The dominant rock type of the Fen carbonite complex is damkjernite, a lamprophyric ultrabasic rock that occurs in diatreme and hypabyssal facies. Damkjernite bears no chemical resemblance to kimberlite, but is chemically similar to alnöite, monchiquite and ouachitite. Phenocrysts include Ti-phlogopite, amphibole, acmitic pyroxene and nepheline; albite, nepheline and sphene occur in the groundmass. Calcite silicate textures suggest liquid immiscibility during crystallization. Calculated values of aSiO2 and crystallization T for the groundmass allow equilibration of the damkjernite magma with peridotite at T = 1200-1250°C, P = 15-25 kbar. Lherzolite nodules were accidentally included by the magma on its way to the surface, following its partial crystallization in a subcrustal magma chamber. ( 87Sr 86Sr)0 of six rocks correlates positively with CO2, Sr and Na Na + K, and inversely with SiO2 and oxidation state, suggesting assimilation of crustally contaminated carbonatite. We suggest that the damkjernite formed by partial melting of peridotite in the presence of a CO2-rich vapor phase. Carbonatite magma separated from the silicate melt in the subcrustal magma chamber, intruded into the crust, and exchanged Sr isotopes with its country rocks. During later intrusion, the damkjernite remelted the carbonatites and was contaminated by them.