Pb-diffusion in monazite: Constraints from a high- T contact aureole setting

Abstract

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 m 2/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.