Computer Science
Scientific paper
Sep 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002phdt........22k&link_type=abstract
Thesis (PhD). WASHINGTON UNIVERSITY, Source DAI-B 63/09, p. 4089, Mar 2003, 129 pages.
Computer Science
Scientific paper
Chromium isotopes are scientifically interesting for many reasons. For example, 53Mn decays to 53Cr which has allowed for the development of a fine-scale chronometer, 54Cr is a neutron-rich isotope believed to be generated in Type Ia supernovae which sheds light on local nucleosynthetic processes, and variations in 53Cr may be due to incorporation of differing amounts of live 53Mn. This 53Mn may be linearly, radially heterogeneous (Lugmair and Shukolyukov, 1998). Therefore, to investigate these issues, the isotopic composition of the implanted solar wind-bearing component was measured by thermal ionization mass spectrometry of acid etches of plagioclase grains from two Apollo 16 lunar soils. Small aliquots of the etches were spiked with Cr and Ca for isotope dilution. Spiking for Ca allows for the determination of etch depth and the Cr/Ca ratio which acts as a proxy for solar wind content. From these isotope dilution data, three important observations were made. First, the etch depth is in line with the solar wind implantation depths of noble gases (e.g. Eberhardt et al., 1970). Second, the Cr/Ca ratio falls off quickly with depth, peaks in the Etch 1 fraction of 62281 at 0.0006 and 0.0009 for 60601 and returns to the lunar Cr/Ca ratio of 0.00007 by weight by Etch 5. The coronal ratio is 0.224 (Anders and Grevesse, 1989). Third, the absolute abundance of Cr peaks in Etch 1 and falls off with progressive etches. Therefore, it is inferred that the solar wind-bearing component has been successfully isolated from the background lunar signature. The isotopic composition of this component is 3.9 ± 0.4 ɛ53Cr and 8.5 ± 0.6 ɛ54Cr for 62281 and 4.5 ± 0.1 and 15.0 ± 0.3, respectively, for 60601. These data are surprising. It will be shown that contributions from contamination, spallation and isobaric interferences are unlikely to be significant. The Cr data presented here are not linearly mass-dependent. Therefore, the linear, mass-dependent processes of volatilization, sputtering and vapor deposition are inadequate to explain these results. Alternative theories and resultant implications are discussed with emphasis on the hypothesis that these Cr results are attributable to spallation in the solar atmosphere.
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