Composition of Magmatic Melts from the Southern Baikal Volcanic Region: A Study of Inclusions in Olivine from Trachybasalts

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) (Fig1). Inclusions were studied in minerals from the trachybasalts of Kadyr-sug (sample Kd-1) and Sagan volcanoes (sample Be-1) and a lava flow of the protovalley of the B. Yenisei River (sample Be-2). The lavas contain porphyritic grains of olivine and, more rarely, pyroxene. The size of the phenocrysts varies from 1 to 3 mm. The groundmass is usually completely crystallized and consists of plagioclase microlites, small grains of pyroxene, olivine, and opaque minerals, sulfide globules, and apatite needles. Minerals were analyzed on a Camebax Microbeam electron microprobe. The chemical compositions of these rocks and minerals are given in Table 1. Melt 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 (12.3–13.4wt %), TiO2 (4.3–5.1 wt %), and P2O5 (up to 0.8 wt %) were detected among the daughter crystals of the melt inclusions (Fig 2, Table 2). The glasses of melt inclusions on the surface were analyzed on a Camebax Microbeam electron Microprobe. The homogenized glasses (16 analyses, Table 3) 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 H