Browsing by Author "Sharp, Z. D."

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    Gas Chemistry and Nitrogen Isotope Compositions of Cold Mantle Gases from Rungwe Volcanic Province, Southern Tanzania
    (Elsevier, 2013-02) De Moor, J. M.; Fischer, Tobias; Sharp, Z. D.; Hilton, David R.; Barry, Peter H.; Mangasini, Frank; Umaña, Carlos J. R.
    We report the first complete bulk gas chemistry and nitrogen isotope data for geothermal volatiles from the Rungwe Volcanic Province, located in the western branch of the East African Rift north of Lake Malawi. Temperatures of springs and gas emissions at Rungwe vary from 13 °C to 65 °C with the highest temperatures observed at the springs in the northern and southern lowlands. The vigorously degassing cold CO2 vents and springs have temperatures between 13 °C and 36 °C and are located at higher elevation than the hot springs. The gas compositions are ~ 99% CO2, 0.0008 to 0.0078 mmol/mol H2, 0.0004 to 0.062 mmol/mol He, 0.08 to 0.77 mmol/mol Ar, 3.1 to 28.5 mmol/mol N2, 0.4 to 3.73 mmol/mol O2, < 0.002 to 1.541 mmol/mol CH4, < 0.001 to 0.009 mmol/mol CO, and are poor in H2S (0.045 to 0.201 mmol/mol). The CO2 flux at a local gas collection plant is estimated to be 1.6 × 105 mol/year. Gas geothermometry indicates a range of equilibration temperatures from > 250 °C (from CO2–Ar) to ~ 60 °C (from H2–Ar), which is interpreted to reflect deep equilibration with hot saline fluids and shallow re-equilibration of kinetically fast gas geothermometers with cold meteoric recharge from the highlands. N2–He–Ar systematics show that the gases fall on a well-defined mixing line between upper mantle or sub-continental lithospheric mantle and air saturated water endmembers. Details of an improved method for analyzing nitrogen isotope compositions in gas samples are presented. Nitrogen isotope compositions (δ15N values) range between + 2‰ and − 5.9‰, overlapping with the upper mantle range, with only one sample location displaying δ15N values greater than air (0‰). The results emphasize the importance of the East African Rift as a potential, but poorly constrained, contributor of sub-continental lithospheric mantle volatiles to the Earth's surface even in regions that are currently volcanically dormant, but are seismically active.
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    Geochemistry and Degassing Systematics of Silicate Magma at Ol Doinyo Lengai, Tanzania
    (2009-11) De Moor, J. M.; Fischer, Tobias; King, Penelope L.; Hilton, David R.; Sharp, Z. D.; Barry, Peter H.; Umaña, Carlos J. R.; Mangasini, Frank
    Ol Doinyo Lengai (OL) volcano is unique in that it produces natro-carbonatite lavas. However, every ~25 years the volcano explosively erupts nephelinitic ash. OL entered an explosive phase in September 2007, which lasted until November 2008, and carbonatite activity resumed early in 2009. This study assesses the composition of the 2007-2009 eruptive products and volatiles to characterize degassing and magmatic processes during the explosive eruption. Ash samples collected in 2008 and 2009 are extremely crystal-rich with scarce scoria. Bulk compositions show that the ash is dominated by alkali- and volatile-rich silicate ash with a secondary carbonatite component (SiO2 37.3%, CO3 4.3%, MgO 1.8%, CaO 15.4%, Na2O 11.2%, K2O 3.5%, S 0.14%, Cl 0.20%). Electron microprobe analyses of vesicular scoria show that the matrix glass (SiO2 41.0%, Na22 but enriched in incompatible elements compared to nepheline-hosted glass inclusions (SiO2 43.2%, Na2O 15.8%). S correlates positively with Cl and F in nepheline-hosted glass inclusions (S 0.2-0.4%, Cl 0.3-0.5%, F 0.3-0.8%) showing that these species behaved incompatibly and were not saturated in the parental melt. Matrix glass extends to higher S concentrations (up to 0.7%) at relatively constant Cl and F (Cl ~0.5%, F ~0.7%) resulting in increasing S/Cl and S/F in the residual melt. This is interpreted to reflect Cl and F saturation in the melt due to further crystallization and partitioning of these species into the gas phase while S was undersaturated. Reflectance FTIR shows that the matrix glass has no detectible H2O and ~3% CO2. Glass inclusions have
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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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    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.