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Carbon isotope anatomy of a single graphite crystal in a metapelitic migmatite revealed by high-spatial resolution SIMS analysis
No full textHigh-spatial resolution carbon isotope analyses of natural graphite using secondary ion mass spectrometry (SIMS), together with conventional mass spectrometry techniques, demonstrate isotopic heterogeneity within single graphite crystals precipitated from a partially melted metamorphic rock. SIMS (13)C/(12)C measurements were calibrated using an internal graphite standard previously analyzed by conventional isotope ratio mass spectrometry, which gave a reproducibility of 0.3aEuro degrees (1 sigma) at a spatial resolution of 2-3 mu m. This resolution helped to identify an unusual carbon isotope distribution in a single graphite crystal from a metapelitic leucosome, showing remarkable core to rim variations with sharp delta(13)C steps up to 10aEuro degrees. The results suggest that the graphite crystal grew from one edge to other forming layers perpendicular to the c-axis. The sharp isotopic steps indicate the presence of disequilibrium carbon isotope zoning in graphite and points to the possible existence of carbon isotope sector zoning. Intra-crystalline carbon isotope disequilibrium in graphite is believed to have resulted from the difference in diffusivity between (12)C and (13)C in the growth medium to the interface of graphite precipitation in different growth sectors
Stardusts in Meteorites : Precursors of Planets
Full text linkPresolar circumstellar grains have been surveyed in 18 primitive meteorites. More than hundreds presolar grains have been identified. Presolar silicates are the most abundant species among presolar grains. The typical size is ~300 nm and the abundance is ~50 ppm in the most primitive chondrites. Main source of silicate presolar grains is from AGB and red giant stars. The average O-isotopic composition of presolar silicates is enriched in 17O relative to the solar composition. The counterpart to form solar isotope ratios having 17O-depleted compositions are missing in the chondrites. The missing matter would be supernovae ejecta but it is difficult to identify because the grain size is expected to be ~10 nm. ©2006 American Institute of PhysicsORIGIN OF MATTER AND EVOLUTION OF GALAXIES: International Symposium on Origin of Matter and Evolution of Galaxies 2005: New Horizon of Nuclear Astrophysics and Cosmolog
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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