COMPOSITION OF MAGMATIC MELTS FROM THE SOUTHERN BAIKAL VOLCANIC REGION: A STUDY OF INCLUSIONS IN OLIVINE FROM TRACHYBASALTS

Abstract

First evidence was obtained on the composition of melt and fluid inclusions in olivine phenocrysts from the trachybasalt of the southern Baikal volcanic region (eastern Tuva lava highland). Inclusions were studied by the methods of homogenization with subsequent analysis by electron and ion microprobes. Amphibole with high TiO2 content (up to 9.8 wt %) and clinopyroxene with low SiO2 (42.3-43.5 wt %) and high Al2O3 (12.3-13.4 wt %), TiO2 (4.3-5.1 wt %), and P2O5 (up to 0.8 wt %) were detected among the daughter crystals of the melt inclusions. The homogenized glasses (16 analyses) show small variations in SiO2 (46.8-50.8 wt %), TiO2 (2.1-3.6 wt %), MgO (6.3-9.8 wt %), and CaO (6.7-9.4 wt %) and high contents of alkalis (3.1-5.8 wt % Na2O and 1.8-2.5 wt % K2O) and P2O5 (0.44-0.95 wt %). The average concentrations of volatiles in the basaltic melts analyzed are (wt %) 0.46 H2O, 0.09 S, 0.07 F, and 0.04 Cl. Using CO2 fluid inclusions, the minimum pressure values were estimated for the crystallization of the olivine phenocrysts as 4.6-5.0 kbar, which corresponds to depths of 16.5-18.0 km. The calculation of the minimum CO2 content in the alkali basalt melt yields a high value of 1.13 +/- 0.14 wt %. The ion microprobe analysis of 17 trace elements in the glasses of melt inclusions demonstrated significant incompatible element enrichment in the magmas of the Baikal rift. In this respect, they are sharply different from the tholeiitic basalts of oceanic rifts and similar to oceanic island basalts.