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[SMP25-P06] Dmisteinbergite (metastable CaAl2Si2O8) in nanogranitoid inclusions in granulites
Keywords:granulite, nanogranitoid inclusion, metastable phase
Melt inclusions called nanogranitoids occur in garnet and other refractory minerals in granulites in continental collision orogens. They indicate partial melting of host rocks during high P-T metamorphism. They also offer some insights into the subsequent cooling and exhumation processes. Nanogranitoid inclusions sometimes include glass as well as spherulitic and dendritic crystals of quartz and feldspar and euhedral to subhedral phenocrysts of quartz. The quartz phenocrysts commonly show SEM-CL zoning with bright Ti-rich interiors and dark Ti-poor overgrowths. The boundaries between the interiors and overgrowths are relatively sharp (< 10 µm). We recently confirmed the occurrence of dmisteinbergite, a metastable phase with anorthite composition, in nanogranitoid inclusions in granulites from Antarctica and Sri Lanka and other areas. Here we report on its modes of occurrence.
Dmisteinbergite occurs as fibrous to platy crystals in nanogranitoid inclusions not only in mafic granulite but also in pelitic granulite. It coexists with plagioclase, K-feldspar, quartz, biotite, epidote, and even andalusite. The occurrence of dmisteinbergite was suggested by its composition, shape, bright SEM-CL image, and coexistence with sodic plagioclase, and confirmed by its Raman spectrum. Dmisteinbergite has been found in a burnt coal dump, pseudotachylytes, and meteorites (Gorelova et al., 2023). Recently it was also found in nanogranitoids in granulites (Wannhoff et al., 2022). Even though anorthite is the only stable polymorph, it never nucleates as a first phase in the supercooled melt, and metastable high-temperature polymorphs of CaAl2Si2O8 are formed instead (Abe et al., 1991). The occurrence of metastable dmisteinbergite is consistent with the occurrences of glass and spherulitic to dendritic quartz in nanogranitoid inclusions, and is indicative of large supersaturation (supercooling). Such a situation may have been produced by a quick shift of solidus of trapped melt to high temperatures by degassing of hydrous melt inclusions induced by cracking of host garnet by deformation during exhumation of granulites. The most significant point is, however, the preservation of such non-equilibrium states by quick cooling, which is consistent with the sharp zone boundary of zoned quartz phenocrysts. Thus, quick cooling tectonic process is inferred for the granulites after the high-temperature metamorphism in the depth of continental collision orogens.
References: Abe et al. (1991) Physics and Chemistry of Minerals, 17, 473–484; Gorelova et al (2023) Geoscience Frontiers, 14 101458; Wannhoff et al. (2022) American Mineralogist, 107, 2315–2319.
Dmisteinbergite occurs as fibrous to platy crystals in nanogranitoid inclusions not only in mafic granulite but also in pelitic granulite. It coexists with plagioclase, K-feldspar, quartz, biotite, epidote, and even andalusite. The occurrence of dmisteinbergite was suggested by its composition, shape, bright SEM-CL image, and coexistence with sodic plagioclase, and confirmed by its Raman spectrum. Dmisteinbergite has been found in a burnt coal dump, pseudotachylytes, and meteorites (Gorelova et al., 2023). Recently it was also found in nanogranitoids in granulites (Wannhoff et al., 2022). Even though anorthite is the only stable polymorph, it never nucleates as a first phase in the supercooled melt, and metastable high-temperature polymorphs of CaAl2Si2O8 are formed instead (Abe et al., 1991). The occurrence of metastable dmisteinbergite is consistent with the occurrences of glass and spherulitic to dendritic quartz in nanogranitoid inclusions, and is indicative of large supersaturation (supercooling). Such a situation may have been produced by a quick shift of solidus of trapped melt to high temperatures by degassing of hydrous melt inclusions induced by cracking of host garnet by deformation during exhumation of granulites. The most significant point is, however, the preservation of such non-equilibrium states by quick cooling, which is consistent with the sharp zone boundary of zoned quartz phenocrysts. Thus, quick cooling tectonic process is inferred for the granulites after the high-temperature metamorphism in the depth of continental collision orogens.
References: Abe et al. (1991) Physics and Chemistry of Minerals, 17, 473–484; Gorelova et al (2023) Geoscience Frontiers, 14 101458; Wannhoff et al. (2022) American Mineralogist, 107, 2315–2319.