Immiscible melt droplets in garnet, as represented by ilmenite-magnetite-spinel spheroids in an eclogite-garnet peridotite association, Blansky Les granulite massif, Czech Republic
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
- alkaline earth metals
- Bohemian Massif
- Central Europe
- chemical composition
- crystallization
- Czech Republic
- eclogite
- Europe
- fugacity
- garnet group
- garnet peridotite
- Gfohl Unit
- granites
- granulites
- high-grade metamorphism
- igneous rocks
- ilmenite
- immiscibility
- isotope ratios
- isotopes
- magnetite
- major elements
- mantle
- melts
- metals
- metamorphic rocks
- metamorphism
- Moldanubian
- Nd-144/Nd-143
- neodymium
- nesosilicates
- orthosilicates
- oxides
- oxygen
- P-T conditions
- peridotites
- plutonic rocks
- pressure
- rare earths
- silicates
- sphericity
- spinel
- Sr-87/Sr-86
- stable isotopes
- strontium
- ultrahigh pressure
- ultramafics
- upper mantle
- Blansky Les Massif
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.