Precise determination of Phanerozoic zircon Pb/Pb age by multicollector SIMS without external standardization
Precise determination of Phanerozoic zircon Pb/Pb age by multicollector SIMS without external standardization
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DOI：
10.1029/2009gc002400
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发表时间：
2009-04-10
影响因子：
3.5
通讯作者：
Chamberlain, Kevin R.
中科院分区：
文献类型：
--
作者：
Li, Xian-Hua;Liu, Yu;Chamberlain, Kevin R.
锆石一直被认为是最好的地质年代计和地球科学中最重要的计时器。现代微束技术如西姆斯和LA-ICPMS已成功地应用于锆石U-Pb年龄的现场测定，空间分辨率为20-30 μ m或更低。由于Pb和U之间的分馏效应，通过外部标准化表征良好的天然锆石标准的基质匹配校准是精确微束U-Pb年龄测定的主要要求，这通常导致外部年龄误差超过1%。另外，具有封闭U-Pb系统的锆石可以通过Pb-207/Pb-206同位素比值直接定年，而不需要外部标准化，这对于年龄大于1.0 Ga的锆石是一种常见的做法，但对于相对年轻（< 1.0 Ga，特别是中生代）的锆石，由于分析精度的限制，这一做法并不适用。本文介绍了用新一代大半径扇形磁收集器Cameca IMS-1280西姆斯测量古生代锆石Pb-207/Pb-206的方法。结合多接收器模式、核磁共振（NMR）磁控制器和氧驱技术，Pb-207/Pb-206比值的测量精度达到< 0.1%和0.1 ~ 0.2%，Pb/Pb年龄误差达到< 0.4%和1 ~ 3%（不包括U衰变常数的不确定性），最新新元古代和晚古生代至中生代的年龄分别为锆石。因此，多接收器西姆斯能够直接测定年轻至中生代的锆石Pb/Pb年龄，具有地质意义的不确定性。该技术可用于薄切片中锆石的直接定年。此外，它对其他一些富铀矿物（即，斜锆石和锆英石）不适合西姆斯U-Pb外标法测年。
Zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences. Modern microbeam techniques such as SIMS and LA-ICPMS have been successfully applied to in situ U-Pb zircon age determinations, at spatial resolutions of 20-30 mu m or less. Matrix-matched calibration by external standardization of well-characterized natural zircon references is a principal requirement for precise microbeam U-Pb zircon age determination due to fractionation effects between Pb and U, which usually result in an external age error exceeding 1%. Alternatively, zircons with a closed U-Pb system can be directly dated by measurement of Pb-207/Pb-206 isotopic ratio without external standardization, which has been a common practice for zircons older than 1.0 Ga, but not for relatively young (< 1.0 Ga and particularly Phanerozoic) ones because of limitations of analytical precision. We describe in this paper a method of Pb-207/Pb-206 measurement on Phanerozoic zircons using a new generation of large radius magnetic sector multicollector Cameca IMS-1280 SIMS. In combination with multicollector mode, a Nuclear Magnetic Resonance (NMR) magnet controller and oxygen flooding techniques, we achieve precisions of Pb-207/Pb-206 ratio of < 0.1% and 0.1 similar to 0.2%, propagating to Pb/Pb age errors < 0.4% and 1 - 3% (excluding U decay constant uncertainties), for zircons of latest Neoproterozoic and late Paleozoic to Mesozoic age, respectively. Therefore, the multicollector SIMS is capable of direct determination of zircon Pb/Pb ages as young as Mesozoic age with uncertainties of geological significance. This technique is useful for direct dating of zircons in thin sections. Moreover, it has significance for dating of some other U-rich minerals (i.e., baddeleyite and zirconolite) that are not suitable for SIMS U-Pb dating by external standardization.