Extreme alkali bicarbonate- and carbonate-rich fluid inclusions in granite pegmatite from the Precambrian Rønne granite, Bornholm Island, Denmark
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Thomas, R and Davidson, P and Schmidt, C, Extreme alkali bicarbonate- and carbonate-rich fluid inclusions in granite pegmatite from the Precambrian Ronne granite, Bornholm Island, Denmark, Contributions to Mineralogy and Petrology, 161, (2) pp. 315-329. ISSN 0010-7999 (2010) [Refereed Article]
Our study of fluid and melt inclusions in quartz and feldspar from granite pegmatite from the Precambrian RÃ¸nne granite, Bornholm Island, Denmark revealed extremely alkali bicarbonate- and carbonate-rich inclusions. The solid phases (daughter crystals) are mainly nahcolite [NaHCO3], zabuyelite [Li2CO3], and in rare cases potash [K2CO3] in addition to the volatile phases CO2 and aqueous carbonate/bicarbonate solution. Rare melt inclusions contain nahcolite, dawsonite [NaAl(CO3)(OH)2], and muscovite. In addition to fluid and melt inclusions, there are primary CO2-rich vapor inclusions, which mostly contain small nahcolite crystals. The identification of potash as a naturally occurring mineral would appear to be the first recorded instance. From the appearance of high concentrations of these carbonates and bicarbonates, we suggest that the mineral-forming media were water- and alkali carbonate-rich silicate melts or highly concentrated fluids. The coexistence of silicate melt inclusions with carbonate-rich fluid and nahcolite-rich vapor inclusions indicates a melt-melt-vapor equilibrium during the crystallization of the pegmatite. These results are supported by the results of hydrothermal diamond anvil cell experiments in the pseudoternary system H2O-NaHCO3-SiO2. Additionally, we show that boundary layer effects were insignificant in the Bornholm pegmatites and are not required for the origin of primary textures in compositionally simple pegmatites at least. Â© 2010 Springer-Verlag.
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