The Strength of Garnet Sm-Nd Geochronology: High Age Precision on Small Samples

Garnet geochronology has value in elucidating the rate and duration of metamorphic processes including P-T-t paths, deformation, and devolatilization. Despite its utility, Sm-Nd geochronology can be frustrated by two well documented limitations: 1. small Nd sample size due to low Nd concentrations in garnet resulting in poor analytical precision, and 2. the susceptibility of garnet to contamination by REE rich inclusions which can result in low Sm/Nd ratios. Here we present recent data from Sifnos, Greece; the Norweigian Caledonides; the Wepawaug Schist, Connecticut; and the Tauern Window, Austria to illustrate how we have overcome these two limitations and demonstrate the strengths of modern garnet Sm-Nd geochronology.

Recent advances in thermal ionization mass spectrometry including the use of NdO+ analysis have improved the precision of ¹⁴³Nd/¹⁴⁴Nd on small samples. Using a Ta₂O₅ activator slurry, we achieve sub-10ppm internal precision and 13-20ppm long term external precision on 4 nanogram Nd standards and natural samples. The new analytical advances permit high precision analysis of very small garnet separates (>4mg) – or microzones within individual garnets – depending on garnet Nd concentration, which is often <1ppm . Several studies have shown that with HF-based partial dissolution cleansing of garnets, sufficiently high ¹⁴⁷Sm/¹⁴⁴Nd ratios (between 1.0 and 5.0) are usually achieved. We have found much greater success in partial dissolution cleansing of crushed separates (100-200 mesh), rather than powdered separates. 13ppm (2sigma) precision on garnet ¹⁴³Nd/¹⁴⁴Nd analysis will produce sub-million year age precision (2sigma) for any garnet whose ¹⁴⁷Sm/¹⁴⁴Nd exceeds about 1.5. A significant challenge remains in cleanly separating Nd from garnets with Nd concentrations <0.1ppm. For such samples, additional chemical procedures are required to achieve high yields and eliminate ionization inhibiting elements and substances. The ability to achieve sub-million year age precision even on very small garnet samples represents the primary advantage of Sm-Nd garnet geochronology.