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, emphasizing 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.