THE ORBIT AND EXPOSURE HISTORY OF THE PIPLIA-KALAN EUCRITE

Cosmogenic radionuclides, particle tracks and rare gases have been mea sured in two fragments of the Piplia Kalan eucrite that fell in Rajast han, India on 1996 June 20. The cosmic-ray exposure age of the meteori te is calculated to be 23 Ma, which is similar to ages of some other e ucrites. The track density in feldspars and pyroxenes varies between 0 .2 x 10(6) to similar to 4.5 x 10(6) cm(-2) The mass ablation of the m eteorite, based on the distribution of track density in near-surface s amples of the two fragments, is calculated to be similar to 75%, which corresponds to an entry velocity of similar to 17 km/s. The orbital p arameters of the eucrite have been computed from the radiant of the me teor trail and the geocentric velocity. The best estimates are a = 2.4 7 AU, e = 0.62 and i = 7.54 degrees, which is similar to the orbital e lements of other meteorites, most of which have been inferred to origi nate within 2.6 AU of the Sun. The activity of the radionuclide Al-26 agrees with the expected production rate; whereas the shortlived radio nuclides Na-22, Mn-54, Sc-46 etc. have levels that are consistent with the galactic cosmic-ray fluxes that are expected during the solar min imum period before the time of fall. All the cosmogenic effects (ie., radio-and stable-nuclides and particle tracks) are consistent with the meteoroid having had a simple, one-stage exposure history in interpla netary space. Lower radiogenic ages of U, Th-He (0.7 Ga) and K-Ar (3.6 Ga) indicate severe losses of He-4 and Ar-40, as observed in most euc rites. A Pu-Xe age, concordant with Angra dos Reis, shows that Piplia belongs to the ''old'' eucrite group.