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  • 1
    Publication Date: 2019-01-08
    Description: Stable isotope compositions of methane (δ13C and δD) and of short-chain alkanes are commonly used to trace the origin and fate of carbon in the continental crust. In continental sedimentary systems, methane is typically produced through thermogenic cracking of organic matter and/or through microbial methanogenesis. However, secondary processes such as mixing, migration or biodegradation can alter the original isotopic and composition of the gas, making the identification and the quantification of primary sources challenging. The recently resolved methane 'clumped' isotopologues Δ13CH3D and Δ12CH2D2 are unique indicators of whether methane is at thermodynamic isotopic equilibrium or not, thereby providing insights into formation temperatures and/or into kinetic processes controlling methane generation processes, including microbial methanogenesis. In this study, we report the first systematic use of methane Δ13CH3D and Δ12CH2D2 in the context of continental sedimentary basins. We investigated sedimentary formations from the Southwest Ontario and Michigan Basins, where the presence of both microbial and thermogenic methane was previously proposed. Methane from the Silurian strata coexist with highly saline brines, and clumped isotopologues exhibit large offsets from thermodynamic equilibrium, with Δ12CH2D2 values as low as -23‰. Together with conventional δ13C and δD values, the variability in Δ13CH3D and Δ12CH2D2 to first order reflects a mixing relationship between near-equilibrated thermogenic methane similar to gases from deeper Cambrian and Middle Ordovician units, and a source characterized by a substantial departure from equilibrium that could be associated with microbial methanogenesis. In contrast, methane from the Devonian-age Antrim Shale, associated with less saline porewaters, reveals Δ13CH3D and Δ12CH2D2 values that are approaching low temperature thermodynamic equilibrium. While microbial methanogenesis remains an important contributor to the methane budget in the Antrim Shale, it is suggested that Anaerobic Oxidation of Methane (AOM) could contribute to reprocessing methane isotopologues, yielding Δ13CH3D and Δ12CH2D2 signatures approaching thermodynamic equilibrium.
    Type: Article , PeerReviewed
    Format: text
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  • 2
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Late Jurassic dioritic plutons from the Bristol Lake region of the eastern Mojave Desert share several geochemical attributes with high-alumina basalts, continental hawaiite basalts, and high-K are andesites including: high K2O concentrations; high Al2O3 (16–19 weight %); elevated Zr/TiO2; LREE (light-rare-earth-element) enrichment (La/YbCN=6.3–13.3); and high Nb. Pearce element ratio analysis supported by petrographic relations demonstrates that P, Hf, and Zr were conserved during differentiation. Abundances of conserved elements suggest that dioritic plutons from neighboring ranges were derived from similar parental melts. In the most voluminous suite, correlated variations in elemental concentrations and (87Sr/86Sr)i indicate differentiation by fractional crystallization of hornblende and plagioclase combined with assimilation of a component characterized by abundant radiogenic Sr. Levenberg-Marquardt and Monte Carlo techniques were used to obtain optimal solutions to non-linear inverse models for fractional crystallization-assimilation processes. Results show that the assimilated material was chemically analogous to lower crustal mafic granulites and that the mass ratio of contaminant to parental magma was on the order of 0.1. Lack of enrichment in 18O with differentiation is consistent with the model results. Elemental concentrations and O, Sr, and Nd isotopic data point to a hydrous REE-enriched subcontinental lithospheric source similar to that which produced some Cenozoic continental hawaiites from the southern Cordillera. Isotopic compositions of associated granitoids suggest that partial melting of this subcontinental lithosphere may have been an important process in the development of the Late Jurassic plutonic arc of the eastern Mojave Desert.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract A siliceous dolomitic marble xenolith within a mid-crustal Jurassic diorite exhibits mineralogical and stable isotopic evidence for infiltration of water-rich fluid. Adjacent to endoskarn which bounds the xenolith, forsterite has been replaced by clinohumite as a result of a devolatilization reaction driven by addition of aqueous fluid and extraction of heat. Isotopic compositions of calcite also record infiltration of aqueous fluid concentrated near contacts with endoskarn. Marble calcite δ18OSMOW values range from +19.5‰ at the center of the xenolith to +12.6‰ adjacent to endoskarn. Calcite δ13CPDB decreases sympathetically from +3.0 to +1.4‰. The calculated equilibrium composition of coexisting C−O−H−F fluid and a quantitative characterization of the whole-rock reaction which produced clinohumite and consumed graphite are used to place inequality contraints on the composition of the infiltrated fluid. Continnum mechanical transport models based on the resulting fluid compositions suggest that a small time-integrated Darey flux of 44 cm3/cm2 coupled with diffusive transport in the fluid was sufficient to produce both the isotopic shifts and the net-transfer reactions evidenced in the xenolith. The calculations demonstrate the importance of graphite as an indicator of time-integrated flux. The maximum possible CH4 content of the infiltrated fluid is sufficiently high to impart a 2.0‰ uncertainty in the δ13C of the fluid. The isotopic composition of the fluid is consistent with a magmatic origin when this uncertainty is taken into account.
    Type of Medium: Electronic Resource
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  • 4
    Publication Date: 1992-03-01
    Description: Late Jurassic dioritic plutons from the Bristol Lake region of the eastern Mojave Desert share several geochemical attributes with high-alumina basalts, continental hawaiite basalts, and high-K are andesites including: high K_2O concentrations; high Al_2O_3 (16–19 weight %); elevated Zr/TiO_2; LREE (light-rare-earth-element) enrichment (La/Yb_CN=6.3–13.3); and high Nb. Pearce element ratio analysis supported by petrographic relations demonstrates that P, Hf, and Zr were conserved during differentiation. Abundances of conserved elements suggest that dioritic plutons from neighboring ranges were derived from similar parental melts. In the most voluminous suite, correlated variations in elemental concentrations and (87Sr/86Sr)_i indicate differentiation by fractional crystallization of hornblende and plagioclase combined with assimilation of a component characterized by abundant radiogenic Sr. Levenberg-Marquardt and Monte Carlo techniques were used to obtain optimal solutions to non-linear inverse models for fractional crystallization-assimilation processes. Results show that the assimilated material was chemically analogous to lower crustal mafic granulites and that the mass ratio of contaminant to parental magma was on the order of 0.1. Lack of enrichment in 18O with differentiation is consistent with the model results. Elemental concentrations and O, Sr, and Nd isotopic data point to a hydrous REE-enriched subcontinental lithospheric source similar to that which produced some Cenozoic continental hawaiites from the southern Cordillera. Isotopic compositions of associated granitoids suggest that partial melting of this subcontinental lithosphere may have been an important process in the development of the Late Jurassic plutonic arc of the eastern Mojave Desert. ©1992 Springer-Verlag
    Print ISSN: 0010-7999
    Electronic ISSN: 1432-0967
    Topics: Geosciences
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  • 5
    Publication Date: 2011-02-01
    Description: Stable isotopes record the evolution of planetary systems, beginning with stars coalescing from molecular clouds, followed by the nucleosynthesis of elements in stars, and proceeding to the accretion and differentiation of planets. Current stable isotope measurements range in scale from isotopic mapping of the Milky Way Galaxy with spectrographs on telescopes to the analysis of stardust with ion probes.
    Print ISSN: 1811-5209
    Electronic ISSN: 1811-5217
    Topics: Geosciences
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  • 6
    Publication Date: 1992-08-01
    Description: A siliceous dolomitic marble xenolith within a mid-crustal Jurassic diorite exhibits mineralogical and stable isotopic evidence for infiltration of water-rich fluid. Adjacent to endoskarn which bounds the xenolith, forsterite has been replaced by clinohumite as a result of a devolatilization reaction driven by addition of aqueous fluid and extraction of heat. Isotopic compositions of calcite also record infiltration of aqueous fluid concentrated near contacts with endoskarn. Marble calcite δ18O_SMOW values range from +19.5‰ at the center of the xenolith to +12.6‰ adjacent to endoskarn. Calcite δ13C_PDB decreases sympathetically from +3.0 to +1.4‰. The calculated equilibrium composition of coexisting C−O−H−F fluid and a quantitative characterization of the whole-rock reaction which produced clinohumite and consumed graphite are used to place inequality contraints on the composition of the infiltrated fluid. Continnum mechanical transport models based on the resulting fluid compositions suggest that a small time-integrated Darey flux of 44 cm3/cm2 coupled with diffusive transport in the fluid was sufficient to produce both the isotopic shifts and the net-transfer reactions evidenced in the xenolith. The calculations demonstrate the importance of graphite as an indicator of time-integrated flux. The maximum possible CH_4 content of the infiltrated fluid is sufficiently high to impart a 2.0‰ uncertainty in the δ13C of the fluid. The isotopic composition of the fluid is consistent with a magmatic origin when this uncertainty is taken into account. ©1992 Springer-Verlag
    Print ISSN: 0010-7999
    Electronic ISSN: 1432-0967
    Topics: Geosciences
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  • 7
    Publication Date: 2019-07-13
    Description: The distribution of isotopes within O2 molecules can be rapidly altered when they react with atomic oxygen. This mechanism is globally important: while other contributions to the global budget of O2 impart isotopic signatures, the O(3P) + O2 reaction resets all such signatures in the atmosphere on subdecadal timescales. Consequently, the isotopic distribution within O2 is determined by O3 photochemistry and the circulation patterns that control where that photochemistry occurs. The variability of isotopic ordering in O2 has not been established, however. We present new measurements of 18O18O in air (reported as delta36 values) from the surface to 33 km altitude. They confirm the basic features of the clumped-isotope budget of O2: Stratospheric air has higher delta36 values than tropospheric air (i.e., more 18O18O), reflecting colder temperatures and fast photochemical cycling of O3. Lower delta36 values in the troposphere arise from photochemistry at warmer temperatures balanced by the influx of high-delta36 air from the stratosphere. These observations agree with predictions derived from the GEOS-Chem chemical transport model, which provides additional insight. We find a link between tropical circulation patterns and regions where delta36 values are reset in the troposphere. The dynamics of these regions influences lapse rates, vertical and horizontal patterns of O2 reordering, and thus the isotopic distribution toward which O2 is driven in the troposphere. Temporal variations in delta36 values at the surface should therefore reflect changes in tropospheric temperatures, photochemistry, and circulation. Our results suggest that the tropospheric O3 burden has remained within a +/-10 percent range since 1978.
    Keywords: Meteorology and Climatology; Geophysics
    Type: GSFC-E-DAA-TN38469 , Journal of Geophysical Research: Atmospheres (ISSN 2169-897X); 121; 20; 12541-12559
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  • 8
    Publication Date: 2019-07-13
    Description: Doppler weather radar imaging enabled the rapid recovery of the Sutter's Mill meteorite after a rare 4-kiloton of TNT-equivalent asteroid impact over the foothills of the Sierra Nevada in northern California. The recovered meteorites survived a record high-speed entry of 28.6 kilometers per second from an orbit close to that of Jupiter-family comets (Tisserand's parameter = 2.8 +/- 0.3). Sutter's Mill is a regolith breccia composed of CM (Mighei)-type carbonaceous chondrite and highly reduced xenolithic materials. It exhibits considerable diversity of mineralogy, petrography, and isotope and organic chemistry, resulting from a complex formation history of the parent body surface. That diversity is quickly masked by alteration once in the terrestrial environment but will need to be considered when samples returned by missions to C-class asteroids are interpreted.
    Keywords: Lunar and Planetary Science and Exploration; Instrumentation and Photography; Earth Resources and Remote Sensing
    Type: ARC-E-DAA-TN8799 , Science; 338; 6114; 1583-1587
    Format: application/pdf
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