Constraints on the duration of high-pressure metamorphism in the Tauern Window from diffusion modelling of discontinuous growth zones in eclogite garnet

Journal article

Authors/Editors

Proyer, Alexander

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Publication Details

Author list: Dachs E, Proyer A

Publisher: Wiley

Place: MALDEN

Publication year: 2002

Journal: Journal of Metamorphic Geology (0263-4929)

Journal acronym: J METAMORPH GEOL

Volume number: 20

Issue number: 8

Start page: 769

End page: 780

Number of pages: 12

ISSN: 0263-4929

eISSN: 1525-1314

Languages: English-Great Britain (EN-GB)

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Abstract

An eclogite sample from the Grossgockner region of the Hohe Tauern, Austria contains garnet with a pronounced compositional discontinuity between a Mn-rich core and an Fe-rich rim. This jump in composition was caused by a garnet-consuming reaction followed by growth of the garnet rim + omphacite and marks the prograde transition from epidote amphibolite to eclogite facies metamorphism. Garnet growth ended at peak metamorphic conditions of 570 degreesC, 17 kbar, but intracrystalline diffusion continued until about 450 degreesC, 4 kbar on the retrograde path. This garnet overgrowth texture represents a natural diffusion couple and a time span of 1 Myr was calculated from the diffusion pro le developing out of the original sharp compositional step. For typical crustal densities, this time corresponds to a minimum average velocity in the range 4.6-7.4 cm yr(-1) (for vertical movement), which is one of the fastest exhumation rates reported. The diffusion of all divalent cations of four profiles was modelled, both analytically and numerically. Both approaches gave comparable results, but the times computed for each element were always discrepant up to a factor of 2. Variations of diffusion coefficients within 2 in analytical calculations remedied this and gave consistent upper time limits. Numerical modelling does not require the simplifications introduced in the analytical approach. On the other hand, error propagation was computationally unfeasible with this method.

Keywords

diffusion modelling, garnet, geospeedometry, Tauern eclogites

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