A new method for U-Pb geochronology of cassiterite by ID-TIMS applied to the Mole Granite polymetallic system, eastern Australia

摘要

We present a new method for precisely determining the U-Pb isotopic composition of cassiterite by isotope dilution-thermal ionisation mass spectrometry (ID-TIMS). Complete sample dissolution and spike-sample equilibration is accomplished in a pressure vessel with HBr. Isolation of small quantities of U and Pb from the tin-rich matrix is based on a novel TRU-spec column separation with total procedural blanks of<9 pg of Pb and< 0.09 pg of U. The method was applied to measure the U-Pb isotopic compositions of cassiterite from the Yankee deposit of the Mole Granite in eastern Australia. A Total-Pb/U isochron age (~(238)U/~(206)Pb, ~(207)Pb/~(206)Pb and ~(204)Pb/~(206)Pb) produces the most precise age estimate of this cassiterite at 246.48 ± 0.51 Ma, which is within error of a previously published ID-TIMS U-Pb age of hydrothermal xenotime from the same location. Initial ~(206)Pb/~(204)Pb (21.28 ± 0.4) and ~(207)Pb/~(204)Pb (15.759 ± 0.034) ratios of the Yankee cassiterite, determined from the isochron regression, are evolved relative to those expected for average crust and measured in Mole Granite feldspars, suggesting a non-magmatic, U-rich component in the mineralising fluids. We also present new U-Pb isotopic compositions for the AY-4 cassiterite from the Furong deposit, South China, which has been used as a U-Pb calibration reference material in several previous studies. A Total-Pb/U isochron age derived from three measured aliquots yields a crystallisation age of 151.9 ± 2.2 Ma for the AY-4 cassiterite, younger than obtained previously by ID-TIMS dating (Yuan et al., 2011).