Estimation of a maximum Lu diffusion rate in a natural eclogite garnet

Abstract

Lutetium zoning in garnet within eclogites from the Zermatt–Saas Fee zone, Western Alps, reveal sharp, exponentially decreasing central peaks. They can be used to constrain maximum Lu volume diffusion in garnets. A prograde garnet growth temperature interval of ~450–600 °C has been estimated based on pseudosection calculations and garnet–clinopyroxene thermometry. The maximum pre–exponential diffusion coefficient which fits the measured central peak is in the order of D₀= 5.7*10⁻⁶ m²/s, taking an estimated activation energy of 270 kJ/mol based on diffusion experiments for other rare earth elements in garnet. This corresponds to a maximum diffusion rate of D (~600 °C) = 4.0*10^-22 m²/s. The diffusion estimate of Lu can be used to estimate the minimum closure temperature, T_c, for Sm-Nd and Lu-Hf age data that have been obtained in eclogites of the Western Alps, postulating, based on a literature review, that D (Hf) < D (Nd) < D (Sm) ≤ D (Lu). T_c calculations, using the Dodson equation, yielded minimum closure temperatures of about 630 °C, assuming a rapid initial exhumation rate of 50°/m.y., and an average crystal size of garnets (r = 1 mm). This suggests that Sm/Nd and Lu/Hf isochron age differences in eclogites from the Western Alps, where peak temperatures did rarely exceed 600 °C must be interpreted in terms of prograde metamorphism.