New U-Pb zircon ages and the duration and division of Devonian time

Newly determined U-Pb zircon ages of volcanic ashes closely tied to biostratigraphic zones are used to revise the Devonian time-scale. They are: (I) 417.6:1:: 1.0 Ma for an ash within the conodont zone of Icriodus woschmidti/I. w. hesperius (early Lochkovian); (2) 408.3:1:: 1.9 Ma for an ash of early Emsian age correlated with the conodont zones of Po. dehiscens-Lower Po. inversus; (3) 391.4:1:: 1.8 Ma for an ash within the Po. c. costatus Zone and probably within the upper half of the zone (Eifelian); and (4) 381.1:1:: 1.3 Ma for an ash within the range of the Frasnian conodont Palmatolepis punctata (Pa. punctata Zone to Upper Pa. hassi Zone). U-Pb zircon ages for two rhyolites bracketing a palyniferous be