Letter

Nature 460, 620-623 (30 July 2009) | doi:10.1038/nature08205; Received 15 January 2009; Accepted 4 June 2009

Progressive mixing of meteoritic veneer into the early Earth's deep mantle

Wolfgang D. Maier1,2, Stephen J. Barnes3, Ian H. Campbell4, Marco L. Fiorentini2, Petri Peltonen5,6, Sarah-Jane Barnes7 & R. Hugh Smithies8

  1. Department of Geology, University of Oulu, Linnanmaa, 90014 Oulu, Finland
  2. Centre for Exploration Targeting, University of Western Australia, 35 Stirling Highway, Crawley 6009, Australia
  3. CSIRO, 26 Dick Perry Avenue, Kensington 6151, Western Australia, Australia
  4. Research School of Earth Sciences, Australian National University, Canberra 0200, Australian Capital Territory, Australia
  5. Geological Survey of Finland, Betonimiehenkuja 4, Espoo 02151, Finland
  6. Department of Geology, University of Helsinki, Gustaf Hällströmin katu 2a, Helsinki 00014, Finland
  7. Sciences Appliques, Université du Québec à Chicoutimi, Chicoutimi G7H 2B1, Québec, Canada
  8. Geological Survey of Western Australia, 100 Plain Street, East Perth 6004, Western Australia, Australia

Correspondence to: Wolfgang D. Maier1,2 Correspondence and requests for materials should be addressed to W.D.M. (Email: wolfgang.maier@oulu.fi).

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 compositions1. 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 interaction2, 3, 4, 5, 6, 7, 8. Most of the available PGE data on komatiites are from late Archaean (approx2.7–2.9 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.

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