Development of Three-dimensional Stress Imaging around Mineral Inclusions in Diamond

\hspace*{5mm}Most natural diamonds trap the mantle minerals and/or fluids as inclusions during its formation in the Earth's interior. Residual stress of diamond can be measured using micro-Raman spectroscopy, as Raman peak of diamond shifts to higher wavenumber when pressure is applied. Two-dimensional Raman mapping of diamond samples have disclosed the stress distribution around mineral inclusions. Due to the anisotropic elastic properties of minerals, however, three-dimensional Raman mapping is necessary for detailed and precise estimations of stress distributions. In the present study, we aimed at three-dimensional visualization of stress distribution using micro Raman spectroscopy. By measuring residual stresses around at least two mineral species with different bulk modulei and thermal expansivitivties, pressure and temperature when the inclusions were trapped in the diamond can be estimated. \hspace*{5mm}Raman spectra of diamond were measured using a confocal Raman microscope equipped with an automatic X-Y-Z stage. High spectroscopic resolution is required for measuring subtle difference (c.a. 0.1 cm-1) in residual stresses in diamond. Furthermore, high stability of spectrometer is also required because it takes long time to obtain three-dimensional mapping. Our previous studies revealed that peak position of Raman spectra contained the periodic oscillation of the peak positions with amplitude of 0.15 cm-1 synchronized the temperature regulation. In the present study, neon atomic lines for the wavenumber standard were collected into the spectrometer simultaneously with the Raman spectrum of samples. \hspace*{5mm}The studied sample was from Internationalnaya pipe (Russia) and contains olivine and chromites as inclusions. The sample was polished to optical grade and a volume containing a couple of inclusions were scanned three dimensionally. By digitizing the Raman shift of diamond, stress distribution around the inclusions can be three-dimensionally visualized and the difference in the maximum residual pressures surrounding mineral inclusions between olivine and chromite were revealed, respectively. Using the obtained maximum residual pressures, the compressibilities and thermal expansivities of diamond and inclusions, source temperature and pressure were estimated. The obtained P-T value was in the stability field of diamond.