Primary inclusions of burbankite in carbonatites from the Fen complex, southern Norway

Abstract

Carbonatites in the Fen intrusive complex (southern Norway) contain abundant burbankite (confirmed by Raman microspectroscopy) as inclusions in calcite, dolomite and, less commonly, fluorapatite and pyrochlore. Typically the inclusions occur in the core of calcite or dolomite grains relatively unaffected by subsolidus processes, and are associated with Fe-poor dolomite or Sr-rich calcite, respectively. Burbankite does not exceed 30 × 50 μm in size and is characteristically absent from the peripheral areas of carbonate grains affected by recrystallization or interaction with fluids. Compositionally, the mineral falls within the following range: (Na1.51–2.16Ca0.58–1.21)(Sr1.50–2.42Ca0.28–0.57LREE0.05–0.64Ba0.06–0.41)(CO3)5 and contains low Th, but no detectable Mg, Fe or F (LREE = light rare-earth elements: Ce > La > Nd > Pr > Sm). Burbankite inclusions at Fen are interpreted as primary and indicative of Na enrichment in their parental carbonatitic magma. Dissociation of burbankite during subsolidus re-equilibration of its host phases with fluids undoubtedly served as one of the sources of LREE for the development of late-stage mineralization in the Fen complex.