Computer Science
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.512g&link_type=abstract
Meteoritics, vol. 30, no. 5, page 512
Computer Science
2
Argon, Chlorine, Chondrites, Exposure Ages, Isotopes, Cosmogenic, Meteorites, Acapulco, Nassirah
Scientific paper
Metal separates were prepared to determine ^36Cl-^36Ar exposure ages for six H4 p.m. falls (with reported bulk exposure ages of 4 to 10Ma), for ten H5 a.m. falls (T(sub)e = 4-10 Ma) and for the Acapulco meteorite (T(^36Cl-^36Ar)= 5.7 Ma). This dating method uses production rate ratios P(^36Cl)/P(^36Ar) and is independent of the shielding-sensitive absolute production rates. It is also known that for protons the production rate ratio is rather insensitive to changes in the energy spectrum; the dependence of this ratio for secondary neutrons is at present less understood. First results were already reported [1]. The cosmic-ray-produced ^3He/^38Ar ratios show a bimodal distribution with two clusters at about 15 and about 9 (Fig. 1). About half of the ^3He is produced via ^3H which is known to diffuse in metal at relatively low temperatures. Therefore, Fig. 1 provides evidence for a quasi-continuous loss of ^3H from such metals. If this loss mechanism is due to solar heating, perihelia <1 AU are indicated for these meteorites. Losses are prominent for H5 a.m. falls, but not for H4 p.m. falls. The orbital implications are consistent with those already known from the time-of-fall parameter (p.m. falls / total falls) which was used in the selection of the H4,H5 sample sets [2]. The exposure age histograms of both H groups show the well known clusters at about 7 Ma. The width of the exposure age peaks differ, however, and the collisional break-up event can be further constrained. Except for Nassirah, all members of the H4 p.m. group fall into the range 7.0 +/- 0.3 Ma. Bulk rock ages (8.2-9.3 Ma) [3] as well as the ^36Cl-^36Ar age (8.3 Ma) of Nassirah are higher and may indicate that this meteorite does not belong to the collisional event. We observe a small but systematic difference in calculated exposure ages by the ^36Cl-^36Ar method, when compared with ages obtained by conventional noble gas production rates. This shift (about 10%) does not appear to be dependent on shielding conditions and, since He, Ne and Ar production rates are essentially based on the ^81Kr (2.1 Ma) radionuclide, can also not be due to a change in the cosmic ray flux. A cross-calibration with the ^36Cl-^36Ar results may provide a means to further refine the exposure age histograms of meteorites. Acknowledgments: This work was supported by IGPP-LLNL and by NASA. References: [1] Graf Th. et al. (1995) LPS XXVI, 487-488. [2] Graf Th. and Marti K. (1995) JGR Planets, submitted. [3] Schultz L. et al. (1990) 17th Intl. Conf. Cosmochron. Isotope Geol. Fig. 1: Distribution of cosmic-ray produced ^3He/^38Ar in metal separates of H4 p.m. falls and H5 a.m. falls. The production rate ratio of ^3He/^38Ar in metal (16 +/- 2) is quite constant over a wide range of shielding conditions. Therefore, the bimodal distribution indicates an almost complete loss of ^3H as opposed to loss of ^3He.
Caffee Marc W.
Finkel Robert C.
Graf Th.
Marti Kurt
Nishiizumi Kuni
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