Extreme alkali bicarbonate- and carbonate-rich fluid inclusions in granite pegmatite from the Precambrian Ronne granite, Bornholm Island, Denmark

摘要

Our study of fluid and melt inclusions in quartz and feldspar from granite pegmatite from the Precambrian Ronne granite, Bornholm Island, Denmark revealed extremely alkali bicarbonate- and carbonate-rich inclusions.The solid phases (daughter crystals) are mainly nahcolite [NaHCO_3], zabuyelite [Li_2CO_3], and in rare cases potash [K_2CO_3] in addition to the volatile phases CO_2 and aqueous carbonate/bicarbonate solution. Rare melt inclusions contain nahcolite, dawsonite [NaAl(CO_3)(OH)_2], and muscovite. In addition to fluid and melt inclusions, there are primary CO_2-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 anvilcell experiments in the pseudoternary system H_2O–NaHCO_3–SiO_2. 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.