DETERMINATIONS OF ZR ISOTOPIC COMPOSITION AND U-PB AGES FOR TERRESTRIAL AND EXTRATERRESTRIAL ZR-BEARING MINERALS USING LASER ABLATION-INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY: IMPLICATIONS FOR NB-ZR ISOTOPIC SYSTEMATICS

Abstract

Isotopic composition of zirconium from eight zircon and two baddeleyite samples including meteoritic zircon have been measured using a laser ablation-multiple collector-inductively coupled plasma mass spectrometer (LA-MC-ICPMS). A frequency-quadrupled Nd-YAG UV laser (266 nm), which produces pit sizes of 10-15 μm was used to ablate solid samples. The resultant precisions of the 92Zr/90Zr, 94Zr/90Zr and 96Zr/90Zr isotopic ratio measurements for zircons and baddeleyites achieved by the present LA-MC-ICPMS technique were 0.01-0.02%, 0.02-0.03% and 0.03-0.04% (2σ, SD), respectively. The obtained Zr isotopic ratios for zircons and baddeleyites show an excellent agreement with those for chemical reagents (Merck AA standard solution, JMC ICP/DCP standard and JMC metallic Zr) obtained by the solution nebulisation technique; normalised to 91Zr/90Zr=0.21814 [Nomura, M., Kogure, K., Okamoto, M., 1983. Isotopic abundance ratios and atomic weight of zirconium. Int. J. Mass Spectrom. Ion Phys., 50, 219-227.], the isotopic ratios for Merck reagent are 92Zr/90Zr=0.333935+/-0.000023, 94Zr/90Zr=0.339158+/-0.000048 and 96Zr/90Zr=0.054626+/-0.000010 (2σ, SD). Neither isotopic variation in 92Zr/90Zr ratio due to radiogenic contribution from 92Nb nor isotopic heterogeneity was found for all the zircon and baddeleyite samples.Combination of the Zr isotopic data and U-Pb age data allows us to estimate the initial abundance of 92Nb/93Nb at the formation stage of the solar system. Based on the Zr and U-Pb (Pb-Pb) isotopic data, initial 92Nb/93Nb ratio at the formation of solar system was found to be <10-4, and this value is consistent with the data obtained by the Nb-rutile in Toluca iron meteorites (92Nb/93Nb=1.6x10-5, [Harper, C.L., 1996a. Evidence for 92gNb in the early solar system and evaluation of a new p-process cosmochronometer from 92gNb/92Mo, The Astrophys. J., 466, 437-456.]). It is worth noting that the Zr isotopic data obtained here have important applications for the Nb-Zr isotopic systematics because the initial 92Nb/93Nb ratio can be calculated more accurately by using Zr-bearing minerals whose formation age could be well analysed by means of U-Pb chronometer.