New analytical procedures to increase the resolution of zircon geochronology by the evaporation technique

In the evaporation technique [Kober, B., 1987. Single zircon evaporation co mbined with Pbf emitter bedding for Pb-207/Pb-206 age investigations using thermal ion mass spectrometry, and implications for zirconology. Contrib. M ineral. Petrol. 96, 63-71] of zircon dating, individual zircon crystals are heated in a series of steps, causing the thermal breakdown of the zircon a long a sharply defined reaction front [Chapman, H.J., Roddick, J.C., 1994. Kinetics of Pb release during the zircon evaporation technique. Earth Plane t. Sci. Lett. 121, 601-611]. Radiogenic Pb-207/Pb-206 ages for each step re present the average Pb isotopic composition of the zircon consumed, resulti ng in geologically meaningless 'mixed ages' where the reaction front inters ects multiple domains within a zircon. Recognition of whether Pb-207/Pb-206 ages are derived from single or heterogeneous components is central to age determinations by the evaporation technique. Replication of an age indicat es that a single component is being dated, as it is unlikely that several c omponents will be repeatedly mixed in identical proportions. Thus increasin g the number of heating steps taken for each zircon increases the resolutio n of information obtained. Presentation of data as age against order of acq uisition allows clear visual interpretation of the degree of mixing behavio ur within each zircon analysed. Ranking the heating steps in order of ascen ding age clarifies trends within the zircon population as a whole, and also allows the use of an R-2 test to delimit those steps which do, or do not, contain mixed components. Mixing behaviour may be further emphasised by sup erimposing the model initial Th/U ratio for each step on the same graph. (C ) 1999 Elsevier Science B.V. All rights reserved.