GZ7 and GZ8 - Two Zircon Reference Materials for SIMS U-Pb Geochronology.

Here, we document a detailed characterisation of two zircon gemstones, GZ7 and GZ8. Both stones had the same mass at 19.2 carats (3.84 g) each; both came from placer deposits in the Ratnapura district, Sri Lanka. The U-Pb data are in both cases concordant within the uncertainties of decay constants and yield weighted mean 206 Pb/238 U ages (95% confidence uncertainty) of 530.26 Ma ± 0.05 Ma (GZ7) and 543.92 Ma ± 0.06 Ma (GZ8). Neither GZ7 nor GZ8 have been subjected to any gem enhancement by heating. Structure-related parameters correspond well with the calculated alpha doses of 1.48 × 1018  g-1 (GZ7) and 2.53 × 1018  g-1 (GZ8), respectively, and the (U-Th)/He ages of 438 Ma ± 3 Ma (2 s ) for GZ7 and 426 Ma ± 9 Ma (2 s ) for GZ8 are typical of unheated zircon from Sri Lanka. The mean U mass fractions are 680 μg g-1 (GZ7) and 1305 μg g-1 (GZ8). The two zircon samples are proposed as reference materials for SIMS (secondary ion mass spectrometry) U-Pb geochronology. In addition, GZ7 (Ti mass fractions 25.08 μg g-1  ± 0.18 μg g-1 ; 95% confidence uncertainty) may prove useful as reference material for Ti-in-zircon temperature estimates.