Sm-Nd systematics of zircon

Sm-147-Nd-143 and Sm-146-Nd-142 systems were studied in nineteen 3.32-4.02 Ga zircon grains from Jack Hills metaconglomerate, 12 of which were previously analyzed for Lu-Hf. Nd concentrations in the zircons are 0.9-20 ppm. The amount of Nd recovered was between 13 and 446 pg. The analytical precision of Nd-142/Nd-144 and Nd-143/Nd-144 ratios varied between 0.004% and 0.08% (2sigma(m)) depending on the sample size. The Sm-147/Nd-143 ratios vary from 0.4 to 1.2. Nd-143/Nd-144 ratios are variably radiogenic, from 0.5125 to 0.5346. Combined interpretation of Sm-141-Nd-143 and U-Pb systems shows that LREE in zircons are a mixture of a primary component with high Sm/Nd (>1.6-2.0), and a secondary, younger component with low Sm/Nd. Determination of initial epsilon(143)Nd of ancient zircons with a precision and accuracy sufficient for petrogenetic interpretations may not be possible because of large errors in the Sm-147/Nd-144 ratio, which propagate to the uncertainty of epsilon(143)Nd. Additionally, the epsilon(143)Nd values are biased by the presence of secondary Nd of unknown age and isotopic composition in more altered zircons. The Sm-147-Nd-143 system is more promising as a gauge for this secondary, alteration-related REE component in zircon. Application of the Sm-146-Nd-142 system in zircon to determine the timing of early terrestrial differentiation is promising. It will require advancement of the analytical procedures to achieve the lowest possible blanks and maximum sensitivity. Ion yield as high as 20-30% will be required. (C) 2004 Elsevier B.V. All rights reserved.