Browsing by Author "Marty, Bernard"

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    The 2005 and 2006 Eruptions Of Ol Doinyo Lengai: Assessing Deep and Shallow Processes At an Active Carbonatite Volcano Using Volatile Chemistry And Fluxes
    (2006-11) Fischer, Tobias; Burnard, Pete; Marty, Bernard; Palhol, Fabien; Mangasini, Frank; Shaw, A. M.
    960's and the oldest natrocarbonatite tuffs have been dated to 1250 years B.P.. Earlier eruptions produced phonolitic and nephelinitc lavas [1]. Since the 1960's the volcano has erupted frequently producing carbonatite lava flows. Explosive eruptions are much less frequent but have occurred in 1966, 1983 [1] and 1993 [3] producing ash, cones and natrocarbonatite tephra. In July 2005, we launched an expedition to the crater to collect gas and rock samples. On July 4, the volcano began erupting low viscosity, low T (540C) high velocity (2 m/sec) lava flows at a rate of about 0.3 m3/sec. By afternoon, the lava was flowing over the eastern crater rim. During the eruption we sampled gases from nearby hornitos at 120 and 168C, yielding pristine magmatic gases characterized by 75 mol% H2O, 22% CO2, < 1% SO2, H2S, HCl and traces of H2, He, Ar, N2, CH4 and CO. CO2-CH4-CO gas equilibrium temperatures are 580C consistent with lava flow temperatures. N2-He-Ar abundances indicate an upper mantle origin of volatiles, confirmed by isotopes [4]. SO2 flux measured by mini DOAS was low (10 t/day). CO2 fluxes calculated using CO2/SO2 are 3000 to 4000 t/day. Volatiles measured in the carbonatite lavas by SIMS show low H2O (< 0.7 wt%), high S (0.2 to 1 wt%) and Cl (0.6 to 1.4 wt%) and variable F (0.06 to 0.7 wt%). CO2 contents are 30 wt% with major and trace elements typical of natrocarbonatite lavas previously reported in [1]. The release of all CO2 (30 wt% or 20 t/day) from eruption lavas would only produce a small fraction of the measured CO2. In March 2006 eyewitnesses [3] reported the occurrence of an explosive eruption and some of us returned to the volcano on May 12. The morphology of the crater had changed and was now filled with lava 2 m deep. The central cone area had collapsed. We sampled a deposit of carbonatite ash containing accretionary lapilli suggesting water-magma or water-ash interaction. The measured SO2 flux was low (approx. 10 t/day). Our data and observations imply that 1) Ol Doinyo Lengai gases originate from the upper mantle and have equilibrium temperatures consistent with carbonatite magmas, 2) the CO2 flux measured during the eruption cannot be produced by the eruption of carbonatite lavas and additional CO2 is released from the mantle, 3) explosive eruptions (such as in 2006) may be triggered by hydromagmatic processes. Alternatively the fountain material interacted with rain at the surface.
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    Noble Gases in Carbonatite Magmatism: Oldonyo Lengai
    (2006-11) Burnard, Pete; Marty, Bernard; Fischer, Tobias; Hilton, David R.; Mangasini, Frank; Makene, C.
    Oldonyo Lengai,Tanzania, is the only volcano on Earth that is actively erupting carbonatitic lavas. In order to further constrain the origin of the Oldonyo Lengai magmas, an expedition to Oldonyo Lengai in July 2005 sampled to volcanic gases. Two fumaroles were sampled, one with a discharging temperature of 124 °C, the other more than 168 °C. The chemical composition of discharging gases is dominated by H2O (approx77 mol%) and CO2 (approx 22 mol%), SO2, H2S and HCl make up less than 1 mol%, combined. The inert gases (N2, He, He/Ne) show that these samples contain virtually no air. He/Ne ratios are between 2000 and 6500 and He/Ar ratios are up to 0.3 [Fischer et al, 2006, this volume]. The 3He/4He ratio of 6.7 - 6.8 Ra is consistent with an upper mantle origin of these gases. We have also measured Ne and Xe isotopic compositions of several aliquots of the sampled gases using a multicollector noble gas mass spectrometer (HELIX-MC). The additional precision afforded by multicollection allows us to identify noble gas isotopic anomalies at the sub 5 per mil level. Despite the excellent purity (low atmospheric content) of the gases, as evidenced by extremely high He/Ne ratios, the isotopic compositions of both Ne and Xe are very close to those of the atmosphere: a 2 per mil excess in 129Xe/130Xe ratio was observed (the remaining Xe isotope ratios being indistinguishable from air) and 20Ne/22Ne up to 10.3 was measured (50 per mil higher than air) in a split of the sample that has He/Ne = 6500. Although isotolically anomalous Ne was observed, it is not possible to determine if this is indeed mantle - derived Ne or if the 20Ne excesses result from kinetic fractionationed air entrained within the volcano's plumbing system: the composition of the three Ne isotopes (20Ne, 21Ne and 22Ne) are consistent with mass fractionation processes. Our results are most readily interpreted as atmospheric entrainment prior to sampling. However, further measurements of the remaining noble gases (principally the 4He/40Ar ratio) and results of on-going experiments on the solubility of noble gases in carbonatite melts will clarify this situation.
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    Oldoinyo Lengai Gas Chemistry From 2005 to 2009: Insights to Carbonatite-Nephelinite Volcanism
    (2008-12) Fischer, Tobias; Burnard, Pete; Marty, Bernard; De Moor, J. M.; Hilton, David R.; Shaw, A. M.; Barry, Peter H.; Umaña, Carlos J. R.; Mangasini, Frank
    The African Rift valleys are sites of carbonatite-nephelinite volcanic complexes. Oldoinyo Lengai (OL), the cone that rises to nearly 3000 m above Tanzania's Rift Valley, is the world's only active carbonatite volcano. Explosive eruptions have occurred at OL in 1966, 1983 [1] and 1993 [2] producing ash, cones and natrocarbonatite tephra. From Sept. 2007 to Nov. 2008, OL erupted explosively forming a ~60 m high ash cone. The magma composition of these eruptions is nephelinite mixed with carbonatite [3]. In June 2009, we observed a carbonatite lava lake at the bottom of the ~100m deep crater. Volcanic products at OL have therefore transitioned from carbonatite erupted in 2005/06 to nephelinite back to carbonatite in three years; a tribute to the highly dynamic nature of the volcano. We collected samples from crater fumaroles in July 2005, May 2006 and June 2009, spanning the volcanoes recent cycle of activity. The gas composition of all samples is dominated by H2O (meteoric) and CO2. S, HCl, and HF contents are < 1 mol%. Hydrogen and CO contents of 0.1 - 0.2 mol% and 0.0015 - 0.025 mol% respectively show the reduced nature of the gases consistent with H2S being the dominant S species. The CO2/S and CO2/HCl ratios of gases are lower than those of carbonatite magmas which contain up to 8000 ppm S and Cl suggesting that carbonatite acts as a condensor for S and Cl (see also [3]). Isotopic compositions of He, N2, Ar, C show that the mantle below OL is characterized by volatiles indistinguishable from those of MORB sources [4]. H2-H2O redox conditions indicate equilibrium with the `rock-buffer' commonly controlling gases associated with silicic magmas [5]. Gas equilibrium temperatures from ~ 400C to 600C are similar to carbonatite magmas (540C). The 2009 gases have CO2/S ratios that are higher by factor of 10 than those collected in the 2005 and 2006, suggesting efficient condensation of S into the erupting carbonatite ~ 100 m below the sampling locality. Alternatively, the low S contents could be attributed to volatile depletion of the underlying silicate magma during explosive eruptions. Abundances of non-condensable gases (CO2, He, N2, Ar) are indistinguishable from those of 2005. This is consistent with the idea that carbonatite magma is a shallow reservoir extending at most several 100's m below the current crater bottom and contributing minimally to the overall volatile budget which is dominated by degassing of the deeper and presumably much larger nephelinite magma. Our data provide important constrains on the nature of carbonatite magmatism and the underlying nephelinite as well as the interaction between these two magmas that produces alternating effusive and explosive eruptive activity. Refs: 1 Dawson, J.B. (1989) Carbonatites: Genesis and Evolution; 2 http://www.mtsu.edu/~fbelton/lengai.html; 3 de Moor et al., AGU Fall 09 abstract. [4] Fischer et al., nature 2009. [5] Giggenbach et al., 1987.
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    Upper-Mantle Volatile Chemistry at Oldoinyo Lengai Volcano and the Origin Of Carbonatites
    (2009-04) Fischer, Tobias; Burnard, Pete; Marty, Bernard; Hilton, David R.; Füri, Evelyn; Palhol, Fabien; Sharp, Z. D.; Mangasini, Frank
    Carbonatite lavas are highly unusual in that they contain almost no SiO(2) and are >50 per cent carbonate minerals. Although carbonatite magmatism has occurred throughout Earth's history, Oldoinyo Lengai, in Tanzania, is the only currently active volcano producing these exotic rocks. Here we show that volcanic gases captured during an eruptive episode at Oldoinyo Lengai are indistinguishable from those emitted along mid-ocean ridges, despite the fact that Oldoinyo Lengai carbonatites occur in a setting far removed from oceanic spreading centres. In contrast to lithophile trace elements, which are highly fractionated by the immiscible phase separation that produces these carbonatites, volatiles (CO(2), He, N(2) and Ar) are little affected by this process. Our results demonstrate that a globally homogenous reservoir exists in the upper mantle and supplies volatiles to both mid-ocean ridges and continental rifts. This argues against an unusually C-rich mantle being responsible for the genesis of Na-rich carbonatite and its nephelinite source magma at Oldoinyo Lengai. Rather, these carbonatites are formed in the shallow crust by immiscibility from silicate magmas (nephelinite), and are stable under eruption conditions as a result of their high Na contents
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    Xe isotopes in carbonatites: Oldonyo Lengai, East African Rift
    (2006-07) Burnard, Pete; Basset, R.; Marty, Bernard; Fischer, Tobias; Palhol, Fabien; Mangasini, Frank; Makene, C.