Screening criteria for reliable U–Pb geochronology and oxygen isotope analysis in uranium-rich zircons: A case study from the Suzhou A-type granites, SE China

Abstract

Zircon is widely used in geochronology and as an isotopic tracer in igneous processes. However, the use of isotopic data obtained from zircon requires evaluating of processes that might compromise the robustness of its isotopic systems. In this study we have assessed how different U contents and degrees of metamictization have affected the SIMS U–Pb and oxygen-isotope signatures of zircon–quartz pairs from the U-rich A-type granites from Suzhou (southern China). Our SIMS results show that zircons with low U concentrations define a U–Pb age of 126.1±0.5Ma and homogeneous mantle δ18O values of 5.34±0.46‰ (2SD). In contrast, zircons with high U contents show a positive correlation between U concentration and U–Pb ages (123–157Ma) and a negative correlation between U concentration and δ18O values (range from 5.3 to 3.7‰). These observations are integrated with Raman spectra and oxygen-isotope analyses of coexisting quartz to reveal that the anomalously high U–Pb ages and low δ18O values reflect the degree of radiation damage to the zircons (quantified by radiation dose in displacements-per-atom, Ddpa). The older SIMS U–Pb ages are a direct result of the matrix effect caused by metamictization, whereas the low δ18O values result from interaction with OH-bearing fluids, the infiltration and diffusion of which was facilitated in the radiation-damaged areas.Coexisting quartz preserves primary O-isotope signatures of the magma better than high-U zircon, and thus provides a constraint for zircon oxygen-isotope values. This study demonstrates that it is essential to evaluate the degree of radiation damage before/after carrying out dating or oxygen-isotope analysis on zircons with high U contents. The results indicate that for the Suzhou pluton, Ddpa<0.03 is a robust discriminant threshold to identify zircons with primary oxygen-isotope ratios, and Ddpa<0.08 is a robust discriminant to screen for reliable U–Pb dating. These different screening values reflect the evidence that oxygen-isotope compositions can be affected at a lower level of post-crystallization disturbance of the zircon lattice than the U–Pb system.