Origin and significance of fluid inclusions in matrix minerals in carbonatites, a case study from the Fen carbonatite complex, Norway

The carbonatites in the Fen complex, SE Norway, contain fluid inclusions in two different microstructural settings: 1) Early inclusions in relict apatite crystals within cumulate xenoliths in the carbonatites, and 2) inclusions of unclear textural status in matrix calcite. Inclusion group 1) was shown earlier to be of mid-crustal, magmatic origin (Andersen 1986). Inclusions of type 2) are indistinguishable from the former group in terms of composition, and have overlapping or marginally higher densities, apparently suggesting a similar deep origin. Independent geological evidence, however, requires these inclusions to have formed in-situ, in the shallow crust. This is feasible only at sub-solidus temperatures of 200-300°C. The consequence of these findings for the applications of fluid inclusion data to carbonatites is discussed. -Author