A METHOD FOR INTERCALIBRATION OF U-TH-PB AND 40AR-39AR AGES IN THE PHANEROZOIC

Abstract

The increasing analytical precision of 40Ar-39Ar geochronological data and its use in studies that combine U-Pb and 40Ar-39Ar 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 40Ar*/40K (usually presented as apparent age) for 40Ar-39Ar 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 40Ar-39Ar ages can be derived. Because both isotopic systems should reflect closure during the instantaneous eruption event, the 24.21 +/- 0.10 Ma 207Pb/235U age should match the age resulting from 40Ar-39Ar 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 207Pb/235U 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.