Recent studies (DeWolf et al., 1993; Harrison et al., 1995; Zhu et al. (1995a,b) have shown that monazite is a particularly valuable phase for in situ SIMS chronometry, given that it has high abundances of Th and U. As with other U-Th enriched phases limitations arise from possible discordance in the U-Th-Pb system, where high precision measurements of U/Pb and Th/Pb ratios remain difficult by SIMS. In high-grade metamorphic rocks, monazite is commonly zoned strongly in Th abundance and in Th/U ratio (DeWolf et al., 1993; Zhu et al. (1995a,b; Watt, 1995). In this study a combination of EPMA and SIMS has been used to make a detailed assessment of Pb-Th-U mobility within individual zoned monazite grains from the high-grade Lewisian terrain The chemical and isotopic zoning structures of these grains are similar to the zoned monazite from the high-grade Wind River terrain of Wyoming (DeWolf et al., 1993).
Two zoned monazites from samples 86004 and 86015 from the central Lewisian region have been traversed at high spatial resolution by EPMA and SIMS. For the traverse across monazite #5 in 86004, the 232Th/238U ratio and 208Pb/206Pb ratios vary from ~110 and ~27 respectively in the intermediate portion of the grain to ~40 and ~10 in the rim. For the traverse across monazite #2 in 86015, the 232Th/238U and 232Th/238U ratios range from ~5 and ~1.2 in the core to up to ~100 and ~24 respectively in the intermediate portion. In both traverses, the 208Pb/206Pb profiles show no resolvable shift relative to the 232Th/238U profiles and the 208Pb/206Pb ratio is linearly correlated within error with the 232Th/238U ratio. There is therefore no resolvable transport of Th relative to U, or between Pb and Th/U in these monazite grains.
Th-Pb profiles of monazite #5 in 86004 and monazite #2 in 86015 have been obtained by EPMA. Th abundance in monazite #5 reaches a maximum of 12 wt % at the rim and varies from 5 to 7 wt% in the bulk core region. For monazite #2, Th varies from ca. 2 wt% in the core to ca. 15 wt% in the rim. Whilst the similarity between Th and Pb profiles across zones for each grain suggests that there is no significant relative movement between Th and Pb, the sharp boundaries between zones evident in the EPM Th line scans limit any Th transport across them. Together, SIMS and EPMA traverses fail to provide any evidence for relative transport of Pb and Th/U on the scale of ~2 mm since the formation of the high Th rim of monazite #5 in 86004 and the formation of the intermediate portion of monazite #2 in 86015.
The extent of Pb transport relative to Th/U in these metamorphic monazites has been investigated with the aid of simple models of Pb transport. The results suggest that 207Pb/206Pb age structure obtained from these two monazites not a result of Pb loss. And it seems unlikely that age discordance will much exceed ~10 Ma assuming that the amount of Pb loss is similar to the amount of Pb exchange.
DeWolf, C.P., Belshaw, N.S. & O'Nions, R.K., Earth Planet. Sci. Lett. 120, 207-220 (1993).
Harrison, T.M., McKeegan, K.D. & LeFort, P., Earth Planet. Sci. Lett. 133, 271-282 (1995).
Watt, Mineral. Magazine 59, 735-743 (1995).
Zhu, X.K., O'Nions, R.K, Belshaw, N.S. & Gibb, A.J., Chem. Geol. (1995a, submitted).
Zhu, X.K., O'Nions, R.K, Belshaw, N.S. & Gibb, A.J., Earth Planet. Sci. Lett. (1995b, submitted).