Effect of Pb loss on the ages of reference zircons QGNG and SL13, and of volcanic zircons from the Early Devonian Merrions and Turondale Formations, New South Wales

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Abstract
Recent papers (Black et al. 1997; Jagodzinski & Black 1999) reported that SL13‐referenced SHRIMP zircon ages are ca 1.3% younger than QGNG‐referenced ages. It is shown here that these authors overestimated their analytical errors and as a consequence failed to register the presence of large and variable fractions of Pb loss within both QGNG (median 40% of spots) and SL13 (25%). If only the oldest 206Pb/238U age groups within each of the same 16 analytical sessions are selected for comparison, the 206Pb/238U ages of QGNG relative to SL13 agree with the Concordia intersection age by isotope dilution to within 0.4% (standard deviation). This observation eliminates the possibility of systematic bias. Jagodzinski and Black (1999) also overlooked Pb loss in their ages for the Early Devonian Turondale and Merrions Formations, which are corrected here for time‐scale use.