MEGACRYSTALLYNE ULTRA-DEPLETED PERIDOTITES OF THE SIBERIAN CRATON LITHOSPHERIC MANTLE: ORIGIN AND EVOLUTION

Petrological, mineralogical and geochemical study of xenoliths of extremely depleted megacrystalline peridotites (MP) from Siberian kimberlites was performed. These xenoliths are composed mostly (> 90 vol. %) by large grains (< 10 cm) olivine with high Mg# (92-95), garnet (< 5 vol. %) mostly presented by strongly subcalcic and Cr-rich pyropes, enstatite (< 3 vol. %), high-Cr (usually Cr₂O₃ > 60 wt.%) chromite (0-1 vol. %), very rare xenoliths (< 5 %) contain rare grains of Cr-diopside, and around 10 % of xenoliths contain diamond and graphite.

Obtained results allow us to suppose that: a) ultra-depleted MP of the Siberian Craton lithospheric mantle (SCLM) were formed in Middle Archean in the depth 120-230 km as a result of high degree of partial melting of primitive mantle material and represented initially the most ancient extremely refractory and depleted residual free from high-melting components and diamond phases of which were in equilibrium with melts close in composition to komatiites; b) shortly after MP formation started processes of their carbonatite metasomatism (including significant part of process of U-type lithospheric diamonds formation), and T of equilibrium at that time (Middle Arhean) at definite depth were significantly (100-120^oC) higher than those for time of kimberlite emplacement (Middle Paleozoic). This peculiarity explains lack of balance between high amounts of chromite inclusions in diamonds and very low contents or even full absence of chromite in MP: decrease of T shifted reaction En+Chr == Ol+Gt to the right with decrease of Chr and increase of Gt in system; c) evolution of MP composition was related with carbonatite and silicate metasomatism of variable intensity. U-type diamond formation was related with initial stages of carbonatite metasomatism and its progressive development produced increase of Ca-component in Gt, then appearance of CPX in system, wehrlitization and carbonation of initial MP. Multistage silicate metasomatism was mainly related with influence of the plume-related melts and dynamic processes on the SCLM MP, increased amounts of Gt, OPX and CPX, and changed texture of rock.

The work is supported by grants: RFBR (16-05-00811), Russian Science Foundation (17-17-01154, 18-17-00249)