A method for intercalibration of U-Th-Pb and Ar-40-Ar-39 ages in the Phanerozoic

The increasing analytical precision of Ar-40-Ar-39 geochronological data an d its use in studies that combine U-Pb and Ar-40-Ar-39 data has necessitate d derivation of a means to intercalibrate ages between the two isotopic sys tems. The primary determinants inhibiting accurate cross calibration are un certainty in decay constant values and uncertainty in Ar-40*/K-40 (usually presented as apparent age) for Ar-40-Ar-39 standards, notably those derived from the Fish Canyon Tuff. By utilizing a monazite- and biotite bearing Ol igocene ash flow tuff (MAC-83) as the primary standard material, precise U- Th-Pb and Ar-40-Ar-39 ages can be derived. Because both isotopic systems sh ould reflect closure during the instantaneous eruption event, the 24.21 +/- 0.10 Ma Pb-207/U-235 age should match the age resulting from Ar-40-Ar-39 b iotite analysis. This forced equivalence will partly offset the effects of decay constant uncertainty. Furthermore, by co-irradiating Fish Canyon Tuff sanidine with MAC-83 biotite, the apparent age for Fish Canyon Tuff sanidi ne is back-calculated to 27.98 +/- 0.15 Ma, relative to the MAC-83 (207)pb/ U-235 age. Hence, intercalibration between the two isotopic systems in a re lative sense can be accomplished, allowing for a direct and valid compariso n of ages. It is also shown that forcing equivalence of the two isotopic sy stems minimizes the effect of decay constant uncertainty for samples of Pha nerozoic age. Copyright (C) 2000 Elsevier Science Ltd.