科研之友
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摘要
Zoned garnet porphyroblasts contain one of the longest continuous records of tectonic processes, with growth histories spanning many millions of years. Well known challenges exist in the physical preparation, purification, and geochronologic analysis of zoned garnets, which have limited the progress and application of this potentially powerful geochronologic tool. Here, we present details of an integrated methodology specifically for the acquisition of high-resolution Sm-Nd geochronologic records from zoned garnets that overcome these challenges. We develop and demonstrate our methods on large test garnets from a shear zone in the Tauern Window in western Austria. Chemically contoured microdrilling permits physical sampling of many discrete age annuli (each as narrow as similar to 100 mu m) within a single garnet. Refined HNO3/HF/HCIO4 partial dissolution methods cleanse coarse crushed (75-150 pm size) garnet annuli of detrimental inclusions yielding high Sm-147/Nd-144 ratios (0.9 to 4.6) and consequently accurate and precise ages. Experiments on powdered garnet (<75 pm size) gave low Sm-147/Nd-144 (<0.18) indicating failure to cleanse powders of inclusions. An example of the potentially significant effect of inclusions on age accuracy is presented. Because total garnet sample loss due to crushing and purification can be 40-75% (depending on inclusion density), and because Nd concentrations in garnet are very low (<1 ppm), resulting Nd mass per annulus can be <10 ng. Recently developed techniques for the analysis of 1-10 ng Nd samples yield Nd-143/Nd-144 with 10-20 ppm (2 RSD) precision, good enough for <+/-1 Ma age precision in most cases when garnet Sm-147/Nd-144>similar to 1.0. In total, this integrated sampling and analytical protocol permits the acquisition of 3-11 discrete high precision (<+/-1 Ma) age annuli in a single 2 cm diameter garnet.
- 出版日期2011-2-24
