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| Titel |
Chemical zonation in garnet: kinetics or chemical equilibrium? |
| VerfasserIn |
Jay Ague, Xu Chu, Jennifer Axler |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250106904
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| Publikation (Nr.) |
 EGU/EGU2015-6587.pdf |
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| Zusammenfassung |
| Chemical zonation in garnet is widely used to reconstruct the pressure (P), temperature (T),
time (t), and fluid (f) histories of mountain belts. Zonation is thought to result largely from
changing P - T - t - f conditions during growth as well as post-growth intracrystalline
diffusion. Chemical zonation is conventionally interpreted to mean that at least
some of the garnet interior was out of chemical equilibrium with the matrix during
metamorphism. In this case, thermally-activated diffusion in garnet is too slow to equalize
chemical potentials. However, in their groundbreaking paper, TajÄmanová et al.
(2014) postulate that in high-grade rocks, chemical zonation may actually reflect
attainment of equilibrium. In this scenario, diffusion is fast but viscous relaxation is slow
such that the zonation patterns directly mirror internal pressure gradients within
garnet. Such zoning would likely be very different than typical concentric growth
zonation.
Furthermore, Baumgartner et al. (2010) hypothesize that given significant variations in
the molar volumes of garnet endmembers, diffusional relaxation may produce internal
pressure gradients if the garnet behaves as a near constant-volume system. Consequently,
growth zoning could be preserved by pressure variations within the garnet that equalize
chemical potentials and slow or stop diffusion (i.e., the garnet is chemically heterogeneous
but maintains internal chemical equilibrium due to the pressure variations). This
mechanism predicts that areas of garnet with small compositional contrasts would
undergo more diffusional relaxation than areas with large contrasts. Moreover,
generation of large internal pressure gradients approaching 1 GPa would be expected to
induce deformation (e.g., fracturing) in regions of large compositional gradients.
Strongly growth-zoned amphibolite facies garnet from the Barrovian zones, Scotland
(Ague and Baxter, 2007) shows neither of these features. The sharp compositional
gradients are instead interpreted to reflect short residence times at peak-T conditions.
Existing diffusion coefficient calibrations predict shockingly short peak-T residence
times |
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