摘要
建立了超低本底单颗粒锆石高精度CA-ID-TIMS U-Pb分析方法.与国内早期实验室报道的全流程Pb本底(30~50 pg)相比,目前本工作全流程Pb本底水平已降低1个数量级以上(2~3 pg),可以更好地满足单颗粒锆石高精度ID-TIMS U-Pb定年要求.同时,详细研究了Pb流程本底的来源,并且对全流程Pb本底获得较高精度的Pb同位素比值测定结果.采用所建立分析流程对清湖(Qinghu),Ple?ovice,Temora 2等标准锆石进行了分析,获得了与文献报道值一致的Pb/U年龄测定结果,^(206)Pb/^(238)U加权平均值年龄测定精度优于0.25%(2σ,n≥7).
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 mL 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% HNO_3 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 HNO_3 and HCl in order to remove the leachate liquid from the partial dissolution step. Zircons were dissolved in 120 mL 29 mol/L HF with a trace of 4 mol/L HNO_3 at 220℃ for 48 hours with a mixed ^（205）Pb-^（235）U spike, dried to salts, and re-dissolved in 120 mL 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 mL 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 mL 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（^（206）Pb/^（204）Pb, 2RSE1%, 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 Ple？ovice, 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 ^（206）Pb/^（238）U ages for these standard zircons are better than 0.25%（n7）.
出处
《科学通报》
EI
CAS
CSCD
北大核心
2016年第10期1121-1129,共9页
Chinese Science Bulletin
基金
国家自然科学基金(41273018)
国家重点基础研究发展计划(2013CB835005,2012CB822005)
岩石圈国家重点实验室研究课题(11401790)资助
