Js. Dougherty-page et Jm. Bartlett, New analytical procedures to increase the resolution of zircon geochronology by the evaporation technique, CHEM GEOL, 153(1-4), 1999, pp. 227-240
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.