Komatiites are ancient volcanic rocks, mostly over 2.7 billion yearsold (from the Archaean era), that formed through high degrees ofpartial melting of the mantle and therefore provide reliableinformation on bulk mantle compositions1. In particular, theplatinum group element (PGE) contents of komatiites provide aunique source of information on core formation, mantle differentiationand possibly core–mantle interaction2–8. Most of theavailable PGE data on komatiites are from late Archaean ( 2.7–2.9 Gyr old) or early Proterozoic (2.0–2.5 Gyr old) samples. Herewe show that most early Archaean (3.5–3.2 Gyr old) komatiitesfrom the Barberton greenstone belt of South Africa and thePilbara craton of Western Australia are depleted in PGE relativeto late Archaean and younger komatiites. Early Archaean komatiitesrecord a signal of PGE depletion in the lower mantle, resultingfrom core formation. This signal diminishes with time owingto progressive mixing-in to the deep mantle of PGE-enrichedcosmic material that the Earth accreted as the ‘late veneer’ duringthe Early Archaean (4.5–3.8 Gyr ago) meteorite bombardment.