Nahcolite in fluid inclusions from the Ryoke metamorphic rocks and its implication for fluid genesis

Abstract

Nahcolite (NaHCO 3 ) 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 2 O NaCl CO 2 CH 4 system, type 2 of H 2 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 loga Na + = 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 + and low Cl − 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.

Silicification of pelitic schist in the Ryoke low‐pressure/temperature metamorphic belt, Southwest Japan: Origin of competent layers in the middle crust

“…The dominant category of inclusions is two‐phase (liquid plus vapor), with degree of volatile filling 20–50 vol.% (visually estimated) except for very rare vapor‐rich inclusions (70–90 vol.%, visually estimated). Hoshino et al. (2006) reported nahcolite‐bearing fluid inclusions in quartz veins from the Ryoke metamorphic rocks in the area to the west of Iwakuni–Yanai area in this study.…”

Fluid types and their genetic meaning for the BIF-hosted iron ores, Krivoy Rog, Ukraine

“…This mineral might have originated from Na-K metasomatism or might have been derived from weathering of the alkaline granites supplying the Krivoy Rog basin with a clastic material (Kulik and Korzhnev, 1997). Hoshino et al (2006) assumed that the nahcolite precipitation resulted from fluid immiscibility (boiling) and the primary fluids responsible for its crystallization contained high amounts of Na + and CO 2 , and low quantities of Cl − .…”

“…However, the presence of the accidentally trapped nahcolite may be an additional constraint. Fluid inclusions containing this solid phase were formed in conditions not exceeding the melting temperature of the entrapped crystal, which is 270°C (Hoshino et al, 2006).…”

Nyerereite and nahcolite inclusions in diamond: evidence for lower-mantle carbonatitic magmas

“…It has two Raman bands at 684 cm À1 and 1046 cm À1 that are indicative of nahcolite (e.g. Hoshino et al, 2006).…”