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 ~100 μm) within a single garnet. Refined HNO3/HF/HClO4 partial dissolution methods cleanse coarse crushed (75–150 μm 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 (<75 μm 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 (< 1 ppm), resulting Nd mass per annulus can be < 10 ng. Recently developed techniques for the analysis of 1–10 ng Nd samples yield 143Nd/144Nd with 10–20 ppm (2 RSD) precision, good enough for <± 1 Ma 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 (<±1 Ma) age annuli in a single 2 cm diameter garnet.