Constraints on average alpha recoil distance during 238U decay in baddeleyite (ZrO2) from atom probe tomography.

Atom probe tomography of ²³⁸U and ²⁰⁶Pb has been applied to baddeleyite crystals from the Hart Dolerite (1791 ± 1 Ma) and the Great Dyke of Mauritania (2732 ± 2 Ma) in an effort to constrain the average nuclear recoil distance of U-series daughter nuclei and thereby correct U-Pb ages determined on small baddeleyite crystals for alpha-recoil loss of Pb. Both crystals were thought to expose natural crystal surfaces providing a boundary where maximum recoil loss could be observed, but both surfaces showed no adjacent variations in Pb concentrations. However, the Great Dyke sample shows U zoning and the associated ²⁰⁶Pb zoning is affected by alpha recoil. A forward modelling approach was used where ²⁰⁶Pb redistribution functions were determined for a range of possible alpha recoil distances and synthetic ²⁰⁶Pb/²³⁸U profiles were determined from the convolution of the observed U profile with the redistribution functions. These can be compared to the observed ²⁰⁶Pb/²³⁸U profile. A complication is that the 400 nm range of sampling is lower than the range of possible alpha recoil redistribution effects. In order to get a realistic match to the observed ²⁰⁶Pb/²³⁸U profile, it was necessary to extrapolate the observed zoning as an oscillatory pattern. This gives a best estimate for the average alpha recoil distance of about 40 nm. [ABSTRACT FROM AUTHOR]