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

    • Alexander Proyer
      Institute of Earth Sciences, University of Graz, Universitaetsplatz 2/II, 8010, Graz, Austria
    • Gerlinde Habler
      Department für Lithosphärenforschung, Fakultät für Geowissenschaften, Geographie und Astronomie, Universität Wien
    • Rainer Abart
      Department für Lithosphärenforschung, Fakultät für Geowissenschaften, Geographie und Astronomie, Universität Wien
    • Richard Wirth
      Geoforschungszentrum Potsdam, Am Telegrafenberg, 14473, Potsdam, Germany
    • Kurt Krenn
      Institute of Earth Sciences, University of Graz, Universitaetsplatz 2/II, 8010, Graz, Austria
    • Georg Hoinkes
      Institute of Earth Sciences, University of Graz, Universitaetsplatz 2/II, 8010, Graz, Austria
  • 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.

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  • http://phaidra.univie.ac.at/o:406773

  • Wissenschaftlicher Artikel

  • Angenommene Version

  • 2013

  • 166

  • 1

  • 211-234

  • Springer Science + Business Media

  • Englisch

  • Embargo

  • 31.03.2014

  • I471-N19 – Fonds zur Förderung der wissenschaftlichen Forschung (FWF)

  • P16194-N06 – Fonds zur Förderung der wissenschaftlichen Forschung (FWF)

  • P22749-N21 – Fonds zur Förderung der wissenschaftlichen Forschung (FWF)

  • 0010-7999

  • Exsolution; Rutile needles; Open-system precipitation; Ultrahigh-pressure; Rhodope

  • Dewey Dezimal Klassifikation → Naturwissenschaften und Mathematik → Geowissenschaften, Geologie → Petrology