How much of Earth's compositional variation dates to processes occurring during 16 planet formation remains an unanswered question. High precision W isotopic data for rocks from two large igneous provinces, the North Atlantic Igneous Province and the Ontong Java Plateau, reveal preservation to the Phanerozoic of W isotopic heterogeneities in the mantle. These heterogeneities, caused by the decay of 182Hf in high Hf/W ratio mantle domains, were created during the first ~50 Ma of Solar System history, and imply that portions of the mantle that formed during Earth’s primary accretionary period have survived to the present.NSF grant EAR1265169, NSF grant EAR1250419.