Small-volume Lu-Hf and U-Pb isotope determination of complex zircons by solution and laser ablation MC-ICP-MS

This study presents solution and laser ablation analysis methods suited to the determination of the Lu-Hf and U-Pb isotope signatures of small volumes of zircon corresponding to sub-nanogram amounts of Hf. A reduced-volume approach to laser ablation is taken with consecutive U-Pb and Lu-Hf isotopic analyses resulting in a total pit depth of 18 μm using a 25 μm laser ablation spot. This results in excavation of ~ 40 ng of zircon, equating to ~ 0.3 ng of Hf and uncertainties ~ 1 εHf (2s). The laser technique presented here is ideally suited to complex, finely-zoned zircons for which drilling to depth would intersect different zones. The possibility of reducing the total laser ablation pit depth to ~ 10 μm is demonstrated by decreasing the integration time for Hf isotope analysis without serious compromise of the uncertainty. Furthermore, the capability to determine the Hf isotope composition of the same amount of Hf using solution MC-ICP-MS is also demonstrated, as is the suitability of analyzing solutions not subject to Hf-HREE separation. For both solution and laser ablation methods, this study investigates possible methods of Yb interference correction, the potential for matrix effects, and the accurate determination of 176Lu/177Hf. Using the approaches described here, acceptable uncertainty levels are achieved to resolve complexity at the level of 25 × 18 μm (diameter × depth), and therefore this method has the potential to yield geologically meaningful results for rocks containing complexly-zoned zircons.