Diffusion-limited REE uptake by eclogite garnets and its consequences for Lu-Hf and Sm-Nd geochronology

Garnets from the Zermatt-Saas Fee eclogites contain narrow central peaks for Lu+Yb+Tm±Er and at least one additional small peak towards the rim. The REE Sm+Eu+Gd+Tb±Dy are depleted in the cores but show one prominent peak close to the rim. These patterns cannot be modeled using Rayleigh fractionation accompanied by mineral breakdown reactions. Instead, the patterns are well explained using a transient matrix diffusion model where REE uptake is limited by diffusion in the matrix surrounding the porphyroblast. Observed profiles are well matched if a roughly linear radius growth rate is used. The secondary peaks in the garnet profiles are interpreted to reflect thermally activated diffusion due to temperature increase during prograde metamorphism. The model predicts anomalously low 176Lu/177Hf and 147Sm/144Nd ratios in garnets where growth rates are fast compared to diffusion of the REE, and these results have important implications for Lu-Hf and Sm-Nd geochronology using garne

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

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 270kJ/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, Tc, 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). Tc 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 = 1mm). 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 metamorphis

Diffusion-limited REE uptake by eclogite garnets and its consequences for Lu–Hf and Sm–Nd geochronology

ISSN:0010-7999ISSN:1432-096