PROBLEMS OF DETERMINING THE THICKNESS OF THE LITHOSPHERE AND THE DIAMOND WINDOW USING THE MONOMINERAL THERMOBAROMETRY FOR PERIDOTITIC GARNETS FROM THE KIMBERLITE PIPE 23RD PARTY CONGRESS

The results of studying 339 garnets from a concentrate of the heavy fraction obtained after thermochemical dissolution of kimberlite samples from the pipe 23rd Congress of the CPSU. The chemical composition of garnets was determined by the EPMA method; the content of rare elements in garnets was determined by the IСP MS method. Based on the content of the main elements, it was determined that the samples is sharply dominated by peridotite parageneses, with a large proportion of garnets belonging to the harzburgite-dunite paragenesis - 51.9% (according to CaO-Cr2O3 contents). Moreover, out of 310 peridotite garnets, 220 grains (71%) have Cr2O3 >5 wt. % and about 17% of garnets belong to the diamond association (Sobolev, 1974). The rare earth composition of garnets, as well as the presence of high-calcium wehrlite garnets (~10%) in the sample, indicates a significant degree of lithospheric reworking by carbonatite melts/fluids. When reconstructing the paleoheterm using the calculated P-T parameters of garnets, the heat flow value was 35.3 mW/M2, and the thickness of the lithosphere was about 250 km, the thickness of the diamond window was about 40 km. However, an ambiguous situation arises in which the method of calculating the “Y edge” (the depleted boundary of the lithosphere, according to Griffin, 1995) may not work correctly. Since the sample almost completely lacks high-titanium garnets (TiO2>0.5%), indicating the transformation of lithospheric mantle rocks by silicate metasomatic agents, the “Y edge” determined at a depth of about 160 km may also indicate the absence of garnet-bearing rocks below this boundary. Also, the “diamond window” can extend to great depths.