REFINING TECHNIQUES FOR OBTAINING PURE GARNET SEPARATES FOR GEOCHRONOLOGY

Garnet geochronology relies on obtaining clean aliquots that do not contain dissolved, inclusion phases with high REE concentrations (e.g., Zrn, Mnz, Ap, Ep). Phases with high concentrations of Sm and Nd are particularly problematic because many have Nd concentrations orders of magnitude greater than garnet. Inclusions in garnet can disrupt isochron fits, shift ages to old values, or reduce age precision by lowering the garnet ¹⁴⁷Sm/¹⁴⁴Nd ratio. Leaching or partial dissolution can effectively remove inclusions; however, optimization is important for obtaining high precision ages. We present multiple partial dissolution methods tested on a single high grade paragneiss sample from the Arthur River Complex, Fiordland, New Zealand.

Garnet contain abundant randomly distributed ca. 600 µm apatite inclusions and the focus of our experiments was to find the most effective method of removal. Published partial dissolution methods utilizing multiple acids were used as a starting point for developing effective new phosphate inclusion removal methods. Unleached garnet has Nd=4.663 ppm, Sm=3.993, and ¹⁴⁷Sm/¹⁴⁴Nd =0.5177. A leach modeled after Baxter et al. (2005) on bulk garnet and a single garnet crystal yielded high Nd concentrations of 3.501 ppm and 4.828 ppm, and low ¹⁴⁷Sm/¹⁴⁴Nd of 0.6329 and 0.5915, respectively. The preferred technique was a single garnet crystal leached in HF for 60 minutes at 120˚C, HNO₃ for 180 minutes at 90˚C, and evaporated in HCLO₄ for 24 hours at 120˚C that yielded the highest ¹⁴⁷Sm/¹⁴⁴Nd ratio (1.86) of 10 different experiments. The preferred technique also yielded a realistically low concentration of Nd for garnet of 1.034 ppm indicating sufficient removal of inclusion phases. A third garnet experiment using H₂SO₄ leaching for 24 hrs yielded low Nd (1.422 ppm) but a low ¹⁴⁷Sm/¹⁴⁴Nd (0.5978). Application on samples of known age is required to evaluate the broader effectiveness of our preferred method. Future work will include Sm and Nd isotopic analysis and REE analysis of apatite included in garnet.