28 Jun 2023
 | 28 Jun 2023
Status: this preprint is currently under review for the journal GChron.

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

Donald Wayne Davis, Steven Denyszyn, and Denis Fougerouse

Abstract. 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.

Donald Wayne Davis et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on gchron-2023-15', Alyssa McKanna, 28 Jul 2023
    • AC1: 'Reply on RC1', Donald Davis, 20 Sep 2023
  • RC2: 'Comment on gchron-2023-15', Alberto Pérez-Huerta, 08 Aug 2023
    • AC2: 'Reply on RC2', Donald Davis, 20 Sep 2023
  • RC3: 'Comment on gchron-2023-15', Michelle Foley, 30 Aug 2023
    • AC3: 'Reply on RC3', Donald Davis, 20 Sep 2023

Donald Wayne Davis et al.

Donald Wayne Davis et al.


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Short summary
Decay of U to Pb in the mineral baddeleyite is used for precisely dating mafic rocks, but some daughter Pb atoms can be ejected out of the crystal, resulting in an age that appears too young. Atom Probe Tomography was used to map the distribution of U and Pb atoms in 3 dimensions within a baddeleyite crystal and estimate the average distance that Pb atoms are displaced by decay of U. This allows us to correct the measured age on a baddeleyite crystal knowing its size and shape.