W-182 AND RE-187-OS-187 SYSTEMATICS OF IRON-METEORITES - CHRONOLOGY FOR MELTING, DIFFERENTIATION, AND CRYSTALLIZATION IN ASTEROIDS. (VOL 62, PG 545, 1998)

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, likel y occurred during segregation of metal from silicate in the iron meteo rite parent bodies. If W isotopic equilibrium was achieved between sil icate and metal during metal segregation, then the W isotopic composit ions 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-III CD group. Together these groups comprise approximately 90% (by weight) of iron meteorites classified so far. With the exception of the IVA g roup, the groups have well-defined Re-187-Os-187 isochron ages which a re interpreted as the time of crystallization of the metal of their pa rent bodies. Within each magmatic group, samples that crystallized bot h early and late in the sequence of each core were analyzed to evaluat e possible evolution of the W isotopic composition during crystallizat ion. All the samples examined display very similar sigma(W) values wit hin a narrow range of compositions between -5.1 and -3.1. Within each group there are no resolvable W isotopic heterogeneities, suggesting t hat, after core crystallization began, less than a few percent of addi tional metal was segregated to the cores. Moreover, the average sigma( W) W values for the five groups have no resolvable difference, ranging from -4.08 +/- 0.69 (IVA) to -3.30 +/- 0.15 (IAB-IIICD). These data i ndicate that all of the iron meteorite parent bodies underwent metal s egregation within 5 Ma of each other, apparently independent of their composition, size, and time of crystallization of the metal. In all gr oups, except the IIIAB group, there is a resolvable time interval betw een the timing of metal segregation, as constrained by the W isotopic data, and the initiation of core crystallization, as given by the Re-O s isotopic systematics.