Mineralogical Peculiarities of Garnet and Magnesiochromite Compositions in Diamond Inclusions from the Zapolyarnaya Kimberlite Pipe (Yakutia)

A detailed investigation was carried out to characterize garnet and magnesiochromite inclusions in diamonds from the Zapolyarnaya kimberlite pipe, located in the Yakutian diamondiferous province, Russia. Optical microscopy, electron probe microanalysis, scanning electron microscopy, Raman spectroscopy, X-ray diffraction, as well as calculations of compositions and residual stresses in the crystals were applied. Seven garnet inclusions were identified within two diamond crystals (3-3 and 3-4), along with other phases. They vary morphologically and in size (50–200 µm), are well-preserved, and appear as isometric grains or irregularly/slightly elongated crystals. Garnet composition, determined from Raman spectral band positions and crystal lattice parameters, evidences a dominance of the pyrope minal. The identified variations in pyrope and almandine content may reflect changes in the chemical composition of the diamond-forming medium at different stages of crystal growth, but generally correspond to a peridotitic paragenesis. Residual stress values of the inclusions and the surrounding diamond areas were calculated using their dependence on frequency shift of the main diamond peak. Inclusions in crystal 3-3 are characterized by residual compressive stress. Residual stresses of garnet inclusions in diamond 3-4 have negative values that may indicate residual tensile stress. Magnesiochromite inclusions were identified in 4 diamond crystals. One crystal was polished to expose the inclusions, while the remaining were located inside the crystals. Their morphology and chemical composition indicate formation at peridotitic paragenesis conditions. Comparison of central and peripheral inclusions demonstrates a multi-stage diamond growth, which is confirmed by the correlation of #Mg and #Cr parameters with the position of bands in the Raman spectra. The obtained data confirm the important role of garnets and magnesiochromites as indicators of diamond crystallization conditions of the Zapolyarnaya kimberlite pipe. This study is supported by program № FMUF-2022-0001 IEM RAS and project 121061600049-4 MSU.