NEW ANALYTICAL PROCEDURES TO INCREASE THE RESOLUTION OF ZIRCON GEOCHRONOLOGY BY THE EVAPORATION TECHNIQUE

Abstract

In the evaporation technique [Kober, B., 1987. Single zircon evaporation combined with Pb+ emitter bedding for 207Pb/206Pb age investigations using thermal ion mass spectrometry, and implications for zirconology. Contrib. Mineral. Petrol. 96, 63-71] of zircon dating, individual zircon crystals are heated in a series of steps, causing the thermal breakdown of the zircon along a sharply defined reaction front [Chapman, H.J., Roddick, J.C., 1994. Kinetics of Pb release during the zircon evaporation technique. Earth Planet. Sci. Lett. 121, 601-611]. Radiogenic 207Pb/206Pb ages for each step represent the average Pb isotopic composition of the zircon consumed, resulting in geologically meaningless `mixed ages' where the reaction front intersects multiple domains within a zircon. Recognition of whether 207Pb/206Pb ages are derived from single or heterogeneous components is central to age determinations by the evaporation technique. Replication of an age indicates that a single component is being dated, as it is unlikely that several components will be repeatedly mixed in identical proportions. Thus increasing the number of heating steps taken for each zircon increases the resolution of information obtained. Presentation of data as age against order of acquisition allows clear visual interpretation of the degree of mixing behaviour within each zircon analysed. Ranking the heating steps in order of ascending age clarifies trends within the zircon population as a whole, and also allows the use of an R2 test to delimit those steps which do, or do not, contain mixed components. Mixing behaviour may be further emphasised by superimposing the model initial Th/U ratio for each step on the same graph.