Rhombohedral carbonates from carbonatites of the Khibina massif, Kola peninsula, Russia

The eastern part of the Khibina alkaline massif, Kola peninsula, Russia, is made up of ijolite, foyaite, olivine melanephelinite, melanephelinite, phonolite, alkali trachyte, and rocks of the carbonatite series. The latter can be divided into (1) biotite - aegirine - apatite rocks containing less than 50% carbonate, (2) early and (3) late carbonatites and (4) carbonate-zeolite veins. These rocks are distinguished by their relative ages and by their major and accessory minerals. The rock-forming minerals include rhombohedral carbonates of Ca, Mn, Fe and Mg: calcite, manganoan calcite, dolomite, ankerite, manganoan ankerite, ferroan kutnohorite, ferroan rhodochrosite, manganoan siderite and siderite. Petrographic and cathodoluminescence observations reveal two textural types of carbonates: primary and secondary (metasomatic). The Sr content is high in primary minerals, but low in those of metasomatic origin. Values of isotope ratios delta(13)C in the range -7.8 to -2.8 parts per thousand (PDB) and delta(18)O in the range +5.9 to +14.3 parts per thousand (SMOW) for the rhombohedral carbonates from the Khibina carbonatites and carbonate-zeolite veins are similar to those from carbonatites elsewhere. For most samples, there is a positive correlation between delta(13)C and delta(18)O, which can be explained by Rayleigh and temperature fractionation. However, in one type of the late carbonatites and the carbonate-zeolite veins, carbonates show wide variations in their delta(13)C and delta(18)O values; this has been interpreted as loss of heavy C to a gas phase. Field relations, petrographic and cathodoluminescence observations suggest that formation of the late carbonatites and carbonate-zeolite veins is more likely to have been from a carbohydrothermal (fluid) system than from a melt.