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Measuring 207Pb/206Pb ratios by atom probe tomography (APT) has provided new insight into the nanoscale behavior of trace components in zircon, and their relationship to time, temperature and structure. Here, we analyze three APT data sets for a 3.77 Ga zircon from the Beartooth Mountains, USA, and apply systematic ranging approaches to understand the spatial and spectral uncertainties inherent in 207Pb/206Pb analysis by APT. This zircon possesses two, 100% concordant U-Pb analyses by secondary ion mass spectrometry (SIMS), indicative of closed U-Pb systematics on the micron scale since crystallization. APT data sets contain sub-spherical Pb-rich (>0.25% atomic) domains with diameter <15 nm. Broadly consistent Pb-rich regions are defined in applying six different permutations of the two most common cluster identification algorithms. Measured 207Pb/206Pb ratios within Pb-rich domains vary between 0.794±0.15 (±2σ) and 0.715±0.052 depending on the ranging approach, cluster definition protocol and number of clusters interrogated. For the bulk APT data sets, 207Pb/206Pb = 0.353±0.18; this is indistinguishable from the bulk 207Pb/206Pb ratio by SIMS (0.367±0.0037), and statistically distinct from the 207Pb/206Pb ratio within clusters. Bulk and clustered 207Pb/206Pb ratios are consistent with Pb clustering at ~2.8 Ga, during protracted metamorphism and magmatism in the Beartooth Mountains. 10.1002/9781119227250.ch16