 1984] helium isotopic ratios were found in hydrothermal fluids of the Kenyan rift [Darling et al., 1995] and Tanzanian geothermal fields [Pik et al., 2006]. [29] 2. The area of mazuku gas emissions comprised between the foot of Nyiragongo volcano and the northern shoreline of Lake Kivu main basin (Figure 1). These emissions are characterized by MORB-like helium isotopes (R/R a = 8) and d 13 C-CO 2 values lighter than those of the Nyiragongo crater gas discharges [Komorowski et al., 2002/ 2003; Tedesco et al., 2007]. [30] 3. The area of the mazuku gas emissions in the surroundings of Sake and Kabuno Bay basin (Figure 1), where a MORB-like helium is associated to the lightest d 13 C-CO 2 of the rift, the latter possibly implying severe fractionation of carbon isotopes and/or addition of biogenic CO 2. [31] 4. The Tingi area in the western side of the rift (Figure 1), which likely represents a transition zone between mantle and crustal domains, within and outside the rift, respectively. As already mentioned, the (R/R air ) c value (1.68) of Tingi gas discharge, corresponding to 21% of juvenile He represented by the (R/R air ) c value of Mudja fluid inclusion (8.13), is indeed significantly lower than those of the nearby Sake gas emissions, but still higher than typical crustal values. However, the d 13 C-CO 2 values are only slightly more negative than typical mantle fluids [Rollinson, 1993]. [32] 5. The outer zone of the rift on the eastern side (Rambo springs), marked by crustal He signature and d 13 C- CO 2 values lighter than those of the mantle. Therefore, Rambo fluids represent the most plausible crustal endmember. In the eastern side of the rift, we did not find any kind of fluid discharges that suggests a transition zone between mantle and crustal domains similar to those found on the western side of the rift represented by the Tingi area. [33] 6. The distal areas of the rift located north of Nyiragongo volcano (Mai ya Moto springs) and on the southern shores of Lake Kivu (Kankule and Muganzo springs, belonging to the extinct Kahusi volcanic province), with negligible to small contributions from juvenile sources. This result partly agrees with the low volcanic seismicity recorded in these areas (Goma Volcano Observatory seismic catalog, ), compared to the high tectonic, riftassociated, seismicity, i.e., a 6.1 M earthquake hit the southwestern part of Lake Kivu on 3 February 2008, 25 km north of Bukavu (Figure 1) [Office for the Coordination of Humanitarian Affairs, 2008]. Accordingly, the volcanic edifices of the Kivu volcanic province lying south of Lake Kivu can be presently considered extinct, as also supported by the low, air-like He isotopic signature found in the Bukavu basin [Tassi et al., 2009]. Similar radiogenicrich He isotopic ratios were measured in the western thermal fluids outside the rift, at Kisuma and Makera springs. The relatively low contribution from mantle-related He measured in these gas discharges likely results from (1) 3 He trapped within the Quaternary volcanics present in the area and/or (2) a regional background flux mantle volatile leaking from the neighbor rift. [34] 7. Lake Kivu, whose immense gas reservoir, stored at depth below m [Tietze et al., 1980; Tuttle et al., 1990; Schoell et al., 1988; Tassi et al., 2009], is produced by a simple binary mixing between (1) a MORB-type component and (2) a crustal source. A simple calculation suggests that the radiogenic (crustal) and mantle (MORB-type) components [Kurz et al., 1982; Mamyrin and Tolstikhin, 1984] in Lake Kivu provide 60% and 40% of the total He stored in the lake, respectively. The extremely high He concentration at Rambo, 100 times that in mazukus, indicates that this area is likely a crustal end-member. The addition of only 0.05% 0.1% of this component to a juvenile, mazukus-like gas phase would be able to produce the He isotopic signature of Lake Kivu. This result also 7of12

8 Figure 3. R/R air versus 4 He (in mmol/mol) binary diagram for gas discharges collected at the Kivu volcanic province. Symbols are as in Figure 2. The hyperbolic mixing trend has been calculated using the following equation: Ax + Bxy + Cy + D = 0, where A, B, C, and D are the end-members values for crater and Rambo fluids. argues that crustal flux into Lake Kivu is likely negligible. Carbon and He isotopic compositions, when compared to previous data [Tietze et al., 1980; Tuttle et al., 1990; Schoell et al., 1988], indicate that no significant and/or measurable changes have occurred within the lake over the past 25 years, suggesting a stable source of gas emissions to the lake over this time period. These findings, mentioned for the first time in this study, have important implications on the long-term stability of the gas reservoir stored in Lake Kivu [Schmid et al., 2005, and references therein]. [35] In Figure 3, the values of the (R/R air ) c ratio plotted versus 4 He concentration (in mmol/mol) clearly show a mixing trend between the crater MORB-type (crater fumaroles) and crustal 4 He-enriched (Rambo springs) end-members. It is worth noting that the mazuku gas samples show a significant He-shift, indicating a measurable enrichment of the 4 He concentration not accompanied by a significant decrease of the (R/R air ) c values, as is expected considering that 4 He is essentially related to gas contributions from a crustal source. To explain this apparent paradox, we propose that the gas phase discharged from mazukus is produced by the mixing of an atmospheric component with almost pure CO 2 derived from mantle (or magmatic) degassing [Vaselli et al., 2002/ 2003]. Therefore, scrubbing processes due to the interaction of the uprising CO 2 -rich component feeding the mazukus with local groundwater, whose presence is likely related to the proximity of Lake Kivu, should cause a passive He enrichment without any significant change of the He isotopic signature. The solubility of CO 2 in water is almost 2 orders of magnitude higher than that of He [Lide, 2001] and, consequently, we suggest that the scrubbing process is capable of strongly affecting the ratio of these two gas species. This scrubbing processes can also cause C isotope fractionation of mantle-related CO 2 [e.g., Vogel et al., 1970; Mook et al., 1974], which could explain, at least partially, the relatively lighter d 13 C-CO 2 values of the mazukus compared with that of the crater emissions (Table 1). In fact, CO 2(g) dissolution in neutral-alkaline groundwater, such as those of Lake Kivu, for example, produces HCO 3(aq) [Schmid et al., 2002/2003] that can cause a significant carbon fractionation in CO 2 up to 7%. Therefore, the removal of only 30% of the CO 2 can change the d 13 C by more than 2%. Nevertheless, on the basis of the available data, the previously hypothesized addition of biogenic CO 2, especially for the samples of the Sake area characterized by a particularly negative d 13 C-CO 2 signature (Table 1), cannot be ruled out. A second unlikely option is that fluids feeding the lakeside mazukus (Central Rift and Sake) could be fed from a completely distinct source at depth, explaining the moderate to large carbon isotope variations. [36] The CO 2 / 3 He ratio is often used as a diagnostic parameter to investigate CO 2 origin since it typically shows distinct values in the crust, the mantle and the atmosphere [e.g., Marty and Jambon, 1987; O Nions and Oxburgh, 1988]. The values of the CO 2 / 3 He ratio in the fumaroles of the Nyiragongo summit crater are (Table 1), up to 30 times the MORB ratio [Des Marais and Moore, 1984; Marty and Jambon, 1987] (Figure 4) and consistent with those measured in clinopyroxene and olivine phenocrysts of Rumoka crater (up to ; Table 1). Such an anomalous high CO 2 / 3 He ratio is not in agreement with the apparent MORB-like signature of the He and C isotopes (Figure 2). Instead, this value likely results from the higher (5 times) CO 2 solubility in basanite and foiditic lavas [Dixon et al., 1997] with respect to that of He, as confirmed by experimental petrology and analysis of Hawaiian North Arch basalts [Lux, 1987; Dixon et al., 1995; Dixon and Stolper, 1995; Dixon and Clague, 2001]. However, the relatively low CO 2 / 3 He ratios of the mazukus (between 1 and ) are related to CO 2 scrubbing in shallow aquifers leading to passive He enrichment, a process previously invoked to justify the He shift of these gas emissions (Figure 3). In agreement with this hypothesis, the d 13 C-CO 2 values of the mazuku gases are more negative (about 2%) than those of the crater fumaroles (Table 1). The relatively high CO 2 / 3 He ratios (up to ) in the dissolved gas phase in Lake Kivu and Kabuno Bay basin (Figure 4) may result from the input into the lake of local CO 2 -rich groundwater that has interacted with CO 2 -dominated mazuku gases. The CO 2 / 3 He ratios (between 1 and ) of the gas samples in the distal areas of the rift and outside the rift with helium and carbon isotopic signatures, largely dominated by crustal interactions and the addition of biogenic CO 2, may be affected by local rock lithology. Because the CO 2 / 3 He ratios that typically characterize crustal fluids exist over a wide range (from 10 9 to )[O Nions and Oxburgh, 1988], making any conclusions based solely on this parameter can be difficult Helium Versus Argon [37] The 40 Ar*/ 4 He ratio is a useful indicator of noble gas fractionation during magma degassing as volatiles are degassed from the mantle during magmatic processes [e.g., Marty, 1995; Sarda and Moreira, 2002]. As shown in Figure 5, the (R/R air ) c and 40 Ar*/ 4 He values of the fluid discharged in the investigated area are produced by the mixing of two end-members: (1) the magmatic-related crater fumaroles (R/R air 8 and 40 Ar*/ 4 He of 0.9) and 8of12

9 Figure 4. CO 2 / 3 He versus d 13 C-CO 2 binary diagram for gas discharges collected at the Kivu volcanic province. Symbols are as in Figure 2. Inset represents, without the possible end-members and at a larger scale, the variation of our samples. (2) the radiogenic-rich Rambo springs (R/R air 0.1 and 40 Ar*/ 4 He ). Dissolved gases in Lake Kivu plot at intermediate position (Figure 5). It is worth noting that the 40 Ar*/ 4 He ratios of the crater fumaroles are much higher than the 40 Ar*/ 4 He ratio produced by the present-day radiogenic decaying process in the mantle (0.27 ± 0.02), calculated on the basis of measured K/(U + Th) ratios in MORB [Jochum et al., 1983] and the bulk earth value of 0.55 for 4.5 Ga of radiogenic production. In this respect, the 40 Ar*/ 4 He ratio is strongly dependent on the magma degassing process regulating the partition equilibrium between the melt and the gaseous phase, because the solubility of He in the magma melt is about a factor of 4 higher than that of Ar [Lux, 1987]. Thus, if the melt has an initial bulk earth 40 Ar*/ 4 He ratio of 0.55 [e.g., Poreda and Farley, 1992], then the first gas off the magma reservoir will have a 40 Ar*/ 4 He ratio of about 2.2. According to these considerations, the measured 40 Ar*/ 4 He ratios of the Nyiragongo crater fumaroles are likely the result of fractionation during degassing processes caused by the differences in solubility of He and Ar within the magma. Conversely, the 40 Ar*/ 4 He ratios may differ from the bulk earth value because of unique K/(U + Th) (ultra-potassic magmas) ratios in the source region. The correlations between the highest excess 40 Ar*, coupled with the highest (R/R air ) c of the region, suggests the measured ratio is the closest to (not equivalent to) the true signature of the mantle. However, the coupling of the lowest (R/R air ) c and 40 Ar* suggests the presence of an ubiquitous crustal end-member. [38] Then the relatively low 40 Ar*/ 4 He ratios ( ) of Rambo springs, representing the crustal end-member, are likely related to the typical 4 He enrichment of fluids circulating in crustal environment [e.g., Torgersen et al., 1988; Hu et al., 1998] Magmatic Degassing and Gas Fractionation [39] There is no attempt to rigorously model the fractionation of CO 2 and He out of the lava because there are only three complete sets of data of samples from the summit fumaroles. Although we collected the fumarolic gases as close as possible to the lava lake, there is only limited knowledge of the original CO 2 / 3 He in this lava and even less data on the mantle concentration of He or CO 2. On the basis of the available data, we can only conclusively state Figure 5. R/R air versus 40 Ar*/ 4 He binary diagram for gas discharges collected at the Kivu volcanic province, where 40 Ar* is the radiogenic argon calculated by using the following equation: 40 Ar* = 36 Ar( 40 Ar/ 36 Ar 295.5). The theoretical 40 Ar*/ 4 He ratio in the MORB (0.27 ± 0.02) was calculated on the basis of measured K/(U+Th) ratio [Jochum et al., 1983]. 9of12

10 that given the observed magmatic compositions present at Nyriagongo, the CO 2 / 3 He in the residual melt will increase with continued degassing because CO 2 is 5 times more soluble than He in these strongly silica-undersaturated lavas [Dixon et al., 1997]. There is some uncertainty in the CO 2 solubility estimate because it is an extrapolation from experimental measurements in moderately undersaturated lavas; however, the magma will evolve toward higher CO 2 / 3 He as observed in the summit fumarole collected over 4 years. Thus, the lowest CO 2 / 3 He most closely reflects the magmatic condition after correction for solubility effects. This interpretation is confirmed by the argon isotope ratio which decreases from a value of 350 to the air ratio within 2 years, despite the high He/Ne ratio (700). As discussed above, argon is much less soluble than helium (about a factor of 4) in all silica-undersaturated lavas and will rapidly degas from an evolving magmatic system [Lux, 1987]. The 40 Ar*/He ratio of 0.8 in the fumarole differs from the bulk earth value of 0.55 (closed system evolution for 4.5 Ga) because of the combined effects of solubility and fractional degassing. A 40 Ar*/He ratio of 0.8 reflects a magmatic ratio of 0.2; if the Nyriagongo magma initially had a bulk earth 40 Ar*/He ratio of 0.55, then the fact that the magmatic value differs from 0.55 would indicate that the lava has partially degassed. A simple Rayleigh fractionation calculation suggests that about 25% of the helium and about 5% of the CO 2 have escaped. This calculation also suggests that the original CO 2 / 3 He is about or very CO 2 rich. Because of the great difference in noble gas and carbon dioxide solubilities, these ratios change quickly and dramatically in degassing Si-undersaturated lavas. Thus, by 2005 and 2007, all of the 40 Ar* has degassed leaving behind only the admixed atmospheric argon. 7. Conclusions [40] Carbon and He isotopic features of the gas discharges at the Nyiragongo volcanic system and neighboring areas depict a sharp distinction between zones dominated by crustal- and mantle-related fluids, respectively. It is a perfect didactic case in which each area shows a characteristic isotopic signature (of He and/or C) in function of the specific source region (or a mixture of source regions) feeding that fluid emission site. The summit crater and the southern flank of the volcano up to the northern shoreline of Lake Kivu discharge deep-seated fluids, where He and CO 2 are dominated by a mantle component. The spatial distribution of mantle and crustal fluids is intimately connected to the rift tectonics controlling the mantle gas emissions. The mantle-like R/R air and d 13 C-CO 2 values do not agree with the idea of a second plume related to the VVP. However, we cannot exclude the presence at depth, within or bordering the rift, of a Rambo-like 4 He-rich crustal fluid, which could (strongly) dilute the (lower) mantle 3 He-rich fluid. These results agree with other studies characterizing the African Cenozoic volcanic provinces as upper mantle or nonplume related, with the only exception of the Ethiopia- Afar system [e.g., Pik et al., 2006], which is likely related to the upwelling of a plume region [Ebinger and Sleep, 1998, George et al., 1998; Nyblade et al., 2000; Furman, 2007]. [41] The high CO 2 / 3 He ratios measured in the most primitive gases collected within the VVP at the fumaroles of the summit crater result from increased CO 2 solubility in the foiditic magma that feed Nyiragongo volcano. The high 40 Ar*/ 4 He ratios result from the differences of solubility in the melt between He and Ar. [42] The crater fumaroles and the mazuku gas emissions have similar R/R air values indicating a common mantle origin. However, the d 13 C-CO 2 values seem to show that the CO 2 discharged at the mazukus is fractionated by scrubbing, although minor addition of biogenic CO 2 cannot be ruled out. Thus, the uprising fluids feeding the mazukus are exposed to a significant interaction with local groundwater and meteoric water, which obviously does not occur along the crater plumbing system. As already suggested by previous investigation [Tedesco et al., 2007], this could also imply that crater and mazuku fluids are not fed by the same reservoir, but more likely by two totally independent reservoirs, one located beneath the crater, the second several kilometers away in the region between Nyiragongo and Lake Kivu. As a consequence, volcanic activity at Nyiragongo volcanic system could involve eruptive episodes from the central crater and/or fissures within the flanks of the volcanic edifice or through peripheral cones. These results indicate that these activities may also take place anywhere else in the active corridor described by both the R/R air and d 13 C-CO 2 isotopes. This means that future volcanic episodes could occur close or even within the heavily populated city of Goma or at the bottom of Lake Kivu without necessarily being accompanied by a significant increase or variation of activity inside the main Nyiragongo crater. This possible (and horrifying) scenario has important implications for future studies and for assessing volcanic hazards related to the activity of the entire Nyiragongo (Virunga volcanic system). [43] Acknowledgments. The authors wish to thank the United Nations agency of OCHA and GVO personnel for their help and support in the field. The International Red Cross is thanked for providing the boat used to sample the dissolved gases from Lake Kivu and Kabuno Bay. This project has been partly financed by UN-OCHA (O. Vaselli) and the GVO (D. Tedesco). 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