The quest for a high-quality zircon standard for microbeam Pb-U-Th geochronology

Full text: Many, if not most, geological studies are critically dependent on effective geochronological control, and microbeam technology is playing an increasingly important role in this respect. Like other microbeam geochronology techniques, SHRIMP Pb-U-Th zircon dating is currently totally dependent on calibrating the analyses of unknown zircons against zircon of known age (the standard). The ideal standard must meet several strict criteria. Naturally, it must have been dated by a technique quite independent of SHRIMP, and that independent age measurement must be both highly accurate and precise. The standard must also represent only a single generation of zircon growth, and have constant Pb/U (and preferably Pb/Th) on all scales from submicron to intergranular. There must have been no post-crystallisation chemical or isotopic disturbance. The standard should also be sufficiently abundant to last indefinitely, and its quality should be so obvious that it will be readily adopted by other laboratories. Over the course of the past two decades about a dozen different zircon samples from a wide range of rocks around the world have been trialed as reference materials at the RSES/AGSO laboratory. A few of these have been selected as in-house standards. As knowledge of both SHRIMP itself and those standards has advanced, the production of progressively more precise data has allowed the question of standard homogeneity to be addressed, and at least most of these standards have proved to be less than ideal in this regard. A recent outcome of this testing process has been the identification of a promising new standard from a small, high-level gabbroic diorite plug near Temora, in the Lachlan Fold Belt, eastern Australia. A reconnaissance sample from this pluton contains 10 ppm of relatively coarse-grained zircon crystals which appear to have remained geologically and isotopically undisturbed since the diorite was emplaced 417 Ma ago. The excellent uniformity of the zircon Pb/U originally identified by SHRIMP has been confirmed by more than a dozen IDTIMS analyses. This isotopic integrity, and apparent lack of any inherited component, are very encouraging. The main outstanding issue is to determine whether the high-quality zircon is present in sufficient quantities for widespread distribution to other laboratories. This is currently being assessed by SHRIMP and IDTIMS analyses of a range of samples from other, more zircon-rich, sites. Copyright (1999) Geological Society of Australia