Influence of inclusions and leaching techniques on Sm-Nd and Lu-Hf garnet chronology

Sm-Nd chronology is often affected by REE-rich inclusions, which lead to ambiguous ages, lower precision or make dating impossible. In order to investigate this problem we conducted Lu-Hf and Sm-Nd garnet dating on a high pressure granulite, a felsic subvolcanic intrusion and a high grade metapelite. We propose a new leaching technique capable of substantially reducing or even eliminating phosphate inclusions. Lu-Hf and Sm-Nd dating of high pressure garnet granulite from Kohistan region of Pakistan yielded concordant ages of 94±1.2 and 95.9±2.6 Ma respectively. Small variations in both Lu/Hf and Sm/Nd ratios among 3 garnet fractions result most likely from small amount of amphibole contamination. Dating of the subvolcanic intrusion from Lanarkshire (Scotland) gave a Lu-Hf age of 419±19 Ma, concordant with the 411±2 Ma Sm-Nd age of Thirlwall (1988). Poor precision of Lu-Hf age is caused by very limited spread in Lu/Hf ratios (0.7). Similarly the metapelite garnet from N. Vietnam was strongly affected by inclusions and yielded Sm/Nd ratios too low to determine an age. Metapelite garnets, which were very rich in monazite, zircon and apatite inclusions, were subjected to leaching experiments. We modified the leaching procedure of Amato et al. (1999). Two leaching steps with concentrated HF, followed by 6N HCl yielded Sm/ 4 Nd ratios of 0.97 and 1.08, respectively. The remaining residue gave a low Sm/Nd ratio due to strong contamination by inclusions undissolved at earlier steps (as indicated by a very high Nd concentration). Two other garnet fractions from the same sample treated with concentrated H2SO4 and subsequently fully dissolved gave identical, very high Sm/Nd of 1.97. All points from both experiments plotted on a single errorchron of age 41.8±4.3 Ma (MSWD = 3.4 and probability of fit = 0.02). The poor statistics are caused by the HF leachate analysis, during which some Nd was leached from inherited monazite and zircon inclusions. After removing this point from the regression an isochron age of 41.7±1.5 Ma is obtained with MSWD = 0.057 and probability of fit = 0.94. Our data demonstrates that H2SO4 leaching can effectively remove phosphate inclusions enhancing precision and reliability of Sm-Nd garnet chronology. References Amato, J.M., Johnson C.M., Baumgartner L.P., Beard B.L., (1999) Earth Planet. Sci. Lett. 171, 425-438. Thirlwall, M.F., (1988) J. Geol. Soc. London 145, 951-967. Geochemistry and Geochronology of Basement Rocks from the Pelagonian Zone, Greece B. ANDERS, T. REISCHMANN AND U. POLLER 1 Dep. of Earth Sciences, Johannes Gutenberg-University, Mainz, Germany (banders@mail.uni-mainz.de) 2 Max-Planck-Institut of Chemistry, Dep. of Geochemistry, Mainz, Germany