Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.356g&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 356
Other
Chondrites, Cosmogenic Nuclides, Exposure Ages, Knyahinya, Metal, Production Rates, St. Severin
Scientific paper
A model for the production of cosmogenic nuclides in chondrites based on a production rate equation with two free parameters (A(sub)i, B(sub)i) for each nuclide i has been presented earlier [1]. The model parameters were determined by fitting measured depth profiles in the large Knyahinya chondrite [2]. New measurements of ^26Al, ^10Be, and ^36Cl in metal separates and Kr in bulk samples of Knyahinya have been carried out [3,4]. Here, we present model calculations for these newly measured nuclides. The A(sub)i and B(sub)i parameters for the production of ^10Be, ^36Cl from metal and ^78,81,83Kr in bulk samples were fitted to the measured depth profiles in Knyahinya and are given in Table 1. The model predictions for ^36Cl are in good agreement with the nearly constant activities observed for St. Severin [5], if corrections for the high Ni content in the metal of this meteorite are applied. However, ^78,83Kr profiles measured in St. Severin [6] show larger variations with shielding depth than predicted by the model. This may partly be due to the complex preatmospheric shape of St. Severin and to uncertainties arising from corrections for the low Rb content of this meteorite. The model predicts that any two production rate ratios are correlated in a way independent of the size and shape of a particular meteoroid. The calculated correlation line in a ^81Kr/^83Kr vs. ^22Ne/^21Ne diagram agrees well (+-3%) with the line obtained from data of many different chondrites [7]. On the other hand, the calculated correlation of ^78Kr/^83Kr vs. ^22Ne/^21Ne yields ^78Kr/^83Kr ratios that are 5-10% lower compared to those observed in samples from near surface locations in small meteorites (^22Ne/^21Ne>1.15) [7]. A(sub)i and B(sub)i parameters for the production of ^36Ar in metal (Table 1) can be derived from those of ^36Cl because >80% of ^36Ar in metal is produced indirectly via decay of ^36Cl and the ratio of the cross-sections for the production of ^36Cl and ^36Ar from Fe is quite constant. Parameters for ^38Ar in metal are obtained from those of ^36Ar by using the known shielding dependence of the ^36Ar/^38Ar ratios. A comparison of the calculated production rates of ^38Ar in bulk samples and metal separates indicates that the production rate ratio P(sub)38(Ca,K)/P(sub)38(Fe,Ni) increases by ~40% with increasing shielding (^22Ne/^21Ne ratios of 1.2 and 1.06, respectively). ^10Be activities in metal show a small decrease with increasing shielding depth except for one unusually low value in a near surface sample. The resulting value for B(sub)10 is 0.77. Model calculations based on ^10Be measurements in the Grant iron meteorite [8] yielded B(sub)10=0, indicating that ^10Be is produced solely by high energy particles. Therefore, the Grant model should be applicable to the Knyahinya data if the interaction mean free path for high energy particles is adjusted properly. The activities predicted in this way show a much larger decrease with increasing shielding (~50%) than observed. A possible solution to the problem may be that the preatmospheric radius of Grant was larger than the 40 cm derived in [9]. References: [1] Graf Th. et al. (1990) GCA, 54, 2521-2534. [2] Graf et al. (1990) GCA, 54, 2511-2520. [3] Reedy et al. (1993) LPSC XXIV, 1195-1196. [4] Toe et al. (1993) Meteoritics, this issue. [5] Nishiizumi et al. (1989) LPSC XIX, 305-312. [6] Lavielle B. and Marti K. (1988) LPSC XVIII, 565-572. [7] Eugster O. (1988) GCA, 52, 1649-1662. [8] Graf et al. (1987) Nucl. Instr. and Methods, B29, 262-265. [9] Signer P. and Nier A. O. (1960) JGR, 65, 2947-2964. Table 1, which appears here in the hard copy, shows model parameters.
Caffee Marc W.
Finkel Robert C.
Gilabert Eric
Graf Th.
Lavielle Bernard
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