TiO2 exsolution from garnet by open-system precipitation: evidence from crystallographic and shape preferred orientation of rutile inclusions

Journal article

Authors/Editors

Proyer, Alexander

Research Areas

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Publication Details

Author list: Proyer A, Habler G, Abart R, Wirth R, Krenn K, Hoinkes G

Publisher: Springer

Place: NEW YORK

Publication year: 2013

Journal: Contributions to Mineralogy and Petrology (0010-7999)

Journal acronym: CONTRIB MINERAL PETR

Volume number: 166

Issue number: 1

Start page: 211

End page: 234

Number of pages: 24

ISSN: 0010-7999

eISSN: 1432-0967

Languages: English-Great Britain (EN-GB)

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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 27.6A degrees 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 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.

Keywords

Exsolution, Open-system precipitation, Rhodope, Rutile needles, ultrahigh-pressure

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