|Title:||Pb‐diffusion in monazite: Constraints from a high‐T contact aureole setting|
|Authors:||C.R.M. McFarlane, and T.M. Harrison|
|Publication:||Earth Planet. Sci. Lett., v. 250, p. 376‐384.|
|Publish Date:||Oct 2006|
We report measurements of Pb diffusion in natural monazite that was subjected to a static, anhydrous, high‐temperature overprint in the contact aureole of the Makhavinekh Lake Pluton (MLP), northern Labrador, Canada. The 1850 Ma monazite grains we studied were reheated to temperatures of 900 ± 25 °C during intrusion of MLP at 1322 Ma. Ion‐microprobe depth‐profiling reveals the outermost ⁓0.3 µm of these grains to contain Pb/U gradients that can be modeled as a result of one‐dimensional concentration‐independent diffusion (i.e., an error‐function profile) at 1322 Ma. These gradients are combined with temperature‐time histories predicted by a numerical model of conductive heat flow in the contact aureole to calculate Pb diffusion coefficients between 3.2 × 10−28 and 1.3 × 10−27 m2/s. This range overlaps with the down‐temperature extrapolation of experimental Pb diffusion data obtained under dry conditions confirming the slow diffusivity of Pb in monazite at crustal temperatures and timescales.