LA-MC-ICP-MS锆石微区原位U-Pb定年技术

In situ U-Pb zircon dating using laser ablation-multi ion counting-ICP-MS

利用激光多接收等离子体质谱(LA-MC-ICP-MS)技术对30～1065Ma的系列锆石进行了详细的定年研究。包含离子计数器的多接收系统使得不同质量数的同位素信号可以同时静态接收,并且不同质量数的峰基本上都是平坦的,进而可以获得高精度的数据,均匀锆石颗粒207Pb/206Pb、206Pb/238U、207Pb/235U比值的测试精度(2σ)均为2%左右,对锆石标准的定年精度和准确度在1%(2σ)左右;不同质量数同位素信号的同时静态接收使得剥蚀时间缩短,剥蚀深度变浅,相比LA-ICP-MS方法,提高了激光剥蚀的空间分辨率。对5个锆石标准和2个实际样品的测试表明,206Pb/238U年龄测定误差在1%(2σ)以内,定年结果在误差范围内与前人报道值完全一致,测试精度达到国际同类实验室先进水平。

High resolution in situ U-Pb zircon geochronology on zoned grains can obtain isotope signatures from multiple growth or thermal events.We present a method using laser ablation-multicollector-inductively coupled plasma-mass spectrometry（LA-MC-ICP-MS） to overcome complications associated with intricately zoned zircon crystals through in situ sampling of zircon volumes as small as 12 μm,25 μm and 40 μm in diameter by about 10 μm in depth.High precision U-Pb age of a series of zircon standard covering a wide age range of 30 to 1 065 Ma was acquired using LA-MC-ICP-MS.The precision of measured Pb/U ratios in homogeneous zircon is about 2%（2σ）,resulting in routinely achieved precision of U-Pb ages obtained by external calibration of～1%（2σ） or better.All masses of interest can be simultaneously recorded with a multi-ion counting system（MIC） operating in static mode,and the short ablation required to achieve such precision results in spatial resolution that is superior to comparable U-Pb zircon analyses by single collector ICP-MS.The resulting present U-Pb age for five zircon reference samples and two geological samples show an excellent agreement with the previously reported ID-TIMS or SHRIMP data.