Assessing Pb Isotopic Equilibrium Between Apatite and K‐Feldspar

Abstract Quantifying common Pb, the non‐radiogenic Pb present in a mineral independent of in situ U decay, is essential for obtaining accurate U–Pb ages in common Pb‐bearing minerals such as apatite. However, constraining the amount and composition of common Pb, as well as the timing of its entrapment, remains a persistent challenge. Common Pb in apatites may be constrained by assuming a terrestrial Pb model and measuring 204 Pb, or by fitting a two‐component mixing line between radiogenic and common components. Here, we utilize an approach that combines in situ K‐feldspar Pb isotopes (a primary common Pb reference due to negligible radiogenic ingrowth) with apatite U–Pb and trace element data. This approach allows us to understand growth relationships between apatite and K‐feldspar, providing a better framework for geo‐thermochronological interpretations. Igneous or high‐grade metamorphic apatite indicates a shared common Pb reservoir with co‐existing K‐feldspar. In contrast, recrystallized, low‐grade metamorphic apatite records distinct common Pb compositions from K‐feldspar in the same rock. Although some ages derived from recrystallized apatite appear statistically significant (e.g., Mean Squared Weighted Deviation ∼1, p ( χ 2 ) ≥ 0.05) when anchored in Tera‐Wasserburg plots using K‐feldspar 207 Pb/ 206 Pb i , they can be geologically inaccurate as the common Pb composition of recrystallized apatite is demonstrably different to the primary magmatic reservoir recorded by K‐feldspar. Rather, unanchored ordinate intercepts in Tera‐Wasserburg plots may better capture secondary common Pb signatures for recrystallized apatite, constraining common Pb at the time of (re)growth. We highlight the advantages of assessing K‐feldspar‐constrained 207 Pb/ 206 Pb corrections using a multi‐proxy geochemical approach, thereby refining thermal histories within complex geological settings.