EOCENE ZIRCON REFERENCE MATERIAL FOR MICROANALYSIS OF U-Th-Pb ISOTOPES AND TRACE ELEMENTS

Abstract

Eocene gem zircons from Reddistone Creek, Glen Innes, Australia, have been assessed for isotopic and chemical homogeneity by Secondary Ion Mass Spectrometry (SIMS), Isotope Dilution Thermal Ionization Mass Spectrometry (ID TIMS), Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS), and Wavelength Dispersive X-ray analysis (WDS). Sensitive High-Resolution Ion Microprobe (SHRIMP) SIMS data show that the U-Pb and Th-Pb isotope systems are homogeneous at the ion microprobe spot mass of a few ng for two zircons, AusZ2 and AusZ5, and that AusZ2 has good homogeneity in U, Th, and Th/U ratio. Twelve ID U-Pb TIMS analyses of separate 200–400 μm chips, measured in two different laboratories, show that AusZ2 has remarkable isotopic homogeneity at the sub-permil level, is concordant, and has a 206 Pb- 238 U age of 38.8963 ± 0.0044 Ma (2σ; n = 12, MSWD = 1.01). A similar data set for AusZ5 shows that this gem has a similarly precise and reproducible 206 Pb- 238 U age of 38.9022 ± 0.0035 Ma (2σ; n = 12, MSWD = 2.4), but exhibits scatter in 207 Pb- 235 U that is in excess of that expected based on the uncertainty of individual analyses (weighted mean 207 Pb- 235 U date of 39.162 ± 0.054 Ma, 2σ; n = 12, MSWD=10.0), with two concordant analyses and 10 slightly discordant analyses (0.7%). The slight discordance of AusZ5 may reflect an initial excess of 231 Pa. The AusZ2 and AusZ5 ID TIMS data sets demonstrate the excellent reproducibility of measurements between labs using the EARTHTIME calibration spikes and solutions. LA-ICP-MS trace element data indicate that AusZ2 is also very homogeneous for the REE and other trace elements, whereas AusZ5 shows trace element heterogeneity. WDS analysis confirms the major element homogeneity of these zircons. Our data show that the AusZ2 zircon will be an excellent Reference Material (RM) for ID TIMS, SIMS, and LA-ICP-MS microanalysis of U-Pb isotopes and trace elements in Cainozoic zircons.