Dislocation motion enhances Pb mobility in experimentally deformed apatite

The distribution of Pb in minerals provides a key window into deciphering the timescales of geologic processes. However, the role of deformation-induced dislocations on Pb mobility and redistribution remains largely unconstrained. We conducted a series of experiments to constrain the processes controlling Pb mobility during deformation of apatite. Torsion experiments on single crystals of Durango apatite at 300 MPa confining pressure and 1100°C resulted in gradual lattice distortion and a network of subgrain boundaries in response to dislocation nucleation, movement, and recovery. Results from a static diffusion experiment at the same P-T conditions with a Pb source are consistent with known rates of volume diffusion. In contrast, torsion of an apatite single crystal coated in a Pb source revealed substantial mobility of Pb during deformation. This sample developed similar deformation-related microstructures, containing increased concentrations of Pb in and near subgrain boundaries. Our results demonstrate that during crystal-plastic deformation of apatite at these experimental conditions, Pb is transported orders of magnitude farther than predicted by published diffusivities, highlighting the importance of active crystal-plastic deformation in enhancing Pb mobility in apatite. We suggest that this enhanced mobility results from the capture and drag of Pb in Cottrell atmospheres associated with mobile dislocations during crystal-plastic deformation.

Our results have important implications for geothermochronological analyses relying on Pb concentrations in apatite, which will be affected by deformation below the Pb diffusion closure temperatures. Similar effects are likely to extend to other trace elements in accessory phases.