EXPOSURE HISTORY OF THE PEEKSKILL (H6) METEORITE

The Peekskill H6 meteorite fell on 1992 October 9. We report extensive measurements of cosmic-ray produced stable nuclides of He, Ne, and Ar , of the radionuclides Na-22, Co-60, C-14, Cl-36, Al-26, and Be-10, an d of cosmic-ray track densities. After correction for shielding via th e Ne-22/Ne-21 ratio, the concentrations of cosmic-ray produced He-3, N e-21 and Ar-38 give an average exposure age of 25 Ma, which is conside red to be a lower limit on the true value. The Be-10/Ne-21 age is 32 M a and falls onto a peak in the H-chondrite exposure age distribution. The activities of Al-26, C-14, Cl-36, and Be-10 are all close to the m aximum values expected for H-chondrites. Together with cosmic-ray trac k densities and the Ne-22/Ne-21 ratio, these radionuclide data place t he samples at a depth >20 cm in a meteoroid with a radius >40 cm. In c ontrast, the Co-60 activity requires a near-surface location and/or a much smaller body. Calculations show that a flattened geometry for the Peekskill meteoroid does not explain the observations in the context of a one-stage irradiation. A two-stage model can account for the data . We estimate an upper bound of 70 cm on the radius of the earlier sta ge of irradiation and conclude that Peekskill's radius was <70 cm when it entered the Earth's atmosphere. This size limit is somewhat smalle r than the dynamic determinations (Brown et al., 1994).