Raman spectroscopic study of garnet inclusions in diamonds from the mantle transition zone
Raman spectra of syngenetic majoritic garnets were obtained from the Brazilian Sno-Luiz diamond suite. These garnets have a low majoritic content (<50%). Spectra were collected front (1) inclusions embedded in the diamonds and (2) inclusions extracted from the diamonds. From the latter, a clear relationship exists between the Raman spectra and the chemical analysis. All the majoritic garnets (Si>3) have a characteristic signature in the frequency region of the SiO4-SiO6, stretching vibrations. This vibration gives a broad peak between 800 and 900 cm(-1) just before in intense band (900-930 cm(-1)) classically assigned to Si-O stretching vibrations of the SiO4 tetrahedra in all known garnets. This broad band is also present in non-majoritic garnets (Si=3) that result front the transformation, within the diamonds, of majoritic garnets into low-pressure garnet + pyroxene intergrowths. We show that the presence of significant TiO2 content in these normal garnets leads to Raman spectra mimicking those of majoritic garnets. The occurrence of a broad shoulder between 900 and 900 cm(-1) as well as broad bands near 960 and 1030 cm(-1) in the Raman spectra are no longer diagnostic features indicative of a majoritic garnet when the TiO2 content or the garnets is high (>1 wt%) as is the case for the Sao Luiz diamonds. The full width at half maximum (FWHM) of the strong Raman mode near 910 cm(-1) is the only reliable signature that discriminates Si=3 from Si>3 garnets, The frequency of the diamond Raman mode used as an in-situ piezometer shows that the actual pressure of the garnet inclusions is between 0.5 and 0.8 GPa.