COMPOSITION OF PHENOCRYSTS OF LAMPROITE LAVA, GAUSSBERG VOLCANO, EAST ANTARCTICA

This paper presents numerous new data on the geochemical composition of olivine, clinopyroxene, and leucite phenocrysts, as well as spinel inclusions in olivine and quench glass from lamproites of the Gaussberg Volcano (East Antarctica). Most of the olivine phenocrysts in the Gaussberg lamproites are high Mg varieties (Fo 89–91) with elevated Ni contents (up to 4900 ppm) and high Ni/Co ratios. According to the data of about 320 analyzes of clinopyroxenes, two groups of phenocrysts belonging to the diopside group have been established. Group I consists mainly of high-Mg varieties (Mg#>80), while group II clinopyroxenes are less magnesian (Mg# 52–80). The main difference between the clinopyroxenes of the two groups is manifested in the increased contents of Al2O3, FeO and reduced TiO2, Cr2O3, and NiO in the compositions of group II compared to group I, as well as different contents of trace elements, which may reflect their crystallization from different types of primary melts. According to the study of about 550 grains of leucite phenocrysts in Gaussberg lamproites, it was shown that they correspond to the ideal stoichiometry of leucite K[AlSi2O6] and are enriched in Na2O (0.05–0.35 wt %), but depleted in K2O (19.9–20.9 wt %) compared to with leucites from lamproites of other provinces. The BaO content reaches 0.3 wt %. %, SrO –0.04 wt. %. The iron content in most leucite phenocrysts varies within 0.7–1.2 wt % Fe2O3, with individual grains with low Fe2O3 contents (<0.5 wt %). In microlites of leucite in the groundmass and rims of phenocrysts, the iron content can reach 2.4 wt % Fe2O3, which may indicate more oxidized conditions at the time of lava eruption.
Based on the study of natural samples, existing experimental data and computational models, the order and conditions of crystallization of Gaussberg lamproites were restored. Crystallization proceeded in the following order: chrome spinel -> chrome spinel + olivine -> olivine + leucite (± chrome spinel) -> olivine + leucite + clinopyroxene (± chrome spinel). The near-liquidus association, represented by high-Mg olivine phenocrysts with inclusions of Cr-spinel, was formed in the temperature range from 1180 to 1250°C. Further crystallization of the melt with the formation of an association of olivine+leucite+clinopyroxene phenocrysts could occur at pressures below 2 GPa and temperatures of 1070–1180°C, corresponding to the presence of water in the magmatic system. Estimates of the redox conditions of crystallization of lamproites, obtained using different oxybarometers, vary in a wide range from QFM-0.5 to QFM+2.3.
Elevated Ni contents in liquidus olivines of Gaussberg indicate high nickel contents in the source. It is shown that the possible formation of ultraalkaline magmas in the region of Gaussberg Volcano occurred during the melting of the continental lithosphere, which was heterogeneous and included both the peridotite mantle and hydrous pyroxenite fragments.