Abstract:
The presence of common lead contamination in zircons used for U-Pb geochronology is a potentially serious source of error. Traditionally, common lead is measured by analysis of 204Pb, and the isotopic composition of lead corrected accordingly. Some analytical methods (e.g. LAM-ICPMS) do not report 204Pb. Correction methods are available for such analyses, but these assume that the only source of discordance in a zircon is the presence of common lead. Using such a correction on a lead analysis that contains a discordance component caused by lead loss will invariably lead to overcorrection, and hence to a meaningless, young age. By assuming that the observed 206Pb/238U, 207Pb/235U and 208Pb/232Th ratios of a discordant zircon can be accounted for by a combination of lead loss at a defined time, and the presence of common lead of known composition, a correction method can be designed that neither uses 204Pb nor assumes concordance. The method proposed here involves a numeric solution to a set of equations relating the content of radiogenic lead in a zircon or other U/Th-enriched mineral to its total lead content, the amount of common lead present, the age of initial crystallization, the age of lead loss and the amount of lead lost in that process. An estimate for the age of lead loss is needed, but in the absence of prior knowledge of this age, the recalculation procedure can be set up in such a way that the bias in initial age caused by a systematic error in the age of lead loss is minimized. Despite this limitation, the method will give less bias in the corrected ages than alternative correction methods.
