High abundances of incompatible elements in MgO-rich Archean basalts from Western Australia, which were thought to have been derived from an enriched mantle source, are probably the result of contamination of primitive magmas by small amounts of older sialic crust. Because these lavas represent some of the more extreme cases of enrichment in incompatible elements observed in Archean basalts, it may be possible to explain much of the trace-element variation previously ascribed to a heterogenous Archean mantle by variable crustal contamination of mantle-derived magmas. Recent calculations of thermal erosion rates predict that high-temperature high-MgO magmas, abundant in the Archean, are most susceptible to contamination; therefore, a cautious reappraisal of isotopic and trace-element data from Archean greenstone belts is required before the real extent of Archean mantle heterogeneity can be established.

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