MELT INCLUSIONS IN MINERALS FROM LAMPROIT ES OF SMOKY-BUTTE (MONTANA,USA)

Two main petrographic varieties of the Smoky Butte rocks (olivine-arma lcolite-phlogopite hyalolamproite and phlogopite-sanidine lamproite) h ave been studied by thermobarogeochemical methods. Thermometric invest igations have showed that primary melt inclusions in olivine are homog enized at T > 1250 degrees C, apatite-hosted inclusions - at 1220-1230 degrees C, diopside-hosted inclusions - at 1160-1205 degrees C, and p hlogopite-hosted ones - at 1085-1210 degrees C. Trapping pressure of i nclusions during crystallization of olivine-2 does not exceed 2 kb. Th e phase composition of inclusions in these minerals is glass + gas bub ble + daughter crystals. In some inclusions, gas bubble is partially o r fully filled by salt phases (Ca-carbonate and others). According to microprobe data, high contents of SiO2 (60-70 wt.%), Al2O3, (10-13 wt. %), alkalis (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 compo nents (< 6 wt.%) are common of the residual glasses of silicate-melt i nclusions in minerals. They compositionally differ in the major elemen t contents from the rocks. In general, during crystallization of the S moky Butte lamproites, primitive mantle-derived magma gradually evolve d towards strong agpaitic compositions rich In SiO2, alkalis, FeO, BaO , TiO2, ZrO2 and poor in Al2O3, CaO, MgO, P2O5. Fluid (or salt melt) w as gradually enriched in CO2 and, possibl,in SO3. Such melt evolution peculiarities favored the appearance of K-Ti-richterite, K-magnesioarf vedsonite, davanite, carbonates, and barite at the latest stages. In a ddition to silicate-melt inclusions, Al-spinel, fresh leucite and sulf ides (pentlandite, probably, Ni-rich monosulfide solid solution, viola rite, chalcopyrite, pyrite) were firstly found in the Smoky Butte lamp roites, New data on geochemistry of the rocks and chemistry of main ro ck-forming minerals are also given for this lamproite locality.