Progressive mixing of meteoritic veneer into the early Earths deep mantle

Komatiites are ancient volcanic rocks, mostly over 2.7 billion years old (from the Archaean era), that formed through high degrees of partial melting of the mantle and therefore provide reliable information on bulk mantle compositions. In particular, the platinum group element (PGE) contents of komatiites provide a unique source of information on core formation, mantle differentiation and possibly core-mantle interaction. Most of the available PGE data on komatiites are from late Archaean (2.7-2.9 Gyr old) or early Proterozoic (2.0-2.5 Gyr old) samples. Here we show that most early Archaean (3.5-3.2 Gyr old) komatiites from the Barberton greenstone belt of South Africa and the Pilbara craton of Western Australia are depleted in PGE relative to late Archaean and younger komatiites. Early Archaean komatiites record a signal of PGE depletion in the lower mantle, resulting from core formation. This signal diminishes with time owing to progressive mixing-in to the deep mantle of PGE-enriched cosmic material that the Earth accreted as the late veneer during the Early Archaean (4.5-3.8 Gyr ago) meteorite bombardment.