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Geological Society, London, Special Publications; 2003; v. 220; p. 83-91;
DOI: 10.1144/GSL.SP.2003.220.01.05
© 2003 Geological Society of London

Advances in the Chronometry of Major Minerals - Prograde Histories

Improving precision of Sm-Nd garnet dating by H2SO4 leaching: a simple solution to the phosphate inclusion problem

Robert Anczkiewicz1,2 & Matthew F. Thirlwall3

1 University College London, Gower Street, London WC1E 6BT, UK rob{at}gl.rhul.ac.uk
2 Institute of Geological Sciences, Polish Academy of Sciences, Kraków Research Centre, ul. Senacka 1, 30-001 Kraków, Poland
3 Department of Geology, Royal Holloway, University of London, Egham, Surrey, TW20 0EX, UK

Sm-Nd garnet geochronology is often hampered by the presence of submicroscopic inclusions of rare earth element-rich phosphates, which lower age precision, lead to inaccurate ages or make dating impossible. We propose a single-step sulphuric acid leaching technique as a very efficient tool in eliminating phosphate inclusions, which helps to achieve more precise and more accurate Sm-Nd garnet dates. Examples from silimanite grade metapelites demonstrate the much higher efficiency of this method in comparison with previously proposed techniques based on HF and HCl. 147Sm/144Nd ratios obtained on garnets leached by sulphuric acid were twice as high as those obtained by HF and HCl leaching. This led to age precision better than 3% for Tertiary samples. Comparison of leached and unleached nearly inclusion-free garnets from high pressure granulites, demonstrates that there is no Sm/Nd fractionation induced by H2SO4 leaching. Our new technique eliminates phosphates, but does not attack garnet. This considerably reduces the necessity for hand-picking and lowers the amount of sample required for analysis making Sm-Nd garnet dating a more easily applied geochronometer.





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