3 research outputs found
The duration of prograde garnet crystallization in the UHP eclogites at Lago di Cignana, Italy
The distinct core-to-rim zonation of different REEs in garnet in
metamorphic rocks, specifically Sm relative to Lu, suggests that Sm-Nd
and Lu-Hf isochron ages will record different times along a prograde
garnet growth history. Therefore, REE zonations in garnet must be
measured in order to correctly interpret the isochron ages in terms of
the garnet growth interval, which could span several m.y. New REE
profiles, garnet crystal size distributions, and garnet growth modeling,
combined with previously published Sm-Nd and Lu-Hf geochronology on a
UHP eclogite of the Zermatt-Saas Fee (ZSF) ophiolite, Lago di Cignana
(Italy), demonstrate that prograde garnet growth of this sample occurred
over a similar to 30 to 40 m.y. interval. Relative to peak metamorphism
at 38 to 40 Ma, garnet growth is estimated to have begun at similar to
11 to 14 kbar pressure at similar to 70 to 80 Ma. Although such a
protracted garnet growth interval is surprising, this is supported by
plate tectonic reconstructions which suggest that subduction of the
Liguro-Piemont ocean occurred through slow and oblique convergence.
These results demonstrate that REE zonations in garnet, coupled to
crystal size distributions, provide a powerful means for understanding
prograde metamorphic paths when combined with Sm-Nd and Lu-Hf
geochronology. (C) 2009 Elsevier B.V. All rights reserved
Burial rates during prograde metamorphism of an ultra-high-pressure terrane: an example from Lago di Cignana, western Alps, Italy
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 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