Effect of impact energy on SIMS U-Pb zircon geochronology

Secondary ion mass spectrometry (SIMS) measurements of Pb isotopes and U-Pb elemental ratios in natural minerals are one of the most important methods of measuring geologic time on the million to billion year timescale. Precise and accurate geochronology requires accurate correction of elemental U and Pb fractionation in the SIMS process. The calibration schemes used to correct for this fractionation have been in use since the 1990s. However, the mechanism by which they work remains poorly understood. The independence of primary ion impact energy from the secondary voltage in the sensitive high-resolution ion microprobe (SHRIMP) is used to examine the ionization efficiency of Pb+ in zircon over a range of impact energies from 3.7kV to 15.7kV. A plateau of around 34cps/nA O-2(-)/ppm Pb-206 is observed between 5.7 and 10.7-kV impact energy. In contrast, useful yield increases steadily as impact energy is decreased. The use of a primary beam made of O-18(2)- ions allows the O-18 implanted by the primary beam to be differentiated from the O-16 in the natural silicate matrix. The oxygen isotopic results, with larger O-18/O-16 ratios at lower energies, suggest that enhanced oxygen activity at the sputtering surface may be due to progressively lower sputtering efficiency at lower energies. This would increase the Pb+ yield from the sample even as the ion yield per nA of primary beam remains constant, as a nA of molecular oxygen at 5kV or less sputters a smaller sample volume than at higher energies. The useful yield for Pb increases from 1% to 3% as the primary energy drops. Copyright (c) 2014 John Wiley & Sons, Ltd.