Precise U–Pb zircon dating at a scale of <5 micron by the CAMECA 1280 SIMS using a Gaussian illumination probe
Precise U–Pb zircon dating at a scale of <5 micron by the CAMECA 1280 SIMS using a Gaussian illumination probe
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DOI：
10.1039/c0ja00113a
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发表时间：
2011-04
影响因子：
3.4
通讯作者：
Yu Liu;Xian‐Hua Li;Qiu-li Li;G. Tang;Q. Yin
中科院分区：
文献类型：
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作者：
Yu Liu;Xian‐Hua Li;Qiu-li Li;G. Tang;Q. Yin
锆石可以说是最好的，当然是最常用的矿物的U-Pb地质年代学。现代大几何结构二次离子质谱（西姆斯）已被常规用于精确测定锆石U-Pb年龄，其横向分辨率为10-30 μm。然而，在现场U-Pb测年的规模约。对于具有复杂结构和化学特征的细粒锆石和/或锆石晶体，5 μm或更小的尺度仍然是地球科学界的一个挑战。本文介绍了一种利用CAMECA ims-1280西姆斯对年龄小于5 μm的侏罗纪锆石进行精确U-Pb定年的方法。高斯模式主要O2−和O−探头的ca。5.2μm和ca. 4.5通过优化一级柱，获得了直径为10 μm，束流强度为100 pA左右的荧光探针。二次离子光学系统经过优化，以确保在锆石中具有高的Pb+灵敏度，使用O2−时产生1.21 cps/ppm/nA，使用O−时产生1.13 cps/ppm/nA（采用氧驱技术）。作为该方法的示范，分析了三个具有良好特征的锆石标准，AS 3（1099 Ma），Plesovice（377 Ma）和青湖（159.5 Ma）的年龄范围。我们证明，这些锆石标准，他们的年龄可以确定的精度和准确度为1-2%，使用一个点<5微米。在小光斑U-Pb锆石年代学中，O−一次射束比O2−射束更受青睐，因为它具有更高的密度，在目标表面产生更小的陨石坑，最终U-Pb年龄的精度和准确性之间的权衡微不足道。对于较年轻的贫铀矿物，O2−可能是首选，以产生足够的Pb+离子进行测量，空间分辨率损失最小。
Zircon is arguably the best, certainly the most commonly used mineral for U–Pb geochronology. Modern large-geometry secondary ion mass spectrometry (SIMS) has been routinely utilized for precise U–Pb zircon age determination at a lateral resolution of 10–30 μm. However, in situ U–Pb dating at a scale of ca. 5 μm scale or less for fine-grained zircons and/or zircon crystals with complex structural and chemical features is still a challenge to the geoscience community. Here we describe a method of precise U–Pb dating for zircons as young as the Jurassic age at a scale of up to <5 μm using the CAMECA ims-1280 SIMS. Gaussian mode primary O2− and O−probes of ca. 5.2 μm and ca. 4.5 μm in diameter with beam intensities of ∼100 pA were obtained, respectively, by optimizing the primary column. Secondary ion optics was optimized to ensure a high Pb+ sensitivity in zircons, producing ∼21 cps/ppm/nA using O2− and ∼13 cps/ppm/nA using O− (with oxygen flooding technique). As a demonstration of this method, three well-characterized zircon standards with a range of ages, AS3 (1099 Ma), Plesovice (377 Ma) and Qinghu (159.5 Ma), were analyzed. We demonstrate with these zircon standards that their ages could be determined with precision and accuracy of 1–2% using a spot <5 micron. The O− primary beam is preferred over the O2− beam for small-spot U–Pb zircon geochronology, because it has higher density and produces smaller craters on the target surface, with insignificant trade off in precision and accuracy of the final U–Pb ages. For U-poor minerals of younger ages, O2− might be preferred in order to generate sufficient Pb+ ions for measurement with minimal loss of spatial resolution.