Correlated helium-3 and tungsten isotopes in iron meteorites:: Quantitative cosmogenic corrections and planetesimal formation times

Core formation in planetesimals can, in principle, be dated using the short-lived Hf-182-W-182 chronometer. However, it has been predicted that burnout and production of W isotopes by nuclear reactions can substantially modify the compositions in iron meteorites when exposed for several hundred million years (Myr) to galactic cosmic-rays. This may severely limit the use of the Hf-W system for determining chronologies of metal segregation as recorded in iron meteorites. Here we present the first experimental evidence of cosmogenic effects on W isotopes in two magmatic iron meteorites, Carbo (IID) and Grant (IIIAB). The W-182/W-184 ratio near the pre-atmospheric centre of the meteorites is lower by similar to 0.5 epsilon compared to values near the pre-atmospheric surface. The W-182/(184) W ratio displays excellent correlations with the 3 He concentration, which in turn provides a proxy for the fluence of the relevant cosmic-ray particles. Using new nuclear physics parameters, a first order correction for cosmic-ray effects on W isotopes is proposed based on the 3 He concentration and on an independent exposure age of the meteorite. This method is then applied to other magmatic iron meteorites (Negrillos, Cape of Good Hope, Navajo and Arispe). When corrected for cosmogenic effects the W isotopic compositions of Carbo, Grant, Negrillos, Cape of Good Hope, and Navajo are similar to the initial composition recorded in Allende CAIs. This shows that at least some magmatic meteorites from groups IIAB, IID, IIIAB, and IVB segregated within less than 1.2 Myr of the Hf-W system closure in Allende CAIs. The isotopic variations among the samples indicate that the iron meteorites studied in this paper segregated over a time interval of similar to 4 Myr, Carbo, being the oldest iron meteorites of this study. (c) 2006 Elsevier B.V. All rights reserved.