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

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.