EarthRef.org Reference Database (ERR) -- Villeneuve et al. 2000

The increasing analytical precision of Ar-40-Ar-39 geochronological data and its use in studies that combine U-Pb and Ar-40-Ar-39 data has necessitated derivation of a means to intercalibrate ages between the two isotopic systems. The primary determinants inhibiting accurate cross calibration are uncertainty 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 Oligocene 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 should 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 biotite 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 sanidine 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 relative sense can be accomplished, allowing for a direct and valid comparison of ages. It is also shown that forcing equivalence of the two isotopic systems minimizes the effect of decay constant uncertainty for samples of Phanerozoic age. Copyright (C) 2000 Elsevier Science Ltd.