Paul Pellas

Plutonium, uranium and rare earths in the phosphates of ordinary chondrites—the quest for a chronometer

Abstract The distributions of Pu, U and the REE (in particular Nd) in single crystals of the calcium phosphates, merrillite and apatite, of ordinary chondrites were studied to establish whether 244 Pu can be used to determine time differences in meteorite formation. Uranium was measured by an induced-fission track technique and the REE by secondary ion mass spectrometry. Limits on Pu concentrations (at the time of track retention) were inferred from fossil track measurements. It is argued here that most previous estimates of Pu concentrations, obtained from fossil tracks in phosphates, failed to correct adequately for spallation-induced tracks and were probably not correct. Plutonium is preferentially enriched in merrillite relative to apatite. In contrast, U concentrations are on the average 15 times higher in apatite than in merrillite. There are grain to grain variations, in the same phosphate phase from a given meteorite, for both elements. On the other hand, in the merrillite of a given meteorite, the REE concentrations are remarkably constant (typical grain to grain variations for type 5 and 6 chondrites are in most cases less than 10%). Average Nd concentrations in the merrillite of 15 different chondrites range from 96 to 164 ppm. REE abundances in apatite show more variations. Average Nd concentrations in the apatite of 13 chondrites range from 8 to 54 ppm and the ratios of Nd concentrations in merrillite relative to apatite range from 2.7 to 11. Despite the higher affinity of both Pu and the REE for merrillite than for apatite, there is no quantitative correlation between the abundances of these elements in merrillite grains from a given ordinary chondrite. Because of this lack of geochemical coherence between Pu and any of the REE, it is concluded that 244 Pu cannot be used to determine the relative formation times of chondrites. Bulk measurements that avoid the problems associated with fossil track measurements are discussed, however they have so far proven to be of little utility.

Fission track age and cooling rate of the Marjalahti pallasite

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 extinct 244 Pu. 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].

The formation age of the Brachina meteorite

Abstract Nuclear particle tracks were studied in various phases from the Brachina meteorite, which was classified until recently as a chassignite. Fission tracks due to the decay of244Pu(T12 = 82m.y.) were observed and indicate that Brachina formed ∼ 4.5 b.y. ago in a parent body which was most probably asteroidal in size. Contrary to what has been previously suggested [7], there is no need to postulate a Martian origin for this meteorite. This conclusion is supported by independent evidence obtained by other groups.