TEM Investigation of Nano-scale Precipitates in Ultrahigh-Pressure Clinopyroxenes

This investigation details the TEM characterization of epitaxially-exsolved nano-scale siliceous precipitates in cores of ultrahigh-pressure (UHP) clinopyroxenes from eclogites of the Bohemian Massif and Western Gneiss Region, and a Kokchetav Massif garnet pyroxenite. Siliceous precipitates are observed in cores of UHP clinopyroxenes from multiple UHP metamorphic terranes, and are often used as indicators for UHP metamorphism. Without atomic-scale characterization, these precipitates have been identified only as quartz or coesite. The close structural and orientation relationships of siliceous phases and their host clinopyroxenes are revealed by High Resolution TEM and Scanning TEM (HRTEM/STEM) images and Selected Area Electron Diffraction. Low pressure, low density siliceous phases of keatite, &agr;-cristobalite, tridymite, siliceous glasses, and albite are reported here, not the previously described higher density silica polymorphs. The phase that precipitates may be controlled by a complex combination of factors. Particularly, calculated vacancy content (Ca-Eskola component) in clinopyroxenes may be the overriding factor in creating localized low pressure micro-environments for nano-precipitate exsolution. Higher vacancy content may be linked with the lowest density silica, tridymite; lower vacancy contents may be linked with somewhat higher density polymorphs of silica, including &agr;-cristobalite and the first confirmed natural occurrences of keatite. Mechanisms of exsolution are closely related to the interfaces that develop between phases. The geometry of the substrate in epitaxial mineral growth may cause growth of phases not stable at existing P-T-X conditions. Little strain in HRTEM/STEM images is observed and shows favorable lattice matching between siliceous precipitates/hosts; interface models detailing a calculated 5.4% difference in unit cells at most support this. Nano-scale size effects are also important---they stabilize nano-scale and metastable phases such as keatite and &agr;-cristobalite outside of bulk mineral stabilities. In combination with vacancy contents and interfacial effects, localized OH contents within pyroxenes and exsolution temperature may also be important factors. The presence of sheet silicates in all three UHP terranes can likely be attributed to retrograde processes.;High tensile strength and low thermal expansivity/compressibility of clinopyroxenes compared to siliceous phases indicates exsolved phases may not affect the rheology of the exhuming rock, but may ultimately affect the interpretation of the P-T path.