Pb–Pb dating of young marbles from Taiwan

Abstract

Direct determination of the depositional age of a sedimentary sequence usually relies on isotopic analyses of authigenic minerals such as glauconite¹ and fine-grained clay minerals^2–4. Moorbath et al.⁵ recently reported the first direct dating of sedimentary carbonates using the whole-rock Pb–Pb method. The success of this method depends on the chemical fractionation of U from Pb and the consequent variation in ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios. The high values and large variation of U/Pb ratios in sedimentary carbonates, long known to marine geochemists, have, however, not been exploited by geochronologists until recently^5,6. Here I report a Pb–Pb isochron age for a relatively young carbonate sequence in Taiwan and demonstrate that the long-lived U–Pb decay scheme can be used to determine events as young as Mesozoic age. The new results also lead to the identification of an early Jurassic (∼170 Myr BP) thermal event, which has never before been recognized in the polymetamorphic schist belt (the Tananao Schist) of Taiwan, but is widespread in southeastern continental China and other parts of eastern Asia^7–10.