Home Physical Sciences Immiscible melt droplets in garnet, as represented by ilmenite–magnetite–spinel spheroids in an eclogite-garnet peridotite association, Blanský les Granulite Massif, Czech Republic
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Immiscible melt droplets in garnet, as represented by ilmenite–magnetite–spinel spheroids in an eclogite-garnet peridotite association, Blanský les Granulite Massif, Czech Republic

  • Stanislav Vrána EMAIL logo , Lukáš Ackerman , Vojtěch Erban and Patricie Halodová
Published/Copyright: January 9, 2016
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American Mineralogist
From the journal American Mineralogist Volume 101 Issue 1

Abstract

Interlayered eclogite and symplectitic garnet rock that is interpreted as former garnetite are found in the Gföhl Unit of the Bohemian Massif. They show unusual Fe–Ti-rich compositions, characterized by TiO2 contents up to 2.34 wt%, and Mg# of 59.8 and 51.6, respectively. Equilibration conditions of 1250 °C and 4.0 GPa are calculated for eclogite. The petrogenesis of this rock association can be best explained as high-temperature and ultrahigh-pressure magmatic cumulates. Highly decoupled Sr-Nd isotopic composition with nearly constant radiogenic 87Sr/86Sr values and a slightly negative e Nd value suggests interaction of aqueous fluid most likely derived from a subducting slab and/or from parental magmas. The garnetite contains large (up to 0.5 mm) Fe–Ti-rich spheroids of ilmenite–magnetite–spinel, interpreted as frozen droplets of a melt incorporated in the growing garnet. The interstices between these garnet crystals are filled by ilmenite–magnetite–spinel aggregates, with concave outer surfaces with trapped Fe–Ti-rich melt. These ilmenite–magnetite–spinel spheroids represent possibly the first record of such an oxidized assemblage in mantle rocks, and probably the first description of Fe–Ti-rich melt in eclogite-garnetite mantle rocks. A calculation based on mineral proportions in the spheroids and mineral composition indicates that the immiscible Fe–Ti-rich melt consisted of 28.7 TiO2, 3.7 Al2O3, 0.2 Cr2O3, 27.9 Fe2O3, 37.0 FeO, 0.8 MnO, and 1.7 MgO wt%. Petrology and geochemistry of the garnetite indicates an unusual composition for an upper mantle melt with a high oxygen fugacity and relatively high Fe content.

Keywords: Ilmenite–magnetite–spinel; Fe–Ti-rich melt; UHP crystallization; garnetite; eclogite; garnet peridotite; Moldanubian Zone; Invited Centennial article

Special collection information can be found at http://www.minsocam.org/MSA/AmMin/special-collections.html.

Acknowledgments

We gratefully acknowledge the contribution of J. Gilotti, and the reviewers R.N. Abbott and H. Brueckner for their detailed and constructive reviews, which contributed to an improved version of the article. We thank P. Gadas, Faculty of Science, Masaryk University, Brno, for microprobe analyses and J. Ďurišová for ICP-MS analyses. M. Rieder critically read an early version of the manuscript. The study was supported by a Project of the Czech Geological Survey, No. 390000. This work was partly supported by the Scientific Programme CEZ: RVO67985831 of the Institute of Geology, Academy of Sciences of the Czech Republic.

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  1. Manuscript handled by Jane Gilotti.

Received: 2015-1-5
Accepted: 2015-6-23
Published Online: 2016-1-9
Published in Print: 2016-1-1

© 2016 by Walter de Gruyter Berlin/Boston

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https://doi.org/10.2138/am-2016-5298
Keywords for this article
Ilmenite–magnetite–spinel; Fe–Ti-rich melt; UHP crystallization; garnetite; eclogite; garnet peridotite; Moldanubian Zone; Invited Centennial article

Articles in the same Issue

  1. Highlights and Breakthroughs
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  3. Highlights and Breakthroughs
  4. Safe long-term immobilization of heavy metals: Looking at natural rocks
  5. Highlights and Breakthroughs
  6. Spinel in planetary systems
  7. Review
  8. Pathways for nitrogen cycling in Earth's crust and upper mantle: A review and new results for microporous beryl and cordierite
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  12. K-bentonites: A review
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