ИСТИНА

Majoritic garnets are widely abundant as inclusions in diamonds worldwide. Pyroxene exsolution textures were registered in garnets from mantle xenoliths in kimberlites, as well as in rocks of ultrahigh-pressure metamorphic complexes, which provides evidence for their crystallization at pressures >7−8 GPa. We studied three large (>5 mm) garnet xenocrysts (Samples 317, 559 and 563) from the Mir pipe (Yakutia) containing regularly oriented pyroxene and olivine lamellae with angles of 71−72° between them. Garnets of all samples are pyrope-rich (75.1−78.6 mol %), with medium CaO (4.5−5.8 wt %) and low Cr2O3 (up to 0.59 wt %) concentrations. Clinopyroxene lamellae are enriched in diopside component (89.9−94.4 mol %). Olivines are characterized by the high Mg# (up to 0.96) and extremely high NiO concentration (1.6 wt % in Sample 559; 2.79 wt % in Sample 317). According to the data of 3D X-ray tomography using a scanner SkyScan1172 and 3D analysis (CT-An software), garnet (Sample 317) contains 9 vol % pyroxene and 0.5 vol % olivine lamellae. Calculation of the compositions of primary garnets demonstrated that Si content in them exceeded 3 f.u. (3.084 in Sample 317; 3.088 in Sample 559; 3.094 in Sample 563) providing evidence for incorporation of majoritic component. The formation of such garnets occurred at pressures >7.5 GPa and high temperatures (Ni in majoritic garnet and olivine). Subsequent decrease of PT parameters to 3.0−3.1 GPa and 950−1000°C resulted in pyroxene and Ni-olivine exsolution in former majoritic garnet.