W-182 AND RE-187-OS-187 SYSTEMATICS OF IRON-METEORITES - CHRONOLOGY FOR MELTING, DIFFERENTIATION, AND CRYSTALLIZATION IN ASTEROIDS

W isotopic compositions (measured by N-TIMS) in five iron meteorite gr oups are used in conjunction with Re-187-Os-187 isotopic data to const rain the timing of metal segregation and crystallization, respectively , in the iron meteorite parent bodies. Fractionation of the parent iso tope, Hf-182(half-life = 9 +/- 2 Ma), from the daughter, W-182, likely occurred during segregation of metal from silicate in the iron meteor ite parent bodies. If W isotopic equilibrium was achieved between sili cate and metal during metal segregation, then the W isotopic compositi ons in the irons should record the timing of this event. Samples were taken from the five largest iron meteorite groups, including the four magmatic groups IIAB, IIIAB, IVA, and IVB and the nonmagmatic IAB-IIIC D group. Together these groups comprise approximately 90% (by weight) of iron meteorites classified so far. With the exception of the TVA gr oup, the groups have well-defined Re-187-Os-187 isochron ages which ar e interpreted as the time of crystallization of the metal of their par ent bodies. Within each magmatic group, samples that crystallized both early and late in the sequence of each core were analyzed to evaluate possible evolution of the W isotopic composition during crystallizati on. All the samples examined display very similar epsilon(w) values wi thin a narrow range of compositions between -5.1 and -3.1. Witzation b egan, less than a few percent of additional metal was segregated to th e cores. Moreover, the average epsilon(w) values for the five groups h ave no resolvable difference, ranging from -4.08 +/- 0.69 (IVA) to -3. 30 +/- 0.15 (IAB-IIICD). These data indicate that all of the iron mete orite parent bodies underwent metal segregation within 5 Ma of each ot her, apparently independent of their composition, size, and time of cr ystallization of the metal. In all groups, except the IIIAB group, the re is a resolvable time interval between the timing of metal segregati on, as constrained by the W isotopic data, and the initiation of core crystallization, as given by the Re-Os isotopic systematics. Copyright (C) 1998 Elsevier Science Ltd.