Kryza R.,Wrocław University |
Crowley Q.G.,Trinity College Dublin |
Larionov A.,Apkarpinsky All Russian Geological Research Institute Vsegei |
Pin C.,University Blaise Pascal |
And 2 more authors.
Gondwana Research | Year: 2012
Thermal annealing followed by acid etching of zircon (chemical abrasion or CA) can be successfully utilised to minimize or eliminate the effects of major and cryptic Pb-loss for SIMS U-Pb zircon dating. The procedure is demonstrated by applying the U-Pb SIMS technique to both untreated and chemically abraded zircons from the Karkonosze Granite, Sudetes, SW Poland. Conventional U-Pb SIMS dating of untreated zircons yields an apparently coherent age population (n=9) with a weighted mean 206Pb/ 238U age of 306±4Ma. Some untreated zircons display anomalously young 206Pb/ 238U ages (c. 225 and 238Ma) and are likely to have suffered substantial Pb-loss. A sub-set of zircons from the same sample was chemically abraded. Physically, zircons treated in this manner display a range in the degree of etching and partial dissolution. Extreme examples developed a 3D network of sub-μm channels which follow high-U (dark CL) zones or linear defects, such as micro fractures or indistinct cleavage planes. U-Pb SIMS dating of treated zircons (n=11) yields a mean 206Pb/ 238U age of 322±3Ma. Two analyses of treated zircons still display younger 206Pb/ 238U ages (c. 297 and 301Ma) ascribed to the effects of Pb-loss.For the analysed sample, U-Pb ages determined from chemically abraded zircons are c. 5% older than those from untreated zircons. This is attributed to effective removal of metamict domains susceptible to Pb-loss. The CA technique also removes micro-inclusions thus lowering common Pb and reducing matrix effects. A cryptic Pb-loss in untreated zircons is only recognised when compared with chemically abraded counterparts or ages determined using other isotope techniques. This clearly demonstrates the utility of CA to high-spatial resolution methods and stresses that Pb-loss is detectable at a range of scales, regardless of the analytical technique used. © 2011 International Association for Gondwana Research.