Fluid and melt inclusions in minerals of West Greenland lamprophyres

Solovova I., Girnis A., Kopylova M.

Institute of Ore deposits, petrography, mineralogy and geochemistry RAS (IGEM), Moscow, Russia

We report a reconnaissance study of melt and fluid inclusions in the rocks of West Greenland (Manitsoq area) assigned to transitional varieties between ultramafic lamprophyres and kimberlites. The lamprophyres are massive dark gray rock with no less than 50% olivine phenocrysts (mg up to 0.9) and less abundant laths of phlogopite, ilmenite, and Cr spinel embedded in an altered fine-grained groundmass of carbonate-dominated composition. The presence of phlogopite indicates that the rock can be classified as a minette or a mica lamprophyre. The groundmass contains euhedral microphenocrysts (~50 micron across) of Mg-Fe carbonate with up to 0.16 wt % F. At least half of the olivine crystals are strongly fractured rounded grains, which are probably xenocrysts. A glomerophyric olivine-phlogopite aggregate almost without groundmass was observed in one sample. This aggregate is considered as a cumulate of liquidus crystals of the lamprophyre. Irrespective of the origin, the olivine grains are strongly altered; however, there are relics of fresh olivine containing melt inclusions.

Table. Compositions of lamprophyre minerals, wt %

Note. 1 √ olivine, 2 √ phlogopite, 3 √ ilmenite, 4 √ chromite, 5 √ secondary (?) carbonate.

Primary melt inclusions 10-20 micron in size were observed in the olivine phenocrysts. The inclusions are often partly decrepitated and surrounded by halos of small satellite inclusions. (figure), which indicates high fluid pressure. The melt inclusions consist of daughter minerals, a small amount of residual glass, and a gas bubble. Carbonates and mica were identified among the daughter crystals. The mica is compositionally similar to the phlogopite phenocrysts and contains up to 4 wt % TiO₂, 0.7 wt % ВаО, and 0.5 wt % F. The residual glass is rich in TiO₂ (up to 4 wt %) and F (up to 0.65 wt %) and contains significant amounts of SrO, BaO, and Cl. Secondary fluid inclusions were observed in olivine along healed fractures. The obtained data suggest that the primary magmas of ultramafic lamprophyres contained considerable amounts of volatiles (CO₂, F, Cl, H₂O) and incompatible elements (Ba, Sr, Ti, etc.). The high mg values of host olivine phenocrysts suggest that the enrichment of the melts is an intrinsic property of their primary mantle melts rather than the result of extensive crystal fractionation.