SUBDUCTED ORGANIC CARBON – EVIDENCE FROM PALEOPROTEROZOIC CARBONATITES IN FINLAND

Plate tectonics can potentially contribute to the redox state of the Earth's surface environments through transport and storage of oxidants and reductants in the mantle. For instance, some authors have proposed a link between the rise of atmospheric oxygen and a global episode of continental breakup during the Early Paleoproterozoic, associated with large-scale subduction of reduced carbon in organic-rich passive margin sediments. Support for the existence of subducted organic material in the mantle has primarily been obtained from studies of kimberlitic diamonds. Our data from Fennoscandian carbonatites provide novel evidence of supracrustal influence on the carbon isotope systematics of upper mantle prior to 1.8 Ga, possibly linked with the evolution of atmosphere.          The isotope ratios of carbon in most Fennoscandian carbonatites fall in the range of typical mantle carbon, with d¹³C varying from -3 to -8 per mil (PDB). Two ca 1.8 Ga carbonatite occurrences in Finland, however, show a strong depletion in ¹³C relative to the average mantle value. Carbonate samples from a carbonatite dike at Halpanen show d¹³C values ranging from -12.2 to -12.4 per mil (n=7) and those from a carbonatite dike swarm at Panjavaara display an even stronger depletion in ¹³C, with values varying from -15.0 to -16.4 per mil (n=20). Sr and Nd isotopic data suggest enriched and nearly chondritic sources for Sr and Nd, respectively, in both Halpanen and Panjavaara carbonatites. Based on geochemical, mineralogical and textural characteristics Halpanen and Panjavaara represent primary mantle-derived carbonatites. The depletion in ¹³C relative to the average mantle value may be related to global tectonic events linked with the oxidation of the atmosphere at 2.4 – 2.1 Ga.