Sections of Mantle Columns and Features of Metasomatic Transformations of the Lithospheric Mantle Beneath the Komsomolskaya-Magnitnaya and Deimos Kimberlite Pipes (Upper Muna Kimberlite Field) Based on Garnet Composition Studies

New geochemical data on the compositions of garnets from heavy mineral concentrates of the Komsomolskaya-Magnitnaya and Deimos kimberlite pipes (Upper Muna kimberlite field, Siberian Craton) are presented. The chemical compositions of 626 garnet grains from the Komsomolskaya-Magnitnaya pipe and 200 garnet grains from the Deimos pipe are determined by electron probe microanalysis (EPMA). Trace and rare earth element concentrations are measured in 308 garnets from the Komsomolskaya-Magnitnaya pipe and 141 garnets from the Deimos pipe using LA-ICP-MS. Pressure-temperature (PT) conditions of garnet formation are determined using monomineralic geothermobarometry (Griffin et al., 1995). Based on the calculated PT parameters and Y concentrations in garnets, the depth to the base of the depleted lithosphere (“Y edge”) and the thickness of the diamond window are estimated. For the garnets from the Deimos pipe, the “Y edge” is constrained at T ≈1180 °C and a depth of ~180 km, whereas for the garnets from the Komsomolskaya-Magnitnaya pipe it is defined at T ≈1190°C and a depth of ~190 km. Mantle column sections beneath the Komsomolskaya-Magnitnaya and Deimos kimberlite pipes are reconstructed using the methodology for reconstructing mantle sections based on peridotitic garnets. The reconstructed sections show a high abundance of garnets derived from depleted rocks (harzburgites and low-Ca harzburgites) at depths of 110–180 km (T = 750–1050°C) for the Komsomolskaya-Magnitnaya pipe and 120–180 km (T = 800–1050°C) for the Deimos pipe. Below ~180 km (for both pipes), lherzolitic garnets become predominant. The high equilibrium temperatures (T = 1300–1550°C) and trace element composition of these garnets indicate the influence of metasomatic processes associated with mafic melts. Paleogeotherm reconstruction indicates that the lithospheric thickness beneath the Upper Muna kimberlite field is ~220 km. The diamond window beneath the Upper Muna kimberlite field extends from depths of 125 km (intersection of the paleogeotherm with the graphite–diamond equilibrium boundary) down to 180–190 km (the base of the depleted lithosphere), yielding a thickness of ~55–65 km.