A new procedure was developed to accurately measure the W isotopic compositions of iron meteorites with a precision of better than ± 0.1 on epsilon W-182 and epsilon W-184. The W isotopic compositions of 33 iron meteorites from both magmatic and non-magmatic groups were determined. Deficits of ∼0.1 part per ten thousand in the abundance of non-radiogenic W-184 in group IVB iron meteorites relative to the silicate Earth were reported for the first time. These are most likely due to incomplete mixing at the planetesimal scale (2-4 km radius bodies) of the products of slow (s) and rapid (r) neutron capture nucleosynthesis in the solar nebula. The correction that must be applied to the Hf-W model age of core formation in IVB irons due to the presence of these nuclear anomalies is -0.5 Myr. Tentative nuclear anomalies in W-180 were also found for IVBs, but more work needs to be done to validate these results.

Epsilon W-182 results from this study are generally consistent with previous studies. The more negative epsilon W-182 values in some iron meteorites than the initial value in CAIs, and variations in epsilon W-182 values within individual magmatic iron meteorite groups are most easily explained as a result of exposure to galactic cosmic rays in space. A correction method was applied to estimate the pre-exposure epsilon W-182 for individual magmatic iron meteorite groups. The corrected epsilon W-182 shows that metal-silicate differentiation occurred very early (within first 2 Myr) for most magmatic groups.

A numerical model was developed to simulate the thermal histories of iron meteorite parent bodies. For the first time, such a thermal model was combined with the Hf-W metal-silicate differentiation age and the parent body size, to constrain the accretion time scale. The estimated accretion time is within 1.5 Myr for most magmatic iron groups, and could be as early as 0.2 Myr for IVB. This is consistent with a recent study by Bottke et al. (2006), showing that the iron meteorite parent bodies may have originated in the region of terrestrial planets where the accretion rates are fast.

We also provide W isotopic constraints on the formation mechanism of non-magmatic irons. The homogeneous epsilon W-182 in IAB-IIICD support the model of formation of metal pools in a chondritic reservoir by a late impact event dated at 4-7 Myr after the condensation of CAIs.