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Immiscible melt droplets in garnet, as represented by ilmenite-magnetite-spinel spheroids in an eclogite-garnet peridotite association, Blansky Les granulite massif, Czech Republic

Stanislav Vrana, Lukas Ackerman, Vojtech Erban and Patricie Halodova
Immiscible melt droplets in garnet, as represented by ilmenite-magnetite-spinel spheroids in an eclogite-garnet peridotite association, Blansky Les granulite massif, Czech Republic
American Mineralogist (January 2016) 101 (1): 82-92

Abstract

Interlayered eclogite and symplectitic garnet rock that is interpreted as former garnetite are found in the Gfohl Unit of the Bohemian Massif. They show unusual Fe-Ti-rich compositions, characterized by TiO (sub 2) contents up to 2.34 wt%, and Mg# of 59.8 and 51.6, respectively. Equilibration conditions of 1250 degrees 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 (super 87) Sr/ (super 86) Sr 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 TiO (sub 2) , 3.7 Al (sub 2) O (sub 3) , 0.2 Cr (sub 2) O (sub 3) 27.9 Fe (sub 2) O (sub 3) , 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.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 101
Serial Issue: 1
Title: Immiscible melt droplets in garnet, as represented by ilmenite-magnetite-spinel spheroids in an eclogite-garnet peridotite association, Blansky Les granulite massif, Czech Republic
Affiliation: Czech Geological Survey, Prague, Czech Republic
Pages: 82-92
Published: 201601
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 54
Accession Number: 2016-025617
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 7 tables
N48°31'60" - N51°04'00", E12°05'60" - E16°49'60"
Secondary Affiliation: Academy of Sciences of the Czech Republic, Institute of Geology, CZE, Czech Republic
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 201613
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