High-sensitivity U–Pb rutile dating by secondary ion mass spectrometry (SIMS) with an O2+ primary beam

Abstract

We present a secondary ionization mass spectrometry (SIMS) technique for U–Pb geochronology of rutile at high spatial resolution and sensitivity using an O2+ primary ion beam coupled with surficial O2 gas deposition (O2 flooding). The O2+ beam is ~10× more intense than conventionally applied O− or O2− beams at the same lateral resolution. Natural and synthetic rutile was determined to be conductive under O2+ bombardment, permitting higher excavation (sputter) rates than conventional SIMS using negatively charged O-beams without detrimental effects of sample charging. The main advantage of O2+ is rapid sputtering at shallow primary ion penetration depths. This minimizes the contribution of surface-derived common Pb, and generates a high secondary ion flux at high sensitivity with useful yields (UY=detected ions/atoms removed from target) for Pb in rutile of ~4 and 3% for O− and O2+, respectively. In addition, O2 flooding reduces spread in the Pb+/U+ vs. UO2+/U+ calibration by mitigating crystal orientation dependent variability of sputter yields. Calibrated against primary rutile standard R10b (1090Ma), O2+-generated SIMS U–Pb and Pb–Pb age averages are accurate within <1% for Early Paleozoic to Archean rutile, without evidence for significant crystal orientation bias. We propose that O2+ bombardment can also be advantageous for SIMS analysis of other conductive minerals such as cassiterite, columbite–tantalite, hematite, ilmenite, and magnetite.