MICRON TO SUBMICRON SCALE MINERALOGY: HIGH-PRESSURE MINERAL INCLUSION AND SOLID SOLUTION IN DIAMOND – RICH ROCKS FROM THE KOKCHETAV MASSIF, KAZAKHSTAN

Diamond-rich garnet-biotite gneiss and dolomite marble from the Kokchetav Massif exhibit complex mineral inclusions and exsolutions of sub-micron scale. Gneissic garnets contain inclusion assemblages of diamond (Dia), Dia + graphite (Gr), mica + Dia and Cpx + Ap + Dia; diamonds are 1-10 ?m in size and have rounded or octahedral forms. Most abundant inclusions in zircon are diamond and garnet with subordinate inclusions of Dia + Grt, Dia + Gr, Dia + Phn, Cpx, coesite, Phn, carbonate, Rt, Ap and Phl. Dolomite marble consists of garnet + diopside + dolomite; inclusions of diamond occur in both garnet and diopside. Whereas inclusions of Dol and Dia were found in garnet and graphite ± diamond in dolomite inclusion. The diopside shows abundant exsolved lamellae of k-feldspar, phengite and minor phlogopite, or phlogopite + quartz. The observed inclusion and exsolution assemblages identify peak-stage mineral parageneses of Dia + Grt + suppersilicic Cpx + Coe ± Phn ± Phl and Dia + Grt + K-bearing diopside and diamond may have crystallized from fluid-bearing supercritical melt. The K₂O, SiO₂ and H₂O contents in clinopyroxene increase with increasing pressure. Substitution [Mg²⁺ Na¹⁺Al³⁺]^VI-VIII by [1.5Si⁴⁺]^VI may occur in supersilicic Cpx. Quartz exsolution in Cpx probably results from a retrograde reaction of 2 CaEskola = CaAl₂SiO₆ + 3SiO₂. Recalculated K₂O content of Kokchetav diopside is up to > 1.5 wt%, and exsolution of K-fs and Phn may express as 1KAlSi₂O₆ + 0.2CaEskola + 18.3CaMgSi₂O₆ + 0.35Mg₂Si₂O₆ + 0.7CaAl₂SiO₆ + 0.7SiO₂ + 0.8H₂O = 0.2KAlSi₃O₈ + 0.8KAl₃Si₃O₁₀(OH) ₂ + 19CaMgSi₂O₆ + 0.1CaSiO₃. The Fe-Mg partitioning between garnet and clinopyroxene and the occurrence of diamond constrain a minimum P for diamond formation at 4-5 GPa at 900-1000 ^oC suggesting continental subduction at mantle depths > 120-150 km.