Importance of establishing sources of uncertainty for the derivation of reliable SHRIMP ages

The ion microprobe, as exemplified by SHRIMP, has long been an invaluable resource for the derivation of geological ages, The derivation of those ages is critically dependent on the identification and individual quantification of all sources of contributing uncertainty In recent years, it has been proposed that the only component of uncertainty arising from the instrument itself is predictable from counting statistics. The adoption of that approach has led to several conclusions including: (i) that zircon U-Pb ages are relatively easily reset, which necessitates the enhanced editing of individual analyses before a grouped age can be obtained; and (ii) that other studies have overestimated analytical uncertainties and, as a consequence, have reported incorrect and/or overly imprecise ages, We present evidence for the presence of additional sources of instrument-related uncertainty that necessitates a different (but not new) approach for the processing of SHRIMP data. Fortunately, this complication does not represent a serious problem, provided that a high-quality zircon-calibration standard has been used for Pb/U calibration. SHRIMP ages obtained some time ago from the Crudine Group of the Hill End Trough (New South Wales) have recently been placed at the centre of this controversy. A significant part of the problem is that most of those ages were based on a standard (SL 13) that is now known to be heterogeneous. The more reliable parts of the original data have been re-reprocessed on the basis of the new evidence. They fail to detect a significant age difference between the bottom and the top of the Merrions Formation, a conclusion that is contrary to earlier expressed opinions.