High precision microsampling and preparation of zoned garnet porphyroblasts for Sm–Nd geochronology

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 ~100μm) within a single garnet. Refined HNO₃/HF/HClO₄ partial dissolution methods cleanse coarse crushed (75–150μm size) garnet annuli of detrimental inclusions yielding high ¹⁴⁷Sm/¹⁴⁴Nd ratios (0.9 to 4.6) and consequently accurate and precise ages. Experiments on powdered garnet (<75μm size) gave low ¹⁴⁷Sm/¹⁴⁴Nd (<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 (<1ppm), resulting Nd mass per annulus can be <10ng. Recently developed techniques for the analysis of 1–10ng Nd samples yield ¹⁴³Nd/¹⁴⁴Nd with 10–20ppm (2 RSD) precision, good enough for <±1Ma age precision in most cases when garnet ¹⁴⁷Sm/¹⁴⁴Nd>~1.0. In total, this integrated sampling and analytical protocol permits the acquisition of 3–11 discrete high precision (<±1Ma) age annuli in a single 2cm diameter garnet.