182W and 187Re- 187Os Systematics of Iron Meteorites: Chronology for Melting, Differentiation, and Crystallization in Asteroids

Abstract

W isotopic compositions (measured by N-TIMS) in five iron meteorite groups are used in conjunction with 187Re- 187Os isotopic data to constrain the timing of metal segregation and crystallization, respectively, in the iron meteorite parent bodies. Fractionation of the parent isotope, 182Hf (half-life = 9 ± 2 Ma), from the daughter, 182W, likely occurred during segregation of metal from silicate in the iron meteorite parent bodies. If W isotopic equilibrium was achieved between silicate and metal during metal segregation, then the W isotopic compositions 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-IIICD group. Together these groups comprise approximately 90% (by weight) of iron meteorites classified so far. With the exception of the IVA group, the groups have well-defined 187Re- 187Os isochron ages which are interpreted as the time of crystallization of the metal of their parent 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 crystallization. All the samples examined display very similar ϵ W values within a narrow range of compositions between −5.1 and −3.1. Witzation began, less than a few percent of additional metal was segregated to the cores. Moreover, the average ϵ 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 indicate that all of the iron meteorite parent bodies underwent metal segregation within 5 Ma of each other, apparently independent of their composition, size, and time of crystallization of the metal. In all groups, except the IIIAB group, there is a resolvable time interval between the timing of metal segregation, as constrained by the W isotopic data, and the initiation of core crystallization, as given by the Re-Os isotopic systematics.