Isotopic analysis of two Archean komatiitic flows from Alexo, Ontario, gives a Pb-Pb isochron age of 2690 ± 15 Ma and a Sm-Nd isochron age of 2752 ± 87 Ma. These ages agree well with U-Pb zircon ages from underlying and overlying volcanics. The variations in element ratios that define the isochrons were not produced during crystallization of the lavas. The spread in U/Pb was caused by submarine alteration soon after eruption, and the spread in Sm/Nd resulted from (a) differences in the composition of the residue of melting, and (b) contamination of the upper komatiite flow through thermal erosion of the lower flow. The 147 Sm/ 144 Nd ratio of uncontaminated komatiite is 0.25 which reflects the depleted nature of its mantle source. The Th/U ratio of about 3.4 is probably also representative of depleted mantle. The initial Nd of +2.44 ± 0.51 indicates that the mantle depletion took place long before magma formation.