Ion microprobe U-Pb dating of zircon with a 15 micrometer spatial resolution using NanoSIMS

We developed a U-238-Pb-206* and Pb-207*-Pb-206* zircon dating method using a Cameca NanoSIMS NS50 ion microprobe. A 7-to 9-nA O- primary beam was used to sputter a 15-mu m crater, and secondary positive ions were extracted for mass analysis using the Mattauch-Herzog geometry. The multicollector system was modified to detect Zr-90(+), Pb-204(+), Pb-206(+), (UO+)-U-238-O-16, and (UO2+)-U-238-O-16 ions simultaneously. A mass resolution of about 4000 at 10% peak height and with a flat peak top was attained, and the sensitivity of Pb was about 4 cps.nA(-1).ppm(-1). A multicrystal zircon standard (QGNG) from South Australia with a U-Pb age of 1842 Ma was used as a reference for Pb+/UO+-UO2+/UO+ calibration, and on the basis of the positive correlation between these ratios, we determined the sample Pb-206/U-238 ratios. Pb-207/Pb-206 ratios were measured by magnetic scanning in single-col lector mode. The standard zircons 91500, from Canada, and SL13, from Sri Lanka, were analyzed against QGNG. Observed U-238-Pb-206* and Pb-207*-Pb-206* ages agreed well with published ages within experimental error. Then, 16 zircon grains in a metamorphic rock from Nagasaki, Japan, were analyzed. Observed ages were compatible with SHRIMP ages, suggesting that the NanoSIMS with a 15-mu m probe diameter is suitable for ion microprobe U-Pb zircon dating.