Halite and sylvite as solid inclusions in high-grade metamorphic rocks

Solid inclusions of halite and sylvite, formed during regional and contact metamorphism have been identified by microscopy and by electron microprobe analysis in rocks from Campolungo, Switzerland and Cornone di Blumone, Italy. The solid inclusions occur in several of the major minerals crystallized during metamorphism and have been observed as idiomorphic crystals and dendrites. The compositions measured in 100 analyses from Campolungo, Switzerland and 40 analyses from Cornone di Blumone, Italy extend across the two-phase region in the system, KCl-NaCl, indicating that the salt inclusions are high temperature precipitates. In both localities compositionally zoned and unzoned crystals have been found. Measured compositions on the temperature maximum of the two-phase region indicate at least 500° C which can be compared with 500°±20° C determined by Mercolli (1982) and Walther (1983) from the Mg content of calcites from Campolungo. The solid inclusions have been trapped apart from CO₂-rich and saline, H₂O-rich fluid inclusions which have been described by Mercolli (1982) as the earliest preserved fluid inclusions in the rocks. The early precipitation of salt minerals at Campolungo indicates that fluids were saturated with NaCl and KCl at 500° C and pressures of 2,000 bars or higher. Similar relationships exist between solid and fluid inclusions in the rocks of Cornone di Blumone which formed at temperatures as high as 800° C and pressures between 0.5 and 1 kilobar (Ulmer 1983). The entrapment of halite and sylvite as solid inclusions preserves the composition of the minerals which may therefore be useful as geothermometers.