Sharygin, VV; Panina, LI; Vladykin, NV
Two main petrographi varieties of the Smoky Butte rocks (olivine-armalcolite-phlogopite hyalolamproite and phlogopite-sanidine lamproite) have been studied by thermobarogeochemical methods. Thermometric investigations have showed that primary melt inclusions in olivine are homogenized at T > 1250 °C, apatite-hosted inclusions - at 1220-1230 °C, diopside-hosted inclusions - at 1160-1205 °C, and phlogopite-hosted ones - at 1085-1210 °C. Trapping pressure of inclusions during crystallization of olivine-2 does not exceed 2 kb. The phase composition of inclusions in these minerals is glass + gas bubble + daughter crystals. In some inclusions, gas bubble is partially or 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 components (< 6 wt.%) are common of the residual glasses of silicate-melt inclusions in minerals. They compositionally differ in the major element contents from the rocks. In general, during crystallization of the Smoky Butte lamproites, primitive mantle-derived magma gradually evolved towards strong agpaitic compositions rich in SiO2, alkalis, FeO, BaO, TiO2, ZrO2 and poor in Al2O3, CaO, MgO, P2O5. Fluid (or salt melt) was gradually enriched in CO2 and, possibly, in SO3. Such melt evolution peculiarities favored the appearance of K-Ti-richterite, K-magnesioarfvedsonite, davanite, carbonates, and barite at the latest stages. In addition to silicate-melt inclusions, Al-spinel, fresh leucite and sulfides (pentlandite, probably, Ni-rich monosulfide solid solution, violarite, chalcopyrite, pyrite) were firstly 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 locality.