Seven high-purity cumulate clinopyroxenes from 2.7 b.y. maficultramafic rock associations from the Abitibi belt, Superior Province, Canada, have been analyzed for major elements and K, Rb, Cs, Ba, Sr and 87Sr/86Sr ratio. Attempts to reconstruct the trace element patterns of the original parent magmas were partially successful; Sr contents (140 ppm), K/Rb (470) and K/Ba (16) ratios are similar to those of modern low-K island arc tholeiites. K/Cs ratios (2700) are significantly lower than island arc tholeiites (17,000) or oceanic island and oceanic ridge basalts (> 30,000); the presentday mantle seems to be more depleted in Cs than in Archean times. Initial Sr isotope ratios of the 7 Archean clinopyroxenes average 0.70114±13(2σ) with relatively little variation; this value is in good agreement with initial ratios published for felsic and mafic rocks of the same age, though the latter show much larger variations and uncertainties. The pyroxene Sr isotope data, in conjunction with data for rocks of other ages, defines the following simple model for mantle evolution: