Computer Science
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.556n&link_type=abstract
Meteoritics, vol. 30, no. 5, page 556
Computer Science
5
Cosmogenic Nuclides, Exposure Age, Meteorites, Dongling, Iron, Nantan, Ningbo, Noble Gases
Scientific paper
Cosmogenic radionuclides have been measured in the iron meteorites Dongling and Nantan. Both meteorites are classified as group III CD and are assumed to belong to the same fall in southern China. Weber et al [1] measured the noble gas concentrations in these meteorites and found that Nantan contains the lowest cosmogenic noble gases ever measured. Assuming that Dongling and Nantan are from the same fall, the low noble gas concentrations in Nantan can be the result of either extremely heavy shielding in the meteoroid or break up of the Nantan and Dongling parent body less than 20,000 years ago [1]. To investigate cosmogenic nuclide production systematics in large objects and to constrain the relationship between the Nantan and Dongling meteorites, we obtained aliquots of samples from Mainz. The cosmogenic radionuclides 10Be and 26Al were measured by AMS at the Lawrence Livermore National Laboratory in Dongling (77 mg) and in two pieces of Nantan ("A" 5192 mg, "S" 4937 mg) as well as in the Ningbo (72 mg) IV A iron meteorite for comparison. Preliminary results are shown in Table 1 along with noble gas concentrations and ratios obtained by Weber et al [1]. Measurements of 36Cl are in progress. Like the cosmogenic noble gases, the concentrations of 10Be and 26Al in both fragments of Nantan are also the lowest activities ever measured. Nantan "S" contains about 70 % lower activity than Nantan "A" although the noble gas concentrations in the two fragments are not distinct. The 10Be and 26Al concentrations in Nantan "A" are a factor of 1670 and in Nantan "S" are a factor of 2610 lower than those in Dongling. The ratios of 26Al and 10Be between the two meteorites are slightly lower than the ratios observed for the noble gases 21Ne (factor of 3000) and 38Ar (4900). However, since the concentrations of cosmogenic 21Ne and 38Ar are similar to background levels, the difference may not be significant. Although the 4He/21Ne ratio has been widely used for meteorite bombardment models to obtain shielding conditions [2, 3] , there is a lack of data for the high ratio which was found for Nantan (1100+/-300). The observed 10Be/26Al ratios are nearly constant over a wide range of 4He/21Ne ratios. According to Masarik and Reedy's theoretical calculation (priv. comm.), Nantan "A" and "S" were shielded at a depth of 150 - 175 cm in the meteoroid. The 21Ne-26Al exposure ages are 68 My for Dongling and 113 My for Ningbo based on a production rate ratio, R = P26Al/P21Ne, of 0.38 [4]. The value of R is rather uncertain and could be up to 50 % higher than the 0.38 value. A higher R would make the 21Ne-26Al exposure ages up to 50 % higher which would bring them into agreement with 4He exposure ages of these meteorites. The 21Ne-26Al ages of Nantan calculated using R = 0.38 are 31 ("A") and 59 My ("S") respectively, since the 21Ne concentration was an average of the two fragments [1]. The present study supports the interpretation that Dongling and Nantan might have originated in the same fall and eliminates the possibility of recent breakup of the meteoroid. References: [1] Weber H. W. et al. (1986) Geochemistry, 5, 320-330. [2] Voshage H. (1984) EPSL, 71, 181-194. [3] Honda M. (1988) Meteoritics, 23, 3-12. [4] Hampel W. and Schaeffer O. A. (1979) EPSL, 42, 348-358.
Caffee Marc W.
Finkel Robert C.
Nishiizumi Kuni
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