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 A
degrees 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 D-0=
5.7*10(-6) m(2)/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 A degrees C) =
4.0*10(-22) m(2)/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)
a parts per thousand currency sign D (Lu). T-c calculations, using the
Dodson equation, yielded minimum closure temperatures of about 630 A
degrees C, assuming a rapid initial exhumation rate of 50A degrees/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 A degrees C must be
interpreted in terms of prograde metamorphism.