Experimental aspects of UHP metamorphism in pelitic systems

Recent experimental studies have addressed mineral phases that can be expec ted to occur in metasedimentary rocks at ultrahigh-pressure conditions. The se studies are based mainly on the P-T location of reaction curves using re versals and the determination of compressibility data as well as the compos itional change of solid solution phases with P and T in fixed mineral assem blages. In the system SiO2, the reactions bounding the coesite stability field as w ell as the rheology of coesitites and the kinetics of the coesite-quartz tr ansformation are discussed. In the system Al2O3-SiO2-H2O, the stability rel ations of kyanite, piezotite, OH-topaz, and diaspore are reviewed. In MgO-A l2O3-SiO2-H2O, investigations on the enstatite polymorphs, talc, the 10 Ang strom phase, MgMgAl-pumpellyite, and pyrope, which can coexist with coesite , are reported. Experimental studies on Mg-chloritoid, Mg-staurolite, and c hlorite also are reviewed. In FeO-Al2O3-SiO2-H2O, chloritoid, staurolite, a nd ferrosilite are addressed. In K2O-Al2O3-SiO2-H2O, K-feldspar, hollandite -type KAlSi3O8, K-cymrite, kalsilite, wadeite-type K2Si4O9, and the KAlSi2O 6 component in clinopyroxenes are considered. In K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH), studies on various white and dark micas are mainly reported, but also information on stilpnomelane and zussmanite is given. Subsequently, v arious berates, borosilicates, and phosphates-most of them are rich in Al a nd therefore have the possibility of occurring in UHP metapelites-are addre ssed. Finally, aragonite, dolomite, and magnesite are considered as well. At the end of the paper a summary is presented that points to the necessity of continuing with various kinds of studies related to the determination o f thermodynamic properties of end-member phases and solid solution series. Such studies will enable us to elucidate the stable topology of phase relat ions in KFMASH and related systems. In addition, such thermodynamic data ar e needed for more reliable geothermobarometry.