Precise U-Pb zircon dating at a scale of <5 micron by the CAMECA 1280 SIMS using a Gaussian illumination probet

摘要

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~1 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/ppmA using O_2~1 and ~13 cps/ppmA using O~1 (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 O_2~- 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, O_2~- might be preferred in order to generate sufficient Pb~+ ions for measurement with minimal loss of spatial resolution.