THE relative abundances of immobile elements (such as calcium, aluminium, titanium, zirconium and the rare-earth elements) in the Archaean ultramafik lavas known as komatiites have provided important information about the composition of the early Earth's mantle, but to establish the origin and tectonic setting of these lavas one also needs the abundances of mobile elements, such as the alkali metals, alkaline-earth elements, and uranium and lead. Up to now, it has not been possible to use these elements in constraining komatiite petrogenesis, because of the pervasive effects of alteration in these ancient lavas; recently, however, some remarkably fresh, 2.7-Gyr-old komatiites have been discovered, which contain unaltered olivine crystals and small glass inclusions1. We report ion micoprobe data for 25 trace elements from these glass inclusions, and show that the ratios of mobile to immobile incompatible elements are similar to those found in modern intraplate basalts, and distinct from modern mid-ocean-ridge and convergent-margin basalts. We infer that these Archaean magmas formed from sources similar to (or slightly depleted relative to) those of modern intraplate basalts, supporting the suggestion 2,3 that komatiites are ancient analogues of modern plume-related magmas.
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