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Archaean ultra-depleted komatiites formed by hydrous melting of cratonic mantle


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.

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Figure 1: Variation of SiO2 and MgO for Commondale liquid and olivine–orthopyroxene cumulates and for other komatiites.
Figure 2: Chondrite normalized rare-earth patterns for liquid and olivine cumulate compositions for Commondale komatiites.
Figure 3: 187Re/188Os versus 187Os/188Os for the Commondale komatiites.
Figure 4: Projections of Commondale liquid compositions and olivine accumulates into two sytems.

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Support for this study comes from a NSF EAR Continental Dynamics grant, the National Research Foundation (South Africa) and the University of Natal Research Fund. C. Herzberg and M. Walter provided comments that improved this paper. We also thank N. Arndt, M. Cheadle, J. Boyd, S. Parman, M. de Wit and B. Mysen for discussions on these komatiites, but the views expressed in this paper are our own.

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Correspondence to A. H. Wilson.

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Wilson, A., Shirey, S. & Carlson, R. Archaean ultra-depleted komatiites formed by hydrous melting of cratonic mantle. Nature 423, 858–861 (2003).

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