CRYSTAL ORIENTATION EFFECTS DURING ION MICROPROBE U-PB ANALYSIS OF BADDELEYITE

Abstract

We show that 206Pb/238U ratios measured in baddeleyite by ion microprobe vary significantly (up to +/-10% or more) and systematically with the relative orientation of the baddeleyite crystal structure and the primary ion beam. Low dispersion between multiple analyses of single crystals demonstrates that it is possible to measure undispersed 206Pb/238U ratios in multi-grain baddeleyite samples if all material is in a single orientation. Owing to the small size of most crystals and their ubiquitous polysynthetic {100} twinning, however, it is extremely difficult to arrange a reasonable number of crystals in identical orientations for analysis. These orientation effects constitute an intractable problem, and place unacceptable limits on the accuracy and precision that can be obtained for 206Pb/238U in baddeleyite by ion microprobe techniques. Comparisons with isotope dilution measurements indicate that any Pb isotope discrimination during analysis of baddeleyite by ion microprobe is negligible, and there is no evidence that Pb isotope ratios vary with crystal orientation. Although the precision of 207Pb/206Pb ages decreases for younger samples, rocks younger than 1 Ga can be dated precisely if the baddeleyite is enriched sufficiently in uranium and if enough analyses are performed. We speculate that orientation effects in baddeleyite might involve channelling of primary ions into the crystal, emission of secondary ions along preferred directions, and/or differential ionisation of secondary species. No orientation-related differences in 206Pb/238U were detected during ion microprobe analysis of zircon or monazite.