Mantle and crustal components in a carbonatite complex, and the evolution of carbonatite magma: Ree and isotopic evidence from the fen complex, southeast Norway

From isotopic and trace-element data, the carbonatites in the Fen complex (SE Norway) may be divided into two groups: (A) sövites dolomite carbonatite, dyke-facies ferrocarbonatite and apatite cumulates [ifε{lunate}_Sr \ ̌ - 12 (( ⁸⁷Sr ⁸⁶Sr)_i \ ̌ 0.7032), δ¹⁸O_SMOw = + 5.7 to + 8‰, δ¹³C_PDB = -3 to -5‰, 1000-7000 ppm Sr, 50-900 ppm Nd]; and (B) tuffisitic ferrocarbonatite and rödberg formed from it by post-magmatic re-equilibration [ε{lunate}_Sr > -10 (( ⁸⁷Sr ⁸⁶Sr)_i > 0.7033), δ¹⁸O_SMOW > +8‰, δ¹³C_PDB > -3‰, 500-1700 ppm Sr, 800-5000 ppm Nd]. ε{lunate}_ND ranges between +2.7 and +4.0 for the carbonatites, but does not show any systematic differences between groups A and B. The data suggest that the magmatic system at Fen was dominated by a component derived from a depleted volume of the upper mantle [ε{lunate}_Sr ≤ -16 ( ⁸⁷Sr ⁸⁶Sr)_i \ ̌ 0.7029), ε{lunate}_Nd ≥ + 4], which was contaminated with crustal material. Prior to melting, the depleted mantle source was enriched in CO₂ by a metasomatic fluid or liquid. This process also led to local redistribution of LREE within the mantle, and to LREE enrichment in melts formed shortly after metasomatism. The parent magma of all the carbonatite units was a CO₂-rich mafic silicate melt (nephelinitic?); crustal contamination took place after the group-A and -B carbonatite liquids had come into existence, in the middle to shallow crust.