Exposure history of the Peekskill (H6) meteorite

Abstract

Abstract— 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 22 Na, 60 Co, 14 C, 36 Cl, 26 Al, and 10 Be, and of cosmic‐ray track densities. After correction for shielding via the 22 Ne/ 21 Ne ratio, the concentrations of cosmic‐ray produced 3 He, 21 Ne and 38 Ar give an average exposure age of 25 Ma, which is considered to be a lower limit on the true value. The 10 Be/ 21 Ne age is 32 Ma and falls onto a peak in the H‐chondrite exposure age distribution. The activities of 26 Al, 14 C, 36 Cl, and 10 Be are all close to the maximum values expected for H‐chondrites. Together with cosmic‐ray track densities and the 22 Ne/ 21 Ne ratio, these radionuclide data place the samples at a depth >20 cm in a meteoroid with a radius >40 cm. In contrast, the 60 Co 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 stage of irradiation and conclude that Peekskill's radius was <70 cm when it entered the Earth's atmosphere. This size limit is somewhat smaller than the dynamic determinations (Brown et al. , 1994).