Browsing by Author "Shaw, A. M."

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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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    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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    Volatile Chemistry of the 2007 to Present Explosive Eruption of Oldoinyo Lengai Volcano, East African Rift
    (2008-11) De Moor, J. M.; Fischer, Tobias; King, Penelope L.; Sharp, Z. D.; Shaw, A. M.; Mangasini, Frank
    We characterize the volatile chemistry of the ongoing explosive eruption at Oldoinyo Lengai (OL) in the Gregory Rift Valley of N Tanzania. Fieldwork was conducted from 4-8 April 2008, during which time OL exhibited Strombolian to ash plume-producing activity. Eight distinct ash lapilli layers were sampled 900m from the crater. Mini-DOAS SO2 flux measurements were conducted on 6, 7, and 8 April. Despite moderate eruptive activity, SO2 concentrations were very low, from ~ 20ppm.m to below detection. A low concentration plume was detected on 7 April, allowing a SO2 flux estimate of 0.2-0.4 tons/day. SIMS analyses of carbonatite lavas erupted in 2005 show very high S concentrations (0.62wt %), suggesting that the low SO2 flux is due to partitioning of S into the melt. Ash leachates were analyzed as a proxy for plume chemistry and to assess health risks associated with mobile elements in the ashes. The solutions had high pH of 10.6 to 11.1. This has implications for pH fluctuations of Lake Natron (pH ~10; located 20km N of the crater), which may correlate with lacustrine ash deposition during passed explosive activity at OL. In the uppermost ash layer (deposited on 4/5/2008; not influenced by rain) dominant mobile ions are Cl (18120mg/kg), SO4 (26616mg/kg), PO4 (2393mg/kg), and F (534mg/kg), Na (101679mg/kg), K (22544mg/kg), Ca (721mg/kg), and Si (189mg/kg). Leachate S/Cl from this pristine ash is 0.49, compared to 0.29 measured by SIMS in lavas from 2005. Using the SO2 flux and the S/Cl in the leachates, the Cl flux was 0.5-0.8 tons/day. High concentrations of leachable ions, particularly F, on ash presents health hazards (F poisoning; water source contamination) to local communities. Concentrations in the underlying ashes are lower (40-129 mg/kg Cl, 965-3223 mg/kg SO4 , 66-104 mg/kg F, 40-335 mg/kg PO4 ) than those in the upper deposit due to leaching by rain prior to deposition of the uppermost ash layer. FTIR spectroscopy of ashes shows at least two carbonate minerals in the uppermost ash sample: calcite and a hydrous carbonate, possibly containing Na, Ca, K and/or Mg, spectrally most similar to nesquehonite (MgCO3.3H2O). Bulk ash samples were analyzed for C and O isotopes to investigate sources of CO2 and carbonate. Samples from 2007 have delta13CPDB ~ -6.30/00 and delta18OSMOW ~9.90/00, which overlap with mantle values (Keller and Hoefs, 1995). Samples from 2008 have delta13C values from -6.53± 0.190/00 to -5.44 ±0.100/00. The more enriched delta13C values can be explained by fractionation by degassing of CO2 that is enriched in 13C relative to the CO2 dissolved in the magma, which agrees with the C isotope compositions of gases from OL (~-2.60/00; Javoy et al., 1988). Oxygen isotope compositions of the ashes collected in 2008 vary systematically from delta18O15.32 ±0.240/00 to 10.46± 0.130/00. This trend may be due to assimilation of altered carbonatite with delta18O values of (>)+160/00 (Keller and Hoefs, 1995) by a magma with delta18O of ~+100/00. Javoy, M., et al. 1998. The Gas Magma Relationship in the 1988 Eruption of Oldoinyo Lengai (Tanzania). EOS Tans. AGU 69,1466. Keller, J., Hoefs, J., 1995. Stable Isotope Characteristics of Recent Carbonatites. in: Bell, K., Keller, J. (Eds.), Carbonatite Volcanism: Oldoinyo Lengai and the Petrogenesis of Natrocarbonatites. IAVCEI Proc. in Volc. 4, 70-86.