First direct radiometric dating of Archaean stromatolitic limestone

Abstract

Here we report the first direct dating of the depositional age of a sedimentary carbonate rock using long-lived radioactive decay schemes. The Mushandike stromatolitic limestone, from the Masvingo (formerly Fort Victoria) greenstone belt of southern Zimbabwe^1,2, yields a Pb/Pb isochron age of 2,839±33 Myr. Published claims³ for an age of ˜3.5 Gyr for the Mushandike limestone were based on its supposed intrusion by the Mushandike granite, for which a Rb–Sr whole-rock age of 3,445±260 Myr had been reported⁴. New Rb–Sr and Pb/Pb isotope data for the Mushandike granite indicate a probable emplacement age of ˜2,900 Myr, and new field evidence (J.L.O., P.T. & J.F.W., in preparation) suggests that the granite forms part of the basement on which the limestone was deposited. Thus the new dates do not support an age of ˜3.5 Gyr for the algal stromatolites at Mushandike. The Pb/Pb isochron method may have wide applicability to dating the time of deposition of Precambrian fossiliferous and unfossiliferous limestones, and thus to the problem of the antiquity of life.