Alpha Recoil Loss of Pb from Baddeleyite Evaluated by High‐Resolution Ion Microprobe (SHRIMP II) Depth Profiling and Numerical Modeling

206Pb/238U ratios have been measured as a function of depth beneath natural surfaces of baddeleyite crystals using a sensitive high-resolution ion microprobe (SHRIMP II). The data are compared to model profiles calculated using a stochastic averaging algorithm to constrain the average alpha recoil distance in baddeleyite. The precision of measured profiles is hampered by relatively high common Pb corrections near the surface and difficulty in estimating penetration depth at the start of measurements. Uncertainties in U concentration gradients within the first 200 nm may also influence the model profiles. Despite these difficulties, the measured U-Pb depth profiles are consistent with an alpha recoil distance in baddeleyite of 24 ± 7 nm. Assuming this recoil distance, Pb loss from near-surface alpha recoil becomes analytically resolvable for tabular crystals with thickness of <15–20 µm. This will not significantly affect ages based on 207Pb/206Pb ratios, but 206Pb/238U ages of bulk crystals may require corrections of 0.1–0.5% depending of grain size and shape. For typical tabular crystals, alpha recoil loss is linearly correlated with surface-area-to-volume ratios allowing simple corrections to be applied if the dimensions of the crystal are accurately determined prior to analyses.