Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.270n&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 270
Other
9
Scientific paper
We report here preliminary results of cosmogenic radionuclides in lunar meteorite Calcalong Creek and shergottite LEW 88516 for study of exposure histories. Table 1 shows ^36Cl and ^10Be results for these two meteorites along with previous measurements of ^36Cl and ^10Be of SNC meteorites. The AMS measurements were performed at LLNL. Measured ^36Cl activities, in dpm/kg meteorite, were normalized to the target element concentration, dpm/kg (8Ca+Fe), for comparison and shown in the table. The ^36Cl saturation activity is ~22 +- 2 dpm/kg (8Ca+Fe) for 4-pi irradiation. Calcalong Creek: This is the first lunar meteorite found outside Antarctica (Hill et al., 1991; Marvin and Holmberg, 1992). ^36Cl and ^10Be activity levels are slightly (10-20%) higher than the production rate of these nuclides on the moon. One possibility is that the meteorite was ejected from near the surface (<70 g/cm^2) of the moon and transferred to the earth. The transition time from moon to earth was ~0.2 My. The other simple case is that the meteorite was ejected from deep (at least a few meters) in the moon, like Yamato 82192, and exposed to cosmic rays as a small body. The transition time in this case was ~2 My. The terrestrial age must be <70 ky for either case. Other cosmogenic nuclide measurements (in progress) are required to constrain the history further. LEW 88516: This meteorite was classified as a shergottite (Mason, 1991). The recovered mass is 13.2 g. We measured ^36Cl and ^10Be in 93.9 mg of homogenized bulk sample. All aspects of petrography and bulk chemical composition of LEW 88516 are remarkably similar to those of ALH 77005 (Boynton et al., 1992; Lindstrom et al., 1992). Since the ^10Be activities of ALH 77005 samples vary from 13.7 to 16.2 dpm/kg with increasing shielding depth (Nishiizumi et al., 1986a), the average of ^10Be in ALH 77005 is slightly lower than ^10Be in LEW 88516. The calculated ^10Be exposure age is ~3.0 My. The normalized ^36Cl activity of LEW 88516 is near saturation for a small object (no significant thermal neutron effect). On this assumption, the terrestrial age of the meteorite is shorter than 50 ky. The ^36Cl terrestrial age of ALH 77005 is ~0.2 My and in good agreement with the ^81Kr terrestrial age of (0.19 +- 0.07) My (Schultz and Freundel, 1986). LEW 88516 and ALH 77005 are separate falls. Probably, however, these two meteorites and Shergotty were ejected in the same event on the parent body, since they have same exposure age within error. Table 1, which in the hard copy appears here, shows the concentration of ^36Cl and ^10Be. References: Boynton W. V. et al. (1992) Lunar Planet. Sci. XXIII, 147-148. Hill D. H. et al. (1991) Nature 352, 614-617. Lindstrom M. M. et al. (1992) Lunar Planet. Sci. XXIII, 783-784. Marvin U. B. and Holmberg B. B. (1992) Lunar Planet. Sci. XXIII, 849- 850. Mason B. (1991) Antarctic Meteorite Newsletter 14 (2), 19. Nishiizumi K. et al. (1986a) Meteoritics 21, 472-473. Nishiizumi K. et al. (1986b) Geochim. Cosmochim. Acta 50, 1017- 1021. Pal D. K. et al. (1986) Geochim. Cosmochim. Acta 50, 2405-2409. Schultz L. and Freundel M. (1984) Meteoritics 19, 310.
Arnold James R.
Caffee Marc W.
Finkel Robert C.
Nishiizumi Kuni
Southon John
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