170-8 The Strength of Garnet Sm-Nd Geochronology: High Age Precision on Small Samples

See more from this Division: Topical Sessions
See more from this Session: Strengthening Links between Metamorphic Conditions and Time: New Advances in High-Temperature Geochronology and Tracing P-T-t Paths of Metamorphic Terranes

Sunday, 5 October 2008: 3:15 PM
George R. Brown Convention Center, 310BE

Ethan F. Baxter1, Anthony D. Pollington1, Leah Y. Mehl1 and Emily M. Peterman2, (1)Earth Sciences, Boston University, Boston, MA
(2)Earth Science, University of California, Santa Barbara, Santa Barbara, CA
Abstract:
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.

See more from this Division: Topical Sessions
See more from this Session: Strengthening Links between Metamorphic Conditions and Time: New Advances in High-Temperature Geochronology and Tracing P-T-t Paths of Metamorphic Terranes