HR: 0800h
AN: V51C-0584    [Abstracts]
TI: FC-1: a Zircon Reference Standard for the Determination of Hf Isotopic Compositions via Laser Ablation ICP-MS
AU: * Coyner, S J
EM: scoyner@ufl.edu
AF: University of Florida, Dept. of Geological Sciences PO Box 112120, Gainesville, FL 32611-2120 United States
AU: Kamenov, G D
EM: kamenov@ufl.edu
AF: University of Florida, Dept. of Geological Sciences PO Box 112120, Gainesville, FL 32611-2120 United States
AU: Mueller, P A
EM: mueller@geology.ufl.edu
AF: University of Florida, Dept. of Geological Sciences PO Box 112120, Gainesville, FL 32611-2120 United States
AU: Rao, V
EM: oaryajiv2002@gmail.com
AF: University of Florida, Dept. of Geological Sciences PO Box 112120, Gainesville, FL 32611-2120 United States
AU: Foster, D A
EM: dfoster@geology.ufl.edu
AF: University of Florida, Dept. of Geological Sciences PO Box 112120, Gainesville, FL 32611-2120 United States
AB: Zircon was extracted from a sample of gabbroic anorthosite of the Duluth gabbro complex (1099.0 ñ 0.6 Ma) at Forest Center, MN. A previous collection from this site has been labeled FC-1 and has been used as a U-Pb age standard for SIMS analyses at the RSES of the ANU. Several other laboratories are also now evaluating FC-1 as a standard for ion probe (SIMS) and LA-ICP-MS methods. Its potential as a Hf isotopic standard for in-situ laser ablation (LA) MC-ICP-MS analyses is reported here. Zircons were divided into 6 fractions based on color and magnetic susceptibility, including: 1) clear, non-magnetic at 0deg, 2) brownish, non-magnetic at 0deg, 3) clear, magnetic at 0deg, 4) non-segregated, magnetic at 0deg, 5) non-segregated, magnetic at 1deg, and 6) non-segregated, magnetic at 3deg. Each fraction was dissolved and Hf purified on BioRad AG50W-X8 resin in 0.05N HF+0.5N HCl. Hf isotopic analyses of all 6 fractions yielded very consistent results with the average 176Hf/177Hf=0.282174 ñ 12 (2 S.D.), relative to JMC-475 176Hf/177Hf=0.282162 ñ 5 (2 S.D.). Laser ablation analyses of individual grains from the six fractions yielded similar results, with an average 176Hf/177Hf=0.282174 ñ 15 (2 S.D.). The ablation experiments were performed on spots and raster lines (both 60 micron beam size), with 8 Hz repetition rate and 70% laser power. These parameters resulted in ion beam intensities around 11-16 V total Hf. The analyses were performed with on-line Lu and Yb isobaric interference corrections, using 176Lu/175Lu=0.02653 and 176Yb/172Yb=0.5870. All isotopic ratios were corrected for mass-bias using 178Hf/177Hf=1.46718. Although the Yb correction on mass 176 is significant (ca. 6% to 16%), the average Hf isotopic ratios obtained by LA are in excellent agreement with the average of the wet plasma analyses of purified Hf. No correlation between 176Hf/177Hf and Lu/Hf ratio is observed, suggesting no detectable inhomogeneity in the Hf isotopic ratios due to Lu decay. These results indicate FC-1 zircons that are 3 degrees magnetic or less are isotopically homogeneous with respect to Hf regardless of color or shape. This means that ample (about 20 milligrams per kg rock) homogeneous zircon can be separated from this sample by standard density/magnetic methods and that FC-1 will be a useful inter-laboratory standard reference material for hafnium isotopic measurements using laser ablation. FC-1 appears to be equivalent to other reported standards in terms of isotopic homogeneity and Lu/Hf ratio (e.g., Temora-1 and 2 and 91500 zircon reference standards) and will be further evaluated for its Hf and REE content.
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting