Use of the extinct 182 Hf-182 W chronometer to constrain the timing of planetary accretion and differentiation rests on the assumption that the solar nebula had homogeneous tungsten isotopic composition. Here, we report deficiencies of $0.1 part in 10,000 in the abundance of 184 W in group IVB iron meteorites relative to the silicate Earth. These are most likely due to incomplete mixing at the planetesimal scale (2Y4 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 182 Hf-182 W model age of core formation in IVB irons due to the presence of these nuclear anomalies is $0.5 Myr.
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