LIGHT PALEOPROTEROZOIC MANTLE CARBON: EVIDENCE FROM FENNOSCANDIAN CARBONATITES

The isotope ratios of carbon in most Fennoscandian carbonatites fall in the range of typical mantle carbon, with d ¹³C varying from -3 to -7 per mil (PDB). Two ca 1.8 Ga carbonatite occurrences in middle Finland, however, show a strong depletion in ¹³C relative to the average mantle value. Carbonate samples from a carbonatite dike at Halpanen show d ¹³C values ranging from -12.2 to -12.4 per mil (n=7) and those from a carbonatite dike swarm at Petäiskoski display an even stronger depletion in ¹³C, with values varying from -15.9 to -16.4 per mil (n=16).

The carbonatite dike at Halpanen is 10 m wide and up to 1.5 km long, crosscutting the surrounding 1.9 Ga Svecofennian gneisses. In addition to calcite, the dike contains scattered apatite phenocrysts and accessory barite, magnetite and monazite. The chemical composition of the dike is characterized by high contents of SrO, BaO, REE (up to 3.63, 0.63 and 0.65 %, respectively) and Y (up to 130 ppm). Preliminary Sr and Nd isotope data suggest an enriched source for Sr and a nearly chondritic source for Nd.

The Petäiskoski carbonatite dike swarm is located some 150 kilometers NE of Halpanen. In total, 10 narrow carbonatite dikes have been observed in an area of 100 square kilometers. The dikes are 2 - 50 cm wide and from meters to tens of meters in length. In addition to calcite, the dikes contain bastnäsite, barite, strontianite and apatite. The dikes are bordered by a 1 to 10 cm wide zone of fenitic alteration in the host rock, with bluish alkaline amphibole, aegirine and calcite. Similarly to Halpanen, the carbonatites of the Petäiskoski dike swarm are highly enriched in SrO, BaO and REE, with concentrations reaching 2.3, 3.9 and 10.0 %, respectively.

The mineralogical, chemical and textural characteristics of the Halpanen and Petäiskoski carbonatite dikes suggest a mantle origin for these rocks. The depletion in ¹³C relative to the average mantle value could be related to subduction of organic-rich crustal material.