SM-ND SYSTEMATICS OF ZIRCON

Abstract

147Sm–143Nd and 146Sm–142Nd 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 142Nd/144Nd and 143Nd/144Nd ratios varied between 0.004% and 0.08% (2σm) depending on the sample size. The 147Sm/144Nd ratios vary from 0.4 to 1.2. 143Nd/144Nd ratios are variably radiogenic, from 0.5125 to 0.5346. Combined interpretation of 147Sm–143Nd 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 ɛ143Nd of ancient zircons with a precision and accuracy sufficient for petrogenetic interpretations may not be possible because of large errors in the 147Sm/144Nd ratio, which propagate to the uncertainty of ɛ143Nd. Additionally, the ɛ143Nd values are biased by the presence of secondary Nd of unknown age and isotopic composition in more altered zircons. The 147Sm–143Nd system is more promising as a gauge for this secondary, alteration-related REE component in zircon. Application of the 146Sm–142Nd 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.