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

0459770 - GLÚ 2017 RIV US eng J - Článek v odborném periodiku
Vrána, S. - Ackerman, Lukáš - Erban, V. - Halodová, P.
Immiscible melt droplets in garnet, as represented by ilmenite-magnetite-spinel spheroids in an eclogite-garnet peridotite association, Blanský les Granulite Massif, Czech Republic.
American Mineralogist. Roč. 101, č. 1 (2016), s. 82-92. ISSN 0003-004X. E-ISSN 1945-3027
Institucionální podpora: RVO:67985831
Klíčová slova: eclogite * Fe-Ti-rich melt * garnet peridotite * garnetite * Ilmenite-magnetite-spinel * Invited Centennial article * Moldanubian Zone * UHP crystallization
Kód oboru RIV: DD - Geochemie
Impakt faktor: 2.021, rok: 2016

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.
Trvalý link: http://hdl.handle.net/11104/0259934