Petrogenesis of Glasses and Microlites in Mantle Xenoliths from Baikal-Mongolia Region : a Review

Petrogenesis of various glasses, microlite assemblages and fluid and melt inclusions in minerals of mantle xenoliths from alkaline volcanics of Baikal-Mongolian region is discussed. Diversity and compositional variations of glasses vary widely and their origin can be addressed to most of previously reported hypotheses. Study of the fluid inclusions indicates that depth of trapping is usually well correlated with the depth of Moho estimated from seismological and petrological data. The density of CO_2 fluid inclusions indicates minimum pressures of their origin of 10-15kbar. Glasses from melt inclusions in minerals of Dzhilinda xenoliths (Vitim flied) can represent direct melts formed by in situ partial melting of peridotite at about 10kbar, the pressure estimated from fluid inclusions, whereas glasses from melt inclusions in minerals of garnet Iherzolite of Bereya Quarry (Vitim flied) can represent melts formed at pressures above 15kbar. The SiO_2 contents of these glasses (51-57 wt.%) are consistent with those of experimental partial melts from low-degree partial melting of peridotite. Origin of SiO_2-rich (up to 74 wt.%) glasses from harzburgite xenoliths is consistent with experimental study by Shaw et al. (1998) indicating that reaction of orthopyroxene with silica-undersaturated melt at ambient pressure results in olivine, clinopyroxene, spinel and intermediate of silicic melt. We describe unusual clinopyroxene-olivine-sanidine symplectites in hightemperature spinel Iherzolites from Burkal locality. Calculated bulk compositions of the symplectites indicate that they can represent breakdown after K-bearing pyroxene solid solution from deep mantle.論