激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)方解石U-Pb定年是一种新兴的地质年代学方法,在解决一系列关键地质问题中具有广阔的应用前景.由于方解石中U和Pb含量低,采用高灵敏度扇形磁场电感耦合等离子体质谱仪(SF-ICP-MS)比四极杆电感耦...激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)方解石U-Pb定年是一种新兴的地质年代学方法,在解决一系列关键地质问题中具有广阔的应用前景.由于方解石中U和Pb含量低,采用高灵敏度扇形磁场电感耦合等离子体质谱仪(SF-ICP-MS)比四极杆电感耦合等离子体质谱仪(Q-ICP-MS)更有优势.研究发现对于Thermo Element XR型号的SF-ICP-MS仪器,当采用Jet+X锥组和N_(2)增敏技术时,U和Pb的灵敏度最佳.提高灵敏度不仅能明显改善方解石定年分析精度,而且可在高空间分辨率条件下(<110μm)实现低^(238)U(<1μg g^(−1))或^(207)Pb(如<10Ma)含量样品的定年测试.对于NIST SRM 614、ARM-3和WC-1这三个标准物质来说,在激光束斑85μm和能量密度~2.0J cm^(−2)的条件下,由激光剥蚀产生的^(206)Pb/^(238)U动态分馏不显著(<2.2%).在最佳仪器条件下,系统分析了四个常用的方解石U-Pb标准物质(WC-1、Duff Brown Tank、JT和ASH-15).结果与已发表的同位素稀释热电离质谱(ID-TIMS)数据相吻合,验证了方法的可靠性.研究进一步证明,基于LA-ICP-MS二维元素成像技术辅助选取样品定年区域可提高方解石U-Pb定年的成功率.展开更多
U-Pb geochronology of calcite using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is an emerging method,with potential applications to a vast array of geological problems.Due to the low level...U-Pb geochronology of calcite using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is an emerging method,with potential applications to a vast array of geological problems.Due to the low levels of U and Pb in calcite,measurement using higher-sensitivity instruments,such as sector field (SF) ICP-MS,have advantages over more commonly used quadrupole (Q) ICP-MS instruments.Using a Thermo Element XR ICP-MS,we demonstrate that the Jet+X cone combination with the N_(2) enhancement technique provides the best sensitivity for measuring U and Pb isotopes.This higher sensitivity improves the precision of calcite U-Pb isotope measurements,and permits dating at high spatial resolutions (<110μm) and for samples containing low contents of^(238)U (<1μg g^(–1)) and/or^(207)Pb (i.e.,young samples<10 Ma).Using a spot size of 85μm with a low fluence (~2.0 J cm^(–2)),the laser-induced elemental fractionation of^(206)Pb/^(238)U in the NIST SRM 614,ARM-3 and WC-1 reference materials are insignificant (<2.2%).Adopting the optimized instrument parameters,we analysed four commonly-used calcite U-Pb reference materials (WC-1,Duff Brown Tank,JT,and ASH-15).The results match well with published isotope dilution data,demonstrating the reliability of our technique.ARM-3,an andesitic glass,is shown to be an appropriate reference material for both^(207)Pb/^(206)Pb calibration and instrument optimization because of its moderate contents of U(~3.75μg g^(–1)) and Pb (~12.7μg g^(–1)).We further demonstrate that the image-guided approach using LA-ICP-MS elemental mapping is an efficient tool in obtaining robust ages.展开更多
文摘激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)方解石U-Pb定年是一种新兴的地质年代学方法,在解决一系列关键地质问题中具有广阔的应用前景.由于方解石中U和Pb含量低,采用高灵敏度扇形磁场电感耦合等离子体质谱仪(SF-ICP-MS)比四极杆电感耦合等离子体质谱仪(Q-ICP-MS)更有优势.研究发现对于Thermo Element XR型号的SF-ICP-MS仪器,当采用Jet+X锥组和N_(2)增敏技术时,U和Pb的灵敏度最佳.提高灵敏度不仅能明显改善方解石定年分析精度,而且可在高空间分辨率条件下(<110μm)实现低^(238)U(<1μg g^(−1))或^(207)Pb(如<10Ma)含量样品的定年测试.对于NIST SRM 614、ARM-3和WC-1这三个标准物质来说,在激光束斑85μm和能量密度~2.0J cm^(−2)的条件下,由激光剥蚀产生的^(206)Pb/^(238)U动态分馏不显著(<2.2%).在最佳仪器条件下,系统分析了四个常用的方解石U-Pb标准物质(WC-1、Duff Brown Tank、JT和ASH-15).结果与已发表的同位素稀释热电离质谱(ID-TIMS)数据相吻合,验证了方法的可靠性.研究进一步证明,基于LA-ICP-MS二维元素成像技术辅助选取样品定年区域可提高方解石U-Pb定年的成功率.
基金co-supported by the National Key R&D Program of China (Grant No. 2018YFA0702602)the National Natural Science Foundation of China (Grant Nos. 41903024, 41525012)the Youth Innovation Promotion Association of the Chinese Academy of Sciences (Grant No. 2022066)。
文摘U-Pb geochronology of calcite using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is an emerging method,with potential applications to a vast array of geological problems.Due to the low levels of U and Pb in calcite,measurement using higher-sensitivity instruments,such as sector field (SF) ICP-MS,have advantages over more commonly used quadrupole (Q) ICP-MS instruments.Using a Thermo Element XR ICP-MS,we demonstrate that the Jet+X cone combination with the N_(2) enhancement technique provides the best sensitivity for measuring U and Pb isotopes.This higher sensitivity improves the precision of calcite U-Pb isotope measurements,and permits dating at high spatial resolutions (<110μm) and for samples containing low contents of^(238)U (<1μg g^(–1)) and/or^(207)Pb (i.e.,young samples<10 Ma).Using a spot size of 85μm with a low fluence (~2.0 J cm^(–2)),the laser-induced elemental fractionation of^(206)Pb/^(238)U in the NIST SRM 614,ARM-3 and WC-1 reference materials are insignificant (<2.2%).Adopting the optimized instrument parameters,we analysed four commonly-used calcite U-Pb reference materials (WC-1,Duff Brown Tank,JT,and ASH-15).The results match well with published isotope dilution data,demonstrating the reliability of our technique.ARM-3,an andesitic glass,is shown to be an appropriate reference material for both^(207)Pb/^(206)Pb calibration and instrument optimization because of its moderate contents of U(~3.75μg g^(–1)) and Pb (~12.7μg g^(–1)).We further demonstrate that the image-guided approach using LA-ICP-MS elemental mapping is an efficient tool in obtaining robust ages.
