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Preservation of coesite in exhumed eclogite; insights from Raman mapping

Patrick J. O'Brien and Martin A. Ziemann
Preservation of coesite in exhumed eclogite; insights from Raman mapping (in From field observation to experimental petrology and back; a special issue to honour Werner Schreyer, Walter Maresch (editor), Edward Grew (editor) and Friedrich Seifert (editor))
European Journal of Mineralogy (October 2008) 20 (5): 827-834

Abstract

Abundant coesite relics have been identified in a single thin section from a carbonate-bearing eclogite from the Saidenbach Reservoir, Erzgebirge, German Variscides. The rock is only very slightly retrogressed despite having experienced granulite facies conditions during exhumation. Coesite, mostly 0.1-0.2 mm in long dimension, occurs mainly in garnet but has also been identified in omphacite. Partial transformation of coesite to palisade quartz, the relief contrast between these two phases, and radial cracks around the inclusions allowed an easy optical identification of coesite. Micro-Raman mapping of specific phases from grids of thousands of individual Raman spectra helped show that: (1) a visible palisade quartz corona between coesite and host is in places lacking, and (2) some inclusions contain aragonite. Based on published results from modeling of elastic behavior of minerals along a pressure-temperature path, a scarcity of palisade quartz suggests that capturing of coesite during garnet growth occurred well into the coesite stability field such that quartz growth at the coesite = quartz univariant curve during decompression was only minimal in unfractured, overpressured inclusions. Another consequence of a high pressure for inclusion capture is that final fracturing of host garnet occurs at lower temperatures. This would explain the presence of aragonite relics in some inclusions. Based on extrapolations from kinetic experiments, aragonite should invert to calcite even faster than coesite to quartz and thus suggests temperatures of 300 degrees C or less for final fracturing.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 20
Serial Issue: 5
Title: Preservation of coesite in exhumed eclogite; insights from Raman mapping
Title: From field observation to experimental petrology and back; a special issue to honour Werner Schreyer
Author(s): O'Brien, Patrick J.Ziemann, Martin A.
Author(s): Maresch, Waltereditor
Author(s): Grew, Edwardeditor
Author(s): Seifert, Friedricheditor
Affiliation: Universitaet Potsdam, Institut fuer Geowissenschaften, Potsdam, Federal Republic of Germany
Pages: 827-834
Published: 200810
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
Accession Number: 2009-003337
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
N50°43'30" - N50°45'00", E13°13'00" - E13°16'00"
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200902
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