Improving precision and accuracy of the Lu-Hf and Sm-Nd garnet dating by acid leaching
Abstract
Garnet dating offers a rare possibility of linking ages to specific PT conditions during metamorphism. Yet Sm-Nd and Lu-Hf geochronometers, routinely applied to garnets, can be imprecise and inaccurate due to the presence of sub-microscopic inclusions. Inclusions lower parent/daughter ratios or, if they have inherited isotopic systematics, lead to erroneous ages. We propose a new H_2SO_4 leaching technique and compare it with the previously used HF-HCl based method. Leaching experiments were performed on samples from the Red River shear zone (N-Vietnam) and from the Franciscan complex. Three garnet fractions from each sample were analysed: one unleached and two leached. Red River shear zone garnets that were not leached yielded 147Sm/144Nd ratios smilar to those of the whole rocks. This was due to the presence of abundant monazite inclusions. After treatment with sulphuric acid, the influence of inclusions was substantially reduced as indicated by high 147Sm/144Nd ratios of c. 2, twice as high as those obtained using HF-HCl leaching. The results were highly reproducible giving a 40±2 Ma age. In comparison with unleached garnet fractions, H_2SO_4 leaching improved 147Sm/144Nd ratios by at least 30%. All samples showed strong correlation between the grain size and leaching efficiency i.e. the finer the grain size the higher the resulting 147Sm/144Nd ratio. Garnet-lawsonite blueschist (Willow Springs Creek) and garnet amphibolite (Berkeley Hills) from the Franciscan Complex yielded highly precise Lu-Hf dates of 147.6±0.7 and 163.9±0.4 Ma, respectively. Unleached garnet fractions gave 176Lu/177Hf ratios between 21 and 22, while leached fractions gave ratios of 27 and 28. An even bigger improvement was observed for eclogite from Jenner, where H_2SO_4 leaching increased the 176Lu/177Hf ratio from 3.4 to 5.5, allowing a well defined age of 155±1.4 Ma. For Lu-Hf dating there was no clear correlation between the grain size and leaching efficiency. The Franciscan samples all failed to provide Sm-Nd ages. For both Sm-Nd and Lu-Hf our new technique is blank insensitive and, since unleached samples lie on the same isochrons as leached samples, appears not to cause elemental fractionation. In comparison with HF-HCl leaching, our results demonstrate considerably higher efficiency of sulphuric acid leaching in eliminating phosphate inclusions. Enhanced 147Sm/144Nd and 176Lu/177Hf ratios significantly improve dating precision.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA....13234A