Titel
TiO2 exsolution from garnet by open-system precipitation: evidence from crystallographic and shape preferred orientation of rutile inclusions
Autor*in
Alexander Proyer
Institute of Earth Sciences, University of Graz, Universitaetsplatz 2/II, 8010, Graz, Austria
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Abstract
We investigated rutile needles with a clear shape preferred orientation in garnet from (ultra) high-pressure metapelites from the Kimi Complex of the Greek Rhodope by electron microprobe, electron backscatter diffraction and TEM techniques. A definite though complex crystallographic orientation relationship between the garnet host and rutile was identified in that Rt[001] is either parallel to Grt<111> or describes cones with opening angle 27.6° around Grt<111>. Each Rt[001] small circle representing a cone on the pole figure displays six maxima in the density plots. This evidence together with microchemical observations in TEM, when compared to various possible mechanisms of formation, corroborates a precipitate origin. A review of exchange vectors for Ti substitution in garnet indicates that rutile formation from garnet cannot occur in a closed system. It requires that components are exchanged between the garnet interior and the rock matrix by solid-state diffusion, a process we refer to as “open-system precipitation” (OSP). The kinetically most feasible reaction of this type will dominate the overall process. The perhaps most efficient reaction involves internal oxidation of Fe2+ to Fe3+ and transfer from the dodecahedral to the octahedral site just vacated by Ti4+:6M2+3TiAl[AlSi2]O12+6M2+2,5TiAlSi3O12=10M2+3.0Al1.8Fe0.2Si3O12+M2++2e−+12TiO2. OSP is likely to occur at conditions where the transition of natural systems to open-system behaviour becomes apparent, as in the granulite and high-temperature eclogite facies.
Stichwort
ExsolutionRutile needlesOpen-system precipitationUltrahigh-pressureRhodope
Objekt-Typ
Sprache
Englisch [eng]
Persistent identifier
https://phaidra.univie.ac.at/o:406773
Erschienen in
Titel
Contributions to Mineralogy and Petrology
Band
166
Ausgabe
1
Seitenanfang
211
Seitenende
234
Verlag
Springer Science + Business Media
Verfügbarkeitsdatum
31.03.2014
Datum der Annahme zur Veröffentlichung
2013
Zugänglichkeit

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