Fossilized high pressure from the Earth's deep interior: The coesite-in-diamond barometer

Mineral inclusions in diamonds provide an important source of information a bout the composition of the continental lithosphere at depths exceeding 120 -150 km, i.e., within the diamond stability field. Fossilized high pressure s in coesite inclusions from a Venezuela diamond have been identified and m easured by using laser Raman and synchrotron x-ray microanalytical techniqu es. Micro-Raman measurements on an intact inclusion of remnant vibrational band shifts give a high confining pressure of 3.62 (+/-0.18) GPa. Synchrotr on single-crystal diffraction measurements of the volume compression are in accord with the Raman results and also revealed direct structural informat ion on the state of the inclusion. In contrast to olivine and garnet inclus ions, the thermoelasticity of coesite favors accurate identification of pre ssure preservation. Owing to the unique combination of physical properties of coesite and diamond, this "coesite-in-diamond" geobarometer is virtually independent of temperature, allowing an estimation of the initial pressure of Venezuela diamond formation of 5.5 (+/-0.5) GPa.