The Pb diffusivity was measured in synthetic NdPO4 crystals, from an epitaxial Nd0.66Pb0.17Th0.17PO4 thin film deposited on (¯101) faces (table 1). Annealing was performed at room pressure at 1200 to 1500C, for 1h to 1 month. Samples were analyzed by using transmission electron microscopy and Rutherford back-scattering spectrometry. Transmission electron microscopy demonstrated that Pb and Th diffused together from the thin film towards the crystal, and Nd moved in the opposite direction; according to the interdiffusion exchange: Pb2++Th4+ ↔ 2Nd3+. Rutherford back-scattering was used to perform depth-profiling. The measured diffusivities obeyed the Arrhenius law, with log[Do(m2/s)] = -3.41 and E = 509kJ/mol. Extrapolation of these data to crustal temperatures yielded very low diffusivities. Thus, the time required for a 50μm grain to lose all of its Pb at 800C was estimated to be longer than the age of the Earth. From these results, and others, it was concluded that most of the perturbations in the U–Th–Pb ages of monazite could not be attributed to Pb diffusion.

Pb Diffusion in Monazite - an Experimental Study of Pb2++Th4+ ↔ 2Nd3+ Interdiffusion. E.Gardés, O.Jaoul, J.M.Montel, A.M.Seydoux-Guillaume, R.Wirth: Geochimica et Cosmochimica Acta, 2006, 70[9], 2325-36

Table 1

Pb2++Th4+ ↔ 2Nd3+ Interdiffusion in NdPO4 Crystals, Normal to (¯101)

continued

Table 1 (continued)

Pb2++Th4+ ↔ 2Nd3+ Interdiffusion in NdPO4 Crystals, Normal to (¯101)