We show that Pb-206/U-238 ratios measured in baddeleyite by ion microprobe
vary significantly (up to +/- 10% or more) and systematically with the rela
tive orientation of the baddeleyite crystal structure and the primary ion b
eam. Low dispersion between multiple analyses of single crystals demonstrat
es that it is possible to measure undispersed Pb-206/U-238 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 numb
er of crystals in identical orientations for analysis. These orientation ef
fects constitute an intractable problem, and place unacceptable limits on t
he accuracy and precision that can be obtained for Pb-206/U-238 in baddeley
ite by ion microprobe techniques. Comparisons with isotope dilution measure
ments indicate that any Pb isotope discrimination during analysis of baddel
eyite by ion microprobe is negligible, and there is no evidence that Pb iso
tope ratios vary with crystal orientation. Although the precision of Pb-207
/Pb-206 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, emi
ssion of secondary ions along preferred directions, and/or differential ion
isation of secondary species. No orientation-related differences in Pb-206/
U-238 were detected during ion microprobe analysis of zircon or monazite. (
C) 2000 Elsevier Science B.V. All rights reserved.