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

We developed a 238U–206Pb⁎ and 207Pb⁎–206Pb⁎ zircon dating method using a Cameca NanoSIMS NS50 ion microprobe. A 7-to 9-nA O− primary beam was used to sputter a 15-μm crater, and secondary positive ions were extracted for mass analysis using the Mattauch–Herzog geometry. The multicollector system was modified to detect 90Zr+, 204Pb+, 206Pb+, 238U16O+, and 238U16O2+ 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 206Pb/238U ratios. 207Pb/206Pb ratios were measured by magnetic scanning in single-collector mode. The standard zircons 91500, from Canada, and SL13, from Sri Lanka, were analyzed against QGNG. Observed 238U–206Pb⁎ and 207Pb⁎–206Pb⁎ 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-μm probe diameter is suitable for ion microprobe U–Pb zircon dating.