EVALUATING U-PB ZIRCON DATA USING A 3-DIMENSIONAL CONCORDIA

When assessing zircon U-Pb data, Wendt's (1984; 1989) 3-dimensional pr ojection for calculating concordia intercept ages has a fundamental ad vantage over other methods: the best-fit plane in three dimensions def ines a sample's age without requiring any advance knowledge about the isotopic composition of the non-radiogenic Pb. However, until now the general validity of this approach has never been investigated using da ta sets measured on terrestrial samples. Best-fit plane calculations w ere made for three terrestrial zircon samples. The t(1) and t(2) conco rdia intercept ages of these samples were found to be statistically eq uivalent to the ages calculated by other means. However, the 3-dimensi onal calculations gave detectable differences in ages and precision es timates as compared to the mean Pb-207/Pb-206 and line regression tech niques; such differences could be important at moderate to high precis ion level. It was also found that the 3-dimensional concordia provides useful information for discerning which analyses should or should not be included within the final data set.