Ultra-low blank analytical procedure for high precision CA-ID-TIMS U-Pb dating of single grain zircons

Abstract

Single grain zircon ID-TIMS U-Pb radiometric method has been widely used in high-precision geochronology. In this study, an ultra-low blank analytical procedure for high precision CA-ID-TIMS U-Pb dating of single grain zircons has been developed. After selection of the zircons under a microscope, multi-grain fractions were put into quartz crucibles and annealed at 900℃ for 60 hours in a muffle furnace. Subsequently, single zircons were rinsed with Milli-Q water and loaded into Teflon microcapsules with 120 μL 29 mol/L HF and placed in an oven at 180℃ for approximately 12 hours. Zircons were then removed from microcapsules, fluxed for several hours (at temperature and in an ultra-sonic bath) in 30% HNO3 and then 6 mol/L HCl; between acid washes, grains were rinsed with Milli-Q water. Meantime, Teflon microcapsules were placed on a hot plate with flux by a mixture of ultra-pure HNO3 and HCl in order to remove the leachate liquid from the partial dissolution step. Zircons were dissolved in 120 μL 29 mol/L HF with a trace of 4 mol/L HNO3 at 220℃ for 48 hours with a mixed 205Pb-235U spike, dried to salts, and re-dissolved in 120 μL 6 mol/L HCl at 180℃ overnight. After conversion of the zircon solution to 3 mol/L HCl, Pb and U were separated by anion exchange chromatography in 50 μL Teflon FEP micro-columns. Purified Pb and U were analyzed by thermal-ionization mass spectrometry on the IGGCAS TRITON Plus mass spectrometer. Both lead and uranium were loaded with 2 μL of a silica gel-phosphoric acid mixture on previously out-gassed single Re filaments. Lead isotopes were measured first by peak-jumping all ion beams into an SEM detector, with a filament temperature of about 1250-1350℃. Once the lead isotopic measurements were finished, filament temperature was increased to about 1400-1500℃ to promote ionization of uranium (as oxide), and U isotopes were measured on an SEM detector in peak-jumping mode. Pb and U isotopic fractionations were monitored throughout the course of this study by daily analysis of the NBS-981 (Pb) and U-500 (U) standards, respectively. Compared to the Pb blank level of 30-50 pg previously reported by the ID-TIMS U-Pb laboratories in China, the total procedural Pb blank in this study is reduced to more than one order of magnitude lower, reaching 2-3 pg. Meantime, the source of the laboratory U and Pb blanks has been thoroughly investigated. Moreover, high precision measurement of the Pb isotopic compositions for the total procedural blank Pb has been achieved (206Pb/204Pb, 2RSE<1%, n=9). These advances enable a more precise and accurate ID-TIMS U-Pb dating for single grain zircons in China. Finally, several standard zircons such as Temora 2, Qinghu and Plešovice, have been analyzed with the method, and the analytical results of Pb/U ages are in good agreement with previously reported values. The analytical precisions (2RSE) of the weighted mean 206Pb/238U ages for these standard zircons are better than 0.25% (n>7).