Fission track age and cooling rate of the Marjalahti pallasite |
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| Authors: | P Pellas C Perron G Crozaz VP Perelygin SG Stetsenko |
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| Institution: | 1. Laboratoire de Minéralogie du Muséum et C.N.R.S., 61 rue Buffon, 75005 Paris France;2. Joint Institute for Nuclear Research, Laboratory of Nuclear Reactions, Dubna U.S.S.R. |
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| Abstract: | Nuclear tracks were studied in olivine and merrillite (phosphate previously called whitlockite) from the Marjalahti pallasite. The merrillite contains an important fission contribution due mainly to the spontaneous decay of now extinct244Pu. The U contents of 29 merrillite grains range from 60 to 140 ppb (median value: 85 ppb). Assuming a reasonable fractionation temperature of ～ 1750 K for the pre-pallasitic material, a lower limit of ～ 5 K/Myr is obtained for the cooling rate, in strong contrast with the previous metallographic result (～ 0.5 K/Myr). This disagreement, together with those observed in the case of mesosiderites, strengthens the need for a revision of the metallographic method of retracing the cooling histories of meteorites, as suggested by Narayan and Goldstein 31]. |
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