Nahcolite in fluid inclusions from the Ryoke metamorphic rocks and its implication for fluid genesis
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Nahcolite (NaHCO<sub>3</sub>) has been found in fluid inclusions in quartz veins hosted by the Ryoke metamorphic rocks in Kasado-Jima (Island), Yamaguchi Prefecture, Japan. The inclusions are classified into three types: type 1 containing vapor +/− liquid of the H<sub>2</sub>O-NaCl-CO<sub>2</sub>-CH<sub>4</sub> system, type 2 of H<sub>2</sub>O-NaCl fluids and type 3 with nahcolite solid. Salinities of the nahcolite-bearing fluids are lower than 4 wt% NaCl eq. Extremely large volume fractions of nahcolite solids, occupying more than 50% of total inclusion volume, and various fluid compositions are indicative of their origin as accidentally trapped ones while the fluids were immiscible below the nahcolite melting temperature (270 °C). A thermodynamic calculation of the nahcolite stability and a phase analysis of the fluids show log<i>a</i><sub>Na<sup>+</sup></sub> = 7.4 − pH as the minimum activity at P = 100 MPa and T = 270 °C where the neutral pH = 5.6. Hence, it can be concluded that the original (prior to immiscible state) liquids might have contained high Na<sup>+</sup> and low Cl<sup>−</sup> yielded by an interaction of a low-salinity fluid and Na-bearing minerals such as plagioclase in the host metamorphic rocks below the fractures now occupied by quartz if nahcolite precipitated from boiling fluids.
- Journal of Mineralogical and Petrological Sciences
Journal of Mineralogical and Petrological Sciences 101(5), 254-259, 2006-10-01
Japan Association of Mineralogical Sciences