Browsing by Author "Barry, Peter H."

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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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    Helium and Carbon Isotope Systematics of Cold “Mazuku” CO2 Vents and Hydrothermal Gases and Fluids from Rungwe Volcanic Province, Southern Tanzania
    (Elsevier, 2013-02) Barry, Peter H.; Hilton, David R.; Fischer, Tobias; De Moor, J. M.; Mangasini, Frank; Umaña, Carlos J. R.
    We report new helium and carbon isotope (3He/4He and δ13C) and relative abundance (CO2/3He) characteristics of a suite of 20 gases and fluids (cold mazuku-like CO2 vents, bubbling mud-pots, hot and cold springs) from 11 different localities in Rungwe Volcanic Province (RVP), southern Tanzania and from 3 additional localities in northern Tanzania (Oldoinyo Lengai Volcano and Lake Natron). At RVP, fluids and gases are characterized by a large range in He-isotope compositions (3He/4He) from 0.97 RA to 7.18 RA (where RA = air 3He/4He), a narrow range in δ13C ratios from − 2.8 to − 6.5‰ (versus VPDB), and a large range in CO2/3He values spanning nearly four orders of magnitude (4 × 109 to 3.2 × 1013). Oldoinyo Lengai possesses upper‐mantle-like He–CO2 characteristics, as reported previously (Fischer et al., 2009), whereas hot springs at Lake Natron have low 3He/4He (~ 0.6 RA), CO2/3He (~ 5–15 × 108) and intermediate δ13C (~−3.7 to − 4.9 ‰). At RVP, fluid phase samples have been modified by the complicating effects of hydrothermal phase-separation, producing CO2/3He and δ13C values higher than postulated starting compositions. In contrast, gas-phase samples have not been similarly affected and thus retain more mantle-like CO2/3He and δ13C values. However, the addition of crustal volatiles, particularly radiogenic helium from 4He-rich reservoir rocks, has modified 3He/4He values at all but the three cold CO2 gas vent (i.e., mazuku) localities (Ikama Village, Kibila Cold Vent and Kiejo Cold Vent) which retain pristine upper-mantle He-isotope (~ 7 RA) and He–CO2 characteristics. The extent of crustal contamination is controlled by the degree of interaction within the hydrothermal system, which increases with distance from each major volcanic center. In contrast, we propose that pristine cold CO2 mazuku gases collected at stratigraphic contacts on the flanks of RVP volcanoes may potentially tap isolated gas pockets, which formed during previous eruptive events and have remained decoupled from the local hydrothermal system. Furthermore, by identifying and utilizing unmodified gas samples, we determine mantle versus crustal provenance of the CO2, which we use to estimate mantle-derived CO2 fluxes at both Rungwe and Lake Natron. Finally, we investigate the origin of the apparent discrepancy in He isotopes between fluids/gases and mafic phenocrysts at RVP (from Hilton et al., 2011), and discuss the tectonic (i.e., rift zone dynamics) and petrogenic conditions that distinguish RVP from other plume-related subaerial rift zones.
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    Helium Isotopes At Rungwe Volcanic Province, Tanzania, and the Origin of East African Plateaux
    (Wiley, 2011-10) Hilton, David R.; Halldórsson, Sæmundur A.; Barry, Peter H.; Fischer, Tobias; De Moor, J. M.; Umaña, Carlos J. R.; Mangasini, Frank; Scarsi, P.
    We report helium isotope ratios (3He/4He) of lavas and tephra of the Rungwe Volcanic Province (RVP) in southern Tanzania. Values as high as 15RA (RA = air 3He/4He) far exceed typical upper mantle values, and are the first observation of plume-like ratios south of the Turkana Depression which separates the topographic highs of the Ethiopia and Kenya domes. The African Superplume - a tilted low-velocity seismic anomaly extending to the core-mantle boundary beneath southern Africa - is the likely source of these high 3He/4He ratios. High 3He/4He ratios at RVP together with similarly-high values along the Main Ethiopian Rift and in Afar provide compelling evidence that the African Superplume is a feature that extends through the 670-km seismic discontinuity and provides dynamic support - either as a single plume or via multiple upwellings - for the two main topographic features of the East Africa Rift System as well as heat and mass to drive continuing rift-related magmatism.
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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-Rich Silicate Melts From Oldoinyo Lengai Volcano (Tanzania): Implications for Carbonatite Genesis and Eruptive Behavior
    (Elsevier, 2012-12) De Moor, J. M.; Fischer, Tobias; King, P. L.; Botcharnikov, Roman E.; Hervig, R. L.; Hilton, David R.; Barry, Peter H.; Mangasini, Frank; Umaña, Carlos J. R.
    This study presents volatile, trace, and major element compositions of silicate glasses (nepheline-hosted melt inclusions and matrix glass) from the 2007–2008 explosive eruption at Oldoinyo Lengai volcano, Tanzania. The bulk compositions of the heterogeneous ash erupted in 2007–2008 are consistent with physical mixing between juvenile nephelinite magma and natrocarbonatite emplaced during the preceding ∼25 years of effusive carbonatite eruption. The melt inclusions and matrix glasses span a wide range of silica-undersaturated compositions, from ∼46 wt% SiO2 and (Na+K)/Al∼3 in the least evolved melt inclusions to 38 wt% SiO2 and (Na+K)/Al up to 12 in the matrix glass. The depletion in SiO2 between melt inclusions and matrix glass is accompanied by strong enrichment in all of the incompatible trace elements measured (Ba, Nb, La, Ce, Sr, Zr, Y), which is consistent with fractional crystallization of a bulk mineral assemblage with SiO2 higher than that of the melt inclusions but inconsistent with silicate melt evolution by assimilation of carbonatite. The melt inclusions are volatile-rich with 2.7 wt% to 8.7 wt% CO2 and 0.7 wt% to 10.1 wt% H2O, indicating that Oldoinyo Lengai is a hydrous system. This is contrary to the long-held assumption that Oldoinyo Lengai is relatively anhydrous, which is based on the observation that natrocarbonatite lavas are water-poor. We argue that natrocarbonatites are derived from hydrous carbonate liquid that degas H2O at low pressure. The silicate glass data show that H2O concentration is negatively correlated with incompatible element enrichment, which we attribute to crystallization of the melt in response to decompression degassing of H2O. The eruptive cycle at Oldoinyo Lengai reflects changes in bulk silicate magma viscosity due to extensive H2O-driven crystallization and explosive eruptions occur when volatiles (i.e. H2O>CO2 gas, and carbonate liquid) cannot separate from the crystal-rich nephelinite magma. Melt H2O content decreases as a function of pressure; however CO2 concentration in the melt inclusions is buffered by the presence of immiscible carbonate liquid. CO2 content increases with melt evolution parameters (e.g. increasing (Na+K)/Al) due to enhanced solubility with alkali enrichment and SiO2 depletion in the melt. The matrix glasses and evolved melt inclusions, on the other hand, experienced low pressure (<50 MPa) CO2 degassing and were not buffered by a coexisting carbonate liquid. Whereas the melt inclusions are the most CO2-rich yet identified, their CO2/Nb ratios are without exception lower than that in MORB, indicating that a volatile-rich mantle source is not required for Oldoinyo Lengai.