GSA Annual Meeting in Seattle, Washington, USA - 2017

Paper No. 48-3
Presentation Time: 2:05 PM


BAUER, Ann, Earth Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Ave Bldg 54-1124, Cambridge, MA 02139 and HORSTWOOD, Matthew, NERC Isotope Geosciences Laboratory, British Geological Survey, Nottingham, NG12 5GG, United Kingdom,

In this study, we present solution and laser ablation analysis methods for coupled Lu-Hf and U-Pb isotope analyses of zircon, focusing on low-volume sampling ultimately corresponding to just 20-40 nanograms of zircon. These techniques allow for the robust analysis of complex zircons in cases where age zonation could easily result in the decoupling of U-Pb and Lu-Hf isotope information, potentially producing spurious initial Hf isotopic results. For both solution and laser ablation methods, we investigated possible methods of Yb interference correction, the potential for matrix effects, and the accurate determination of 176Lu/177Hf. Our reduced-volume laser ablation protocol provides an alternative to split-stream analysis and consumes less volume. We utilize consecutive U-Pb and Lu-Hf isotopic analyses resulting in a total spot size of 25 µm and pit depth of 18 µm, excavating just ~40 ng of zircon. We demonstrate that appropriate levels of uncertainty can be achieved on just ~20 ng total of ablated zircon. We also demonstrate the capability to determine the Hf isotope composition on an equivalent volume of zircon using solution MC-ICP-MS, and determine the suitability of analyzing solutions not subject to Hf-HREE separation. For these measurements, we optimized the sample introduction protocols to consume just 100 µL of solution, resulting in an order of magnitude improvement in required sample size versus conventional approaches. This was achieved with comparable levels of uncertainty and total sample volume as measured by laser ablation, using just 0.3-0.4 ng of Hf while achieving uncertainties ~1 εHf. The successive low-volume U-Pb and Lu-Hf laser ablation method is demonstrated for detrital zircons from the Zimbabwe Craton and the solution method is applied to microsampled granitic zircons. These applications highlight the potential utility of these methods for complex Archean and Hadean zircons.