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

We present a secondary ionization mass spectrometry (SIMS) technique for U-Pb geochronology of rutile at high spatial resolution and sensitivity using an O-2(+) primary ion beam coupled with surficial O-2 gas deposition (O-2 flooding). The O-2(+) beam is similar to 10x more intense than conventionally applied O- or O-2(-) beams at the same lateral resolution. Natural and synthetic rutile was determined to be conductive under O-2(+) bombardment, permitting higher excavation (sputter) rates than conventional SIMS using negatively charged O-beams without detrimental effects of sample charging. The main advantage of O-2(+) 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 similar to 4 and 3% for O- and O-2(+), respectively. In addition, O-2 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 (1090 Ma), O-2(+)-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 O-2(+) bombardment can also be advantageous for SIMS analysis of other conductive minerals such as cassiterite, columbite-tantalite, hematite, ilmenite, and magnetite. ((c)) 2012 Elsevier B.V. All rights reserved.