Cosmic-ray production of tungsten isotopes in lunar samples and meteoritesand its implications for Hf-W cosmochemistry

Excesses and deficiencies in W-182 in meteorites and lunar samples relative to the terrestrial W-182 atomic abundance have been assigned to the decay of Hf-182 (t(1/2) = 9 Ma) and have been used to date metal-silicate fractio nation events in the early solar system. Because the effects are very small , production and burn-out of tungsten isotopes by cosmic ray interactions a re a concern in such studies. Masarik [J. Masarik, Contribution of neutron- capture reactions to observed tungsten isotopic ratios, Earth Planet. Sci. Lett. 152 (1997) 181-185] showed that neutron-capture reactions on tungsten isotopes can account at best for a minor part of the observed deficit of W -182 in Toluca and other iron meteorites. On the other hand, in lunar sampl es and stony meteorites the production of W-182 from Ta-181 may become cruc ial. Here, we calculate this contribution as well as production and consump tion of W182-186 by other neutron-induced reactions. The neutron fluence of each sample is estimated by its nominal cosmic-ray exposure age deduced fr om noble gas data. This approach overestimates the true cosmogenic W isotop ic shifts for samples that might have been irradiated very close to the reg olith surface. A quantitative estimate is often also hampered by a lack of Ta data. Despite these reservations, it appears that in many lunar samples neutron-capture on Ta has caused a large part of the observed W-182 excess. On the other hand, in some samples, especially those with very low exposur e ages, clearly only a minor or even negligible fraction of the W-182 exces s can be cosmogenic. Therefore, the conclusion, based on Hf-W model ages, t hat the Moon formed 50 Myr after the start of the solar system remains vali d. Martian meteorites have lower Ta/W ratios and cosmic ray exposure ages t han most lunar samples. Therefore, cosmogenic production has not significan tly altered the W isotopic composition in Martian meteorites. Observed W-18 2 excesses in Martian meteorites as well as the very large excesses in two eucrites are undoubtedly the result of early Hf-182 decay. (C) 2000 Elsevie r Science B.V. All rights reserved.