Abstract In terms of the relative abundance of K, Rb, Cs, Sr and Ba, modern oceanic and island-arc basalts may be viewed as a gradational family ranging from alkali basalt (enriched in these elements) to low-K-tholeiite, such as ocean-floor basalt (depleted in these elements). In island arcs, the contents of these trace-elements in basalts are proportional to the depth to the seismic zone. Our model for this relationship suggests derivation of alkali basalts from non-depleted mantle by low degrees of partial melting at relatively great depths; the low-K tholeiites are satisfactorily explained as products of high degrees of partial melting of residual or “depleted” mantle at relatively shallow depths. Some of these tholeiites are shown to be contaminated with oceanic sediment, and the usefulness of Cs and Ba concentrations to detect such contamination is established. New trace-element analyses reveal that the basalts of the Archaean (2.7 by) greenstone belts of Canada average 2100 ppm K, 6 ppm Rb, 0.4 ppm Cs, 175 ppm Sr and 70 ppm Ba. These trace-element abundances show that the Archaean basalts are most closely related to the modern low-K tholeiites of island arcs. This analogy leads us to propose that Archaean basalts are derived by high degrees of melting at shallow depths. We find no evidence for the presence of residual or “depleted” mantle during the Archaean. Our model implies a thin lithosphere during the Archaean and we suggest that Archaean volcanics evolved in an environment dominated by multiple thin oceanic plates with arc-trench boundaries and shallow subduction zones.