THE 147Sm–143 Nd system has been successfully exploited for the dating of lunar rocks and meteorites1–3 and has yielded results of comparable precision to those obtained from the 87Rb–87Sr system. Furthermore, recent studies of the Angra dos Reis achondrite3,4,30 have shown that the age obtained by the Sm–Nd method is in excellent agreement with ages calculated from U–Pb-isotope compositions when the new U-decay constants are used5. Ages quoted here are based on the following decay constants: λ87Rb = 1.39×10−11yr−1; λ147Sm = 6.54×10−12yr−1; λ235U = 0.98485×10−9yr−1; λ238U = 0.15513×10−9yr−1. Measurements of the Nd-isotope compositions of recent oceanic and continental basalts6–11, combined with estimates of the Sm/Nd ratio of the bulk Earth8, 12 have led to a much improved assessment of mantle differentiation8, 9, 12. Hamilton et al.13 have obtained a Sm–Nd whole rock isochron from Rhodesian Archaean volcanics, which demonstrates the feasibility of dating terrestrial rocks by this method. The latter investigation also indicates that the Sm–Nd system will probably prove more valuable than the Rb–Sr system for dating ancient volcanics, because of the greater stability of Sm/Nd compared with Rb/Sr during alteration processes. Here we report the results of Sm–Nd dating of the oldest known terrestrial rocks from the Isua supracrustal succession in West Greenland.