This paper summarizes the variety in the mode of occurrence of staurolite and compiles its compositional features. Staurolite generally occurs in pelitic schists that underwent medium-pressure (P) and medium-temperature (T) metamorphism (about 0.5-1.0 GPa, 500-600°C), which is represented by the presence of the staurolite zone in the Barrovian region. However, in not a few cases, staurolite is stable even in eclogite- and granulite- facies conditions, and also in andalusite-stable low-P conditions. Mineral parageneses are important to give a constraint to P-T conditions; i.e. staurolite itself cannot be an indicator for P-T conditions. Especially, the most important point is whether the staurolite-forming environments are SiO2-saturated or not. In the SiO2-undersaturated environments, Mg-rich staurolite [Mg/(Fe+Mg)≥0.30] is stable over a wide range of P-T conditions, and is commonly associated with aluminous phases such as spinel and corundum. For example, Mg-rich staurolite is known to be stable even in ultrahigh pressure conditions, which was shown by the synthetic experiments and observation of natural rocks. Also in the granulite-facies rocks, staurolite occurs as inclusions in garnet, cordierite and plagioclase or as a matrix phase associated with spinel and corundum. Stability P-T field of staurolite is not known, but staurolite itself should be stable even in high-T conditions (≥700°C). Compilation of natural staurolite compositions revealed that exchange between bivalent and trivalent cations should be considered for formula expression. In addition, comparison of staurolites formed under high-P and medium- or low-P conditions did not show a clear relationship between staurolite compositions and pressures. We cannot give a constraint to pressure conditions only from the staurolite compositions. Geothermometer using Fe-Mg exchange reaction between garnet and staurolite has been proposed, but Fe-Mg distribution coefficients are widely scattered even in the rocks that experienced mostly the same temperature metamorphism. Solid-solution properties of staurolite are still not well known.
A new computerized mass spectrometric technique for analyzing gases in fluid inclusions has been developed. A quartz sample is subjected to heating under vacuum. The gases liberated by thermal decrepitation of fluid inclusions are instantaneously introduced into a quadrupole mass spectrometer (QMS) and ion currents of gas species are successively measured in the temperature range from 130 to 650°C. After the measurement, the average of ion currents for gas species over the whole temperature range is calculated. Using the average ion currents and the sensitivity of the QMS for the gases, the bulk composition of inclusion fluids is obtained. The pegmatitic quartz sampled from three pegmatite fields in Japan were analyzed by this method. Inclusion fluids in quartz from the Naegi pegmatite have a molar concentration of H2O higher than 98%. Inclusion fluids from the Ishikawa and Sakihama pegmatite fields are rich in CO2. The former has a molar concentration of CO2 ranging from 2 to 24%, and the latter from 8 to 11%. The analytical results are in good agreement with the observations of fluid inclusions under the microscope. Gas release profiles are different between the samples from the Naegi, Ishikawa and Sakihama pegmatite fields. The Naegi pegmatitic quartz has a single peak of release profiles both in H2O and CO2. The gas release profiles of the Ishikawa quartz are comlex, and those of Sakihama are simple. The diversity of gas release profiles observed between the pegmatitic quartz may be ascribed to the number of formation stages of fluid inclusions. A significant extent of fluid release upon heating was observed at the temperature around 573°C. This is due to fluid inclusion rapture caused by the alpha-beta phase transition of quartz. The presence of H2S as a minor component of inclusion fluids was confirmed from the mass decrepitation above.