SILICATE-MELT INCLUSIONS IN MINERALS OF LAMPROITES FROM SMOKY BUTTE (Montana, USA) Available to Purchase

Two main petrographic varieties of the Smoky Butte rocks (olivine-armaIcolite-phlogopite hyalolamproite and phlogopite-sanidine lamproite) have been studied by thermobarogeochemical methods. Thermometric investigations have shown that primary melt inclusions in olivine homogenize at T >> 1250 °C, apatite-hosted – at 1220–1230 °C, diopside-hosted – at 1160–1205 °C, and phlogopite-hosted inclusions – at 1085–1210 °C. Trapping pressure of inclusions during crystallization of olivine-2 does not exceed 2 kbar. The phase composition of inclusions in these minerals is glass + gas bubble + daughter crystals. In some inclusions, the gas bubble is partially or fully filled with salt phases (Ca-carbonate and others). According to microprobe data, high contents of SiO₂ (60–70 wt.%), Al₂O₃ (10–13 wt.%), alkalies (9–12 wt.%), BaO (0.7–1.9 wt%), ZrO₂, and F (up to 0.5 wt.%), and moderate contents of TiO₂ (2.5–4.5 wt.%) and other femic components (< 6 wt.%) are common of the residual glasses of silicate-melt inclusions in minerals. They differ from the rocks in contents of major elements. In general, during crystallization of the Smoky Butte lamproites, primitive mantle-derived magma gradually evolved toward strong agpaitic compositions rich in SiO₂, alkalies, FeO, BaO, TiO₂, ZrO₂ and poor in Al₂O₃, CaO, MgO, and P₂O₅. Fluid (or salt melt) was gradually enriched in CO₂ and, possibly, SO₃. Such specific features of the evolution of melt favored appearance of K-Ti-richterite, K-magnesioarfvedsonite, davanite, carbonates, and barite at the latest stages. In addition to silicate-melt inclusions, AI-spinel, fresh leucite, and sulfides (pentlandite, Ni-rich monosulfide solid solution, viola rite, chaIcopyrite, and pyrite) were also found in the Smoky Butte lamproites. New data on geochemistry of the rocks and chemistry of main rock-forming minerals are also given for this lamproite occurrence.