Paper No. 1
Presentation Time: 8:00 AM-6:00 PM
USING INTRA-GRAIN VARIATIONS IN OXYGEN ISOTOPE COMPOSITION AND CL IMAGING TO UNDERSTAND U-PB DISCORDANCE IN BADDELEYITE
DAVIES, Joshua H.F.L.1, HEAMAN, Larry M.
2 and STERN, Richard A.
1, (1)Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, University of Alberta, Edmonton, AB T6G 2E3, Canada, (2)Earth and Atmospheric Sciences, University of Alberta, 1-23 Earth Sciences Building, Edmonton, AB T6G2E3, Canada, jdavies1@ualberta.ca
Unlike zircon, baddeleyite routinely produces concordant or near concordant U-Pb ages with no need for pre-analytical treatment. However, minor discordance (<10%) does occur, requiring multiple analyses to produce robust upper intercept crystallization ages. Discordance in baddeleyite is poorly understood and it is not clear if Pb loss is the dominant cause, as is typically the case with zircon. Initial results from baddeleyite diffusion experiments
1 suggest that Pb is difficult to mobilize in the baddeleyite lattice. Secondary zircon overgrowths on baddeleyite during hydrothermal alteration or metamorphism have been suggested as an alternative cause of discordance although this has not been proven. One method for evaluating the effects of baddeleyite alteration is through a combined investigation of intra-grain variations in oxygen isotope composition using Secondary Ion Mass Spectrometry (SIMS) and cathodoluminescence (CL) imaging.
SIMS oxygen isotope analysis of baddeleyite is still in the developmental stage and this presentation reports the first results. Large baddeleyite grains from the Phalaborwa carbonatite complex (South Africa), the Mogok tectonic belt (Burma), and ~250μm grains from syeno-gabbros (Greenland) all reveal 0.1-6‰ within-crystal 18O variations. The variations in O-isotope composition do not appear to be related to crystal orientation or other analytical biases. The grains with largest isotopic variations typically contain the greatest complexity in CL suggesting that variations in 18O are related to hydrothermal alteration. U-Pb TIMS analysis of the Greenland and South African samples produce concordant results, which allows us to draw three tentative conclusions: 1) hydrothermal processes affecting the oxygen isotopic composition of these crystals occurs very soon after crystallization and is not resolvable with U-Pb dating, 2) hydrothermal alteration of baddeleyite in this study did not disturb the U-Pb systematics, and 3) grain selection for TIMS analysis preferentially discards crystals with obvious alteration, therefore not sampling discordant grains.
1. Niihara et al, 2010, AMRC symposium, abstract.
