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  • 2015-2019
  • 1990-1994  (6)
  • 1955-1959
  • 1991  (6)
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Keywords
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  • 2015-2019
  • 1990-1994  (6)
  • 1955-1959
Year
  • 1
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    Precise chronology of the last interglacial period: 234U-230Th data from fossil coral reefs in the Bahamas (1991)
    Geological Society of America
    Details
    Publication Date: 1991-01-01
    Print ISSN: 0016-7606
    Electronic ISSN: 1943-2674
    Topics: Geosciences
    Published by Geological Society of America
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    PAPER CURRENT
  • 2
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    Correlated Si isotope anomalies and large C-13 enrichments in a family of exotic SiC grains (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: A hypothesis is presented to the effect that the distinctive morphological characteristics and comparatively simple Si isotope systematics identify the platy SiC crystals as a genetically related family, formed around a single isotopically heterogeneous presolar star on an association of related stars. The enrichments in C-13 and the Si isotope systematics of the platy SiC are broadly consistent with theoretical models of nucleosynthesis in low-mass, carbon stars on the ASG. The Si isotope array most plausibly reflects mixing between (Si-28)-rich material, inherited from a previous generation of stars, and material enriched in Si-29 and Si-30, produced in intershell regions by neutron capture during He-burning. The absence of a correlation between the Si and C isotopic compositions of the SiC suggests either episodic condensation of SiC, extending over several thermal pulses, in the atmosphere of a single star, or the derivation of the SiC from several stars characterized by different rates of C-13 production.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Earth and Planetary Science Letters (ISSN 0012-821X); 107; 4-Mar
    Format: text
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    NASA TECHNICAL REPORTS
  • 3
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    Diffusion of a multi-species component and its role in oxygen and water transport in silicates (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: The diffusion of a multispecies component is complicated by the different diffusion coefficient of each species and the interconversion reactions among the species. A diffusion equation is derived that incorporates the diffusive fluxes of all species contributing to the component's concentration. The effect of speciation on diffusion is investigated experimentally by measuring concentration profiles of all species developed during diffusion experiments. Data on water diffusion in rhyolitic glasses indicate that H2O molecules predominate over OH groups as the diffusing species at very low to high water concentrations. A simple theoretical relationship is drawn between the effective total oxygen diffusion coefficient and the total water concentration of silicates at low water content.
    Keywords: GEOPHYSICS
    Type: Earth and Planetary Science Letters (ISSN 0012-821X); 103; 4-Jan
    Format: text
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    NASA TECHNICAL REPORTS
  • 4
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    Origin of plagioclase-olivine inclusions in carbonaceous chondrites (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: The origin of plagioclase-olivine inclusions (POIs) from three CV chondrites and one ungrouped chondrite was investigated by examining the chemical, mineralogical, and isotopic characteristics of a group of POIs from these chondrites. Results of these analyses demonstrate that the mixing and the partial melting processes in these inclusions were superimposed on more ancient isotopically heterogeneous material. A comparison of the essential characteristics of POIs and CAIs suggests that the major processes leading to the formation of POIs (such as condensation, dust/gas fractionation, aggregation of chemically and isotopically disparate materials, and partial melting) are common to most CAIs and chondrules. A scenario for the origin of POIs is proposed, showing that the homogeneity of the final assemblage (whether a POI, a CAI, or a chondrite) is primarily a reflection of the thermal history rather than the nature of precursor materials.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geochimica et Cosmochimica Acta (ISSN 0016-7037); 55; 581-599
    Format: text
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    NASA TECHNICAL REPORTS
  • 5
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    Diffusion of water in rhyolitic glasses (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: The process of water diffusion in 11 natural rhyolitic glasses was considered using the results of measurements by FTIR spectroscopy of concentration profiles of both H2O and OH at different temperatures. The diffusion coefficients of H2O and OH, D(H2O) and D(OH), respectively, were calculated from concentration profiles. Results indicate that, at 400-530 C, the value of D(OH) is negligible compared to that of D(H2O), and that OH does not contribute significantly to the diffusion of total water even at water concentrations as low as 0.2 percent, which is minute compared to that of OH. In the activation energy vs radius plot, the D(H2O) followed the same trend as that of noble gases.
    Keywords: GEOPHYSICS
    Type: Geochimica et Cosmochimica Acta (ISSN 0016-7037); 55; 441-456
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    NASA TECHNICAL REPORTS
  • 6
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    Negative thermal ion mass spectrometry of osmium, rhenium, and iridium (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: This paper describes a technique for obtaining, in a conventional surface ionization mass spectrometer, intense ion beams of negatively charged oxides of Os, Re, and Ir by thermal ionization. It is shown that the principal ion species of these ions are OsO3(-), ReO4(-), and IrO2(-), respectively. For Re-187/Os-187 studies, this technique offers the advantage of isotopic analyses without prior chemical separation of Re from Os.
    Keywords: ATOMIC AND MOLECULAR PHYSICS
    Type: Geochimica et Cosmochimica Acta (ISSN 0016-7037); 55; 397-401
    Format: text
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    NASA TECHNICAL REPORTS
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