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.