THE APPLICATION OF SHRIMP TO PHANEROZOIC GEOCHRONOLOGY; A CRITICAL APPRAISAL OF FOUR ZIRCON STANDARDS

Abstract

Derivation of Phanerozoic zircon 206Pb/238U ages by SHRIMP depends on calibration against an independently dated standard. The qualities of four different zircon standards (SL13, QGNG, AS3 and TEMORA 1) are assessed herein. Not all of these behave consistently on SHRIMP with respect to their ages as determined by IDTIMS. SL13, the most commonly used standard over the past decade and a half, is the most heterogeneous in Pb/U. In addition, when SL13 is used as the calibration standard, the varied ages resulting from that heterogeneity are generally younger than ages derived from the other three standards. AS3-calibrated ages are the oldest of the group. Only QGNG and TEMORA 1, when calibrated relative to each other, yield ages on SHRIMP that are consistent with their IDTIMS ages. Of these two, TEMORA 1 has the distinct advantage of producing consistent IDTIMS ages at high precision. Because of these factors and its availability, we recommend its use in geological studies where precise and accurate Pb/U zircon ages are imperative. Approximate conversion factors have been derived to improve quantitative inter-comparison between SHRIMP ages that have been calibrated against the different standards. These refinements significantly advance the role that SHRIMP can play in the numerical calibration of the Phanerozoic timescale.