Computer Science
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.597w&link_type=abstract
Meteoritics, vol. 30, no. 5, page 597
Computer Science
4
Aluminum-26, Beryllium-10, Diogenites, Isotopes, Cosmogenic, Noble Gases, Shielding Effects
Scientific paper
Due to the attenuation of primary particles and the variations in secondary part fluxes with depth, production rates of cosmogenic nuclides are affected by the s shape of the irradiated object. The effects of shielding conditions on the produduction rates of noble gases can be estimated on the basis of the cosmogenic 22Ne/21Ne r [1]. For the production of cosmogenic radionuclides, shielding studies mainly fo on large meteorites like St. Severin [2], Knyahinya [3], Chico [4] and Jilin [5] estimated preatmospheric radii between 25 and 85 cm. The 10Be and 26Al production were also measured in three smaller meteorites, but the cosmogenic 22Ne/21Ne rat were obscured by large amounts of trapped neon [6]. Therefore we carried out a systematic study on the 10Be and 26Al activities as a function of the 22Ne/21Ne in 7 non-Antarctic and 15 Antarctic diogenite samples. Diogenites show exposure long enough (>10 Ma) to have reached saturation levels for 10Be and 26Al and are similar to ordinary chondrites with respect to the target element composition fo production of 10Be, 26Al and Ne isotopes. The measured 10Be and 26Al activities were normalized to average diogenite compo on the basis of ICP and XRF measurements and the experimental production rate eq of [7] and [8]. For the Antarctic samples with known terrestrial ages [9] correc were made for radioactive decay. In figure 1, the resulting 10Be and 26Al production rates are plotted against the 22Ne/21Ne ratios, which were measured on the same The solid lines represent the results of an exponential fitting procedure, from two samples were excluded: EET83246 because of SCR-produced 26Al and LEW88008 be of an anomalously low 26Al/10Be ratio, which is not yet understood. Figure 1 illustrates that the 10Be and 26Al production rates are similarly affect shielding conditions: both 10Be and 26Al decrease about 30 - 40% when going from objects with low 22Ne/21Ne ratios (<1.10) to small objects with high 22Ne/21Ne r (>1.25). Recently, model calculations were carried out for the 10Be and 26Al pro rates in diogenites [10]. The calculated P10 values agree within 10% with the experimental results, but the P26 values were significantly underestimated for sm medium-sized objects. The similar shielding sensitivity of 10Be and 26Al implies that the 26Al/10Be ra relatively independent of shielding conditions, resulting in an average of 3.2 + (which corresponds to a value of 3.0 for average L-chondrite composition). There the 26Al/10Be ratio is a useful parameter to identify meteorites with unusual ex histories and/or terrestrial histories as will be demonstrated elsewhere [11]. References: [1] Eugster O. (1988) GCA, 52, 1649-1662. [2] Tuniz et al. (1984) GC 1867-1872. [3] Graf Th. et al. (1990) GCA, 54, 2511-2520. [4] Garrison D. H. et (1992) Meteoritics, 27, 371-381. [5] Jilin Consortium Study (1985) EPSL, 72, 247 [6] Vogt S. (1988) Ph.D. thesis, Universit t zu K"ln. [7] Moniot R. K. et al. (1 GCA, 52, 499-504. [8] Hampel W. et al. (1980) GCA, 44, 539-547. [9] Nishiizumi K al. (1989) EPSL, 93, 299-313. [10] Michel R. et al. (1994) Planet. Space Sci., i press.
Lindner Louis
Loeken Th.
Schultz Ludolf
van der Borg Klaas
Welten Kees C.
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