Evaluating reference materials and common-Pb corrections for high-resolution apatite UPb geochronology

We report isotope dilution thermal ionization mass spectrometry (ID-TIMS) and laser ablation split stream inductively coupled plasma mass spectrometry (LASS) UPb data for a suite of widely available reference apatites: Fish Canyon Tuff, Mount Dromedary, TEMORA 2, and Duluth Complex anorthosite. We apply different common-Pb correction strategies to the UPb data sets: (1) anchoring to a Stacey and Kramers (1975) model Pb composition; (2) unanchored 2-D ²³⁸U/²⁰⁶Pb-²⁰⁷Pb/²⁰⁶Pb isochron regressions; and (3) unanchored 3-D ²³⁸U/²⁰⁶Pb-²⁰⁷Pb/²⁰⁶Pb-²⁰⁴Pb/²⁰⁶Pb isochron regressions. The different common-Pb corrections yield consistent dates within each ID-TIMS and LASS data set, with 3-D regression method producing the highest precision isochrons. FCT apatite produces an ID-TIMS 3-D isochron age of 28.8 ± 3.7 Ma with (²⁰⁷Pb/²⁰⁶Pb)_i = 0.851 ± 0.021. Mount Dromedary apatite yields an ID-TIMS 3-D isochron age of 98.4 ± 0.5 Ma with (²⁰⁷Pb/²⁰⁶Pb)_i = 0.839 ± 0.003. TEMORA 2 apatite has an ID-TIMS 3-D isochron age of 402 ± 7 Ma and (²⁰⁷Pb/²⁰⁶Pb)_i = 0.839 ± 0.008. Duluth Complex anorthosite apatite yields an ID-TIMS 3-D isochron age of 1077 ± 9 Ma with (²⁰⁷Pb/²⁰⁶Pb)_i = 0.849 ± 0.046. The MSWDs associated with isochrons calculated from both the ID-TIMS and LASS data sets are larger than expected for a single age population, revealing complexities that are otherwise not captured by 2-D isochron methods. In the case of FCT apatite, the ID-TIMS data indicate significant heterogeneity in the initial Pb ratio ((²⁰⁷Pb/²⁰⁶Pb)_i = 0.845–0.856), invalidating this sample as a viable reference apatite for high-precision geochronology. Additionally, the common-Pb compositions of TEMORA 2 and Duluth Complex anorthosite apatites calculated using the ID-TIMS data deviate from bulk Earth Pb evolution models beyond 2σ uncertainty. The data emphasize the utility of unanchored age regressions in generating the highest fidelity apatite UPb dates. Further, TEMORA 2 and Duluth Complex apatite ages are both younger than their corresponding zircon UPb ages, highlighting the need to independently verify the ages of prospective reference apatites.