U-Th-Pb and Trace Element Evaluation of Existing Titanite and Apatite LA-ICP-MS Reference Materials and Determination of 208Pb/232Th-206Pb/238U Date Discordance in Archaean Accessory Phases

Abstract

The last decade has seen a marked increase in U-Th-Pb petrochronological studies focused on titanite and apatite. This has motivated the need for detailed, phase-specific method development and well-characterised reference materials with a wide variety of ages and chemical compositions. This study presents new U-Th-Pb isotope and major-, minor- and trace-element mass fraction data for ten titanite and five apatite reference materials based on integrated EPMA, LA-ICP-MS and isotope-dilution (ID) multi-collector (MC)-ICP-MS characterisation. Cross-comparison of EPMA and LA-ICP-MS data for the same titanite reference material suite demonstrates that careful selection of EPMA primary reference materials is necessary to minimise inaccuracies. We further identify a significant X-ray interference when measuring low Ce abundances in Ti-bearing phases and outline a method for its correction. New ID-MC-ICP-MS and LA-ICP-MS data suggest tri-concordance (defined as concordant ages for the ²³⁸U-²⁰⁶Pb, ²³⁵U-²⁰⁷Pb and ²³²Th-²⁰⁸Pb decay systems) of the MKED-1 titanite reference material and demonstrate the reproducibility of ²⁰⁸Pb/²³²Th measurements in some secondary titanite reference materials. Integration of ²⁰⁸Pb/²³²Th results with U-Pb geochronology provides a meaningful tool to help interpret complex U-Th-Pb isotopic age spectra. We provide example applications of ²⁰⁸Pb/²³²Th LA-ICP-MS measurements toward interpreting Archaean titanite ages from the Acasta Gneiss Complex, Northwest Territories, Canada.