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 (∼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.
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We thank D. Savard for the PGE analysis, and E. Hanski for comments. We also thank N. Arndt and H. Becker for reviews. This study was partly funded by the ARC (to M.L.F.), the University of Western Australia (to W.D.M.), CSIRO (to S.J.B.) and NSERC (to S.-J.B.). P.P. acknowledges support from the Geological Survey of Finland (GTK).
Author Contributions W.D.M., S.J.B., M.L.F., P.P. and R.H.S. provided samples and data, S.-J.B. analysed the South African and Pilbara samples, the first four authors provided the interpretation, and all co-authors contributed in the form of discussion and critical comment.
This file contains Supplementary Methods, Supplementary Figures 3 - 4 with Legends and Supplementary References. (PDF 2187 kb)
This file contains the whole rock data of the Barberton samples. (XLS 59 kb)
This file contains the whole rock data of the Pilbara samples. (XLS 27 kb)
This file contains the analytical accuracy and precision. (XLS 16 kb)
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Maier, W., Barnes, S., Campbell, I. et al. Progressive mixing of meteoritic veneer into the early Earth’s deep mantle. Nature 460, 620–623 (2009). https://doi.org/10.1038/nature08205
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