PALAEOGEOTHERM, METASOMATIC TRANSFORMATIONS AND MANTLE DEPTH SECTIONS BENEATH SOME PIPES OF THE ALAKIT-MARKHA KIMBERLITE FIELD

Clinopyroxene xenocrystals from the heavy fraction mineral concentrate of the Yubileinaya pipe and garnet xenocrystals from Bobkov, CNIGRI, Schukina, Iskorka, Morozovykh, Amakinskaya, Kismet, Sytykanskaya and A16/A17 pipes were studied. The mantle palaeogeotherm beneath the Yubileinaya kimberlite pipe was reconstructed using monomineral clinopyroxene geothermobarometry and FITPLOT software. It was found that the paleogeotherm of the Yubileinaya kimberlite pipe corresponds to a surface heat flux of 34.8 mW/m2. The lithosphere thickness is 230 km and the "diamond window" thickness is 110 km. The "diamond window" thickness, determined by the "Y-edge", is 75 km. Based on the calculated equilibrium temperatures of the garnets and their Y contents, the depths of the basement of the depleted lithosphere ("Y-edge") beneath the studied kimberlite pipes were calculated. Using the reconstructed palaeogeotherm and the "Y-edge", the thickness of the diamond window was calculated for the TSNIGRI pipe - 60 km, Sytykanskaya - 60 km, Iskorka - 55 km, Bobkova - 47 km, Amakinskaya - 47 km, Kismet - 46 km, A16/A17 - 45 km, Schukina - 37 km, Morozovs - 36 km. The chemical composition of garnets indicates three types of metasomatic transformations of the lithospheric mantle - metasomatism with the participation of carbonatite melts/fluids (S-shaped REE spectra), metasomatism with the participation of silicate melts (enrichment of Ti and HFSE) and metasomatism with the participation of water-potassium fluids (enrichment of Zr, without significant enrichment of Y and HFSE). Two rock types were found to predominate: Depleted peridotites and deformed lherzolites metasomatized by silicate melts. The depth distribution of peridotite garnets was used to construct sections of mantle columns beneath kimberlite pipes. It was found that the mantle section beneath the Morozovs, Schukina, A16/A17, Kismet, Amakinskaya, Bobkova pipes from depths below 160-170 km and beneath the Iskorka, Sytykanskaya and CNIGRI pipes from depths below 180-185 km is dominated by garnets of deformed lherzolites. At this depths, the rocks have been subjected to strong metasomatic action of silicate melts. At depths of 125-185 km the rocks have been affected by carbonatite melts/fluids and to a lesser extent phlogopite metasomatism. Thus, the low diamondiferousness of the nearby pipes is associated with both the low thickness of the diamond window and a higher degree of diamond-destroying metasomatism by silicate melts.