Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.596w&link_type=abstract
Meteoritics, vol. 30, no. 5, page 596
Mathematics
Logic
2
Acapulcoites, Cosmogenic Isotopes, Exposure Ages, Lodranites, Meteorite, Allen Hills 81261, Queen Alexander Range 93148, Noble Gases
Scientific paper
We continue our comprehensive studies of the cosmic ray exposure history of lodranites [1] to include new noble gas measurements in the QUE 93148 lodranite and the ALH 81261 acapulcoite. In addition, we model the production rates of cosmogenic nuclides in lodranites and acapulcoites using the HERMES high energy transport code [2], in order to test whether conventional production rates can be extrapolated to this group of small meteoroids which reveal very large values of the shielding parameter 22Ne/21Ne (Table 1). The model calculations are based on the same excitation functions of p- and n-induced reactions as used in recent calculations [3,4]. We extended our studies to acapulcoites, since petrologic, mineralogic, and O-isotopic investigations [5] as well as chemical investigations [6] suggest that lodranites and acapulcoites are residues of varying degree of partial melting, consistent with an origin on a common parent body. Whether a collisional event on the common parent body ejected both types of meteorites can be investigated by an analysis of the transfer times to Earth, specifically their cosmic-ray exposure ages. Because the contents of trapped He, Ne, and Ar in lodranites and acapulcoites are very low we can derive reliable cosmogenic noble gas contents. Using the composition-adjusted production rates for cosmogenic noble gases in achondrites [7], and adopting the shielding-parameter dependence for H-chondrites the exposure ages of [1] are obtained. For lodranites these exposure ages overlap those calculated [8] from 26Al and 10Be measurements. For the acapulcoites our exposure ages agree with those [5] calculated with the Graf-model [9], as well as with the shielding-independent exposure age for Acapulco that is based on the 36Cl-36Ar method[10]. The large spread in the exposure ages can be attributed to the highly variable target element abundances, as multiple measurements on several aliquots show unusually large variations. The fact that the average exposure age T3 is 44% higher than the average T21 in the case of lodranites, and 13% higher for acapulcoites, cannot be explained by the variable chemical composition. Studies of Lodran mineral separates [1] suggest that the Lodran parent meteoroid was enriched in metal (up to 65%). This means that the Ne production rates for bulk samples are overestimated. The average cosmogenic noble gas contents, average shielding, and main target element abundances of lodranites and acapulcoites (Table 1) coincide fairly well. The slightly higher Mg abundance in lodranites translates into a higher Ne production rate and therefore a lower exposure age T21. The higher Ca abundance in acapulcoites leads to a higher Ar production rate and, therefore, a lower T38. The production of cosmogenic 3He, however, is quite insensitive to variations in chemical composition and shielding. We observe good agreement between the T3 values of lodranites (6.2+/-0.5Ma) and acapulcoites (6.8+/-1.1Ma) and conclude that a single collisional event can account for the currently known exposure ages of lodranites and acapulcoites. Acknowledgments: We thank NASA and the MWG for the meteorite samples. This work was supported by the Swiss NSF and partially by the Deutsche Forschungsgemeinschaft. References: [1] Weigel A. et al. (1994) Meteoritics, 29, 548. [2] Cloth P. et al. (1988) JUEL-2203. [3] Michel R. et al. (1995) Planet. Space Sci., in press. [4] Herpers U. et al. (1995) Planet. Space Sci., in press. [5] McCoy T. J. et al. (1995) GCA, submitted. [6] Zipfel J. and Palme H. (1993) LPS XXIV, 1579. [7] Eugster O. and Michel Th. (1995) GCA, 59, 177. [8] Xue et al. (1994) LPS XXV, 1523. [9] Graf Th. et al. (1990) GCA, 54, 2521. [10] Graf Th. et al., this volume. Table 1 shows average cosmogenic noble gas contents, main target element abundances, and exposure ages of eight lodranites and four acapulcoites.
Eugster Otto
Marti Kurt
Michel Rigo
Weigel Andreas
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