Ion microprobe (NanoSIMS 50) Pb-isotope geochronology at

Abstract

Techniques have been developed for ion microprobe Pb-isotope dating of minerals at a lateral scale <5 μm using the Cameca NanoSIMS 50. Probe spots 3–5 μm in diameter comprising 16 keV 16O − ions were utilized in the analyses by magnetic peak-switching of 204Pb +, 206Pb +, 207Pb +, and 208Pb + in NIST SRM 981 and 982 Pb metals, and ∼1 Ga uraninite, xenotime, and zirconolite reference materials. The Pb + mass resolution ( M/Δ M at 1% peak height) was adjusted to ∼5000 to ensure the separation of molecular isobars. Under such conditions, the sensitivity for Pb + in zircon and zirconolite was 3.0–3.5 and ∼10 cps/ppm/nA 16O −, respectively, about one-third the sensitivity obtained with large-geometry ion probes. Instrumental mass fractionation for Pb + was undetectable for SRM Pb metals and zirconolite, but values of up to 1.2% per atomic mass unit (enhancement of lighter isotopes) were recorded for xenotime and uraninite. As a demonstration of the technique, small zirconolite grains from a Cambrian mafic sill were analyzed in situ using a 3 μm diameter probe. A mean 207Pb */ 206Pb * age of 504 ± 18 Ma was determined, and is considered an accurate measure of the crystallization age. Small-spot geochronology with the NanoSIMS at the present time is practical mainly for high-U mineral chronometers.