INTERACTION OF THE UPPER-MANTLE ROCKS WITH DEEP-SEATED FLUIDS AND MELTS IN THE BAlKAL RIFT ZONE

The process of melt-rock interaction in mantle inclusions from the Baikal Rift basalts was studied through autoradiography, gas chromatography, and thermobarogeochemistry. The β-autoradiograms obtained through neutron activation show nonuniform distribution of Cr and Sc in zonal crystals and near fractures and glass streaks in lherzolites. Megacrysts display a nonuniform distribution of LREE, microinclusions and glasses on their walls being enriched in them. Within the same pyroxene grains from lherzolite three types of microinclusions with fluid phases have been detected: 1) the earliest inclusions of dense carbon dioxide liquid (about 1.18 g/cm3) correspond to a pressure up to 13 kbar at 1150 °C; 2) inclusions of CO2 (1.02–1.05 g/cm3) match pressures of 8–10 kbar at 900–1,000 °C 3) CO2-containing inclusions of a density from 0.6 to 0.95 g/cm3 were formed at 2–8 kbar. The PT-conditions determined with the aid of mineral thermobarometers for garnet and spinel lherzolites [1] and estimations given in the work for fluid-gas inclusions facilitate the reconstruction of mantle diapir evolution accompanied by intense interaction between the deep melts and fluids and enclosing mantle rocks. It is highly probable that there is a concentration of melt and carbon-dioxide fluid in the upper part of the column here.