Early core formation in asteroids and late accretion of chondrite parent bodies:: Evidence from 182Hf-182W in CAIs, metal-rich chondrites, and iron meteorites

The Hf-182-W-182 isotopic systematics of Ca-Al-rich inclusions (CAIs), metal-rich chondrites, and iron meteorites were investigated to constrain the relative timing of accretion of their parent asteroids. A regression of the Hf-W data for two bulk CAIs, various fragments of a single CAI, and carbonaceous chondrites constrains the Hf-182/Hf-180 and epsilon(w), at the time of CAI formation to (1.07 +/- 0.10) X 10(-4) and -3.47 +/- 0.20, respectively. All magmatic iron meteorites examined here have initial epsilon(w) values that are similar to or slightly lower than the initial value of CAIs. These low ew values may in part reflect W-182-burnout caused by the prolonged cosmic ray exposure of iron meteorites, but this effect is estimated to be less than similar to 0.3 epsilon units for an exposure age of 600 Ma. The W isotope data, after correction for cosmic ray induced effects, indicate that core formation in the parent asteroids of the magmatic iron meteorites occurred less than similar to 1.5 Myr after formation of CAIs. The nonmagmatic IAB-IIICD irons and the metal-rich CB chondrites have more radiogenic W isotope compositions, indicating formation several Myr after the oldest metal cores had segregated in some asteroids. Chondrule formation similar to 2-5 Myr after CAIs, as constrained by published Pb-Pb and Al-Mg ages, postdates core formation in planetesimals, and indicates that chondrites do not represent the precursor material from which asteroids accreted and then differentiated. Chondrites instead derive from asteroids that accreted late, either farther from the Sun than the parent bodies of magmatic iron meteorites or by reaccretion of debris produced during collisional disruption of older asteroids. Alternatively, chondrites may represent material from the outermost layers of differentiated asteroids. The early thermal and chemical evolution of asteroids appears to be controlled by the decay of Al-26, which was sufficiently abundant (initial Al-26/Al-27 > 1.4 X 10(-5)) to rapidly melt early-formed planetesimals but could not raise the temperatures in the late-formed chondrite parent asteroids high enough to cause differentiation. The preservation of the primitive appearance of chondrites thus at least partially reflects their late formation rather than their early and primitive origin. Copyright (c) 2005 Elsevier Ltd.