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% (2 σ, 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 (2 σ, 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=1.6×10 −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.