1. School of Geological and Computer Sciences, University of Natal, Durban 4041, South Africa
2. Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, Washington DC 20015, USA

Correspondence to: A. H. Wilson¹ Correspondence and requests for materials should be addressed to A.H.W. (Email: wilsona@nu.ac.za).

Abstract

Komatiites are ultramafic volcanic rocks containing more than 18 per cent MgO (ref. 1) that erupted mainly in the Archaean era (more than 2.5 gigayears ago). Although such compositions occur in later periods of Earth history (for example, the Cretaceous komatiites of Gorgona Island²), the more recent examples tend to have lower MgO content than their Archaean equivalents. Komatiites are also characterized by their low incompatible-element content, which is most consistent with their generation by high degrees of partial melting (30–50 per cent³). Current models for komatiite genesis include the melting of rock at great depth in plumes of hot, diapirically rising mantle⁴ or the melting of relatively shallow mantle rocks at less extreme, but still high, temperatures caused by fluxing with water⁵. Here we report a suite of ultramafic lava flows from the Commondale greenstone belt, in the southern part of the Kaapvaal Craton, which represents a previously unrecognized type of komatiite with exceptionally high forsterite content of its igneous olivines, low TiO₂/Al₂O₃ ratio, high silica content, extreme depletion in rare-earth elements and low Re/Os ratio. We suggest a model for their formation in which a garnet-enriched residue left by earlier cratonic volcanism was melted by hydration from a subducting slab.