δ13C AND δ18O VARIATIONS IN PRIMARY AND SECONDARY CARBONATE PHASES: SEVERAL CONTRASTING EXAMPLES FROM PALAEOPROTEROZOIC 13C-RICH METAMORPHOSED DOLOSTONES

Abstract

The Palaeoproterozoic Kuetsjarvi Sedimentary Formation from the Pechenga Greenstone Belt, NW Russia, has been studied with the purpose of determining δ13Ccarb and δ18Ocarb values in various carbonate phases from genetically different 13C-rich dolostones sampled by micro-coring techniques. Samples were collected from quarries and drillholes. This avoids the influence of recent weathering on isotopic composition, and allows rocks from different metamorphic facies intersected by deep drillholes to be sampled. In the low-grade greenschist facies conditions, distinct textural components from the same parental impure dolostones have yielded δ13C values ranging within 1‰, and showing no statistical differences between early dolomicrite and late void- and fenestra-filling dolospar. No significant differences have been observed between analyses of the whole-rock and microcored samples. Genetically different dolomites, however, retain their primary isotopic peculiarities and show δ13C differences exceeding 30‰. No external isotopically light carbon has been introduced during diagenesis, and therefore there is no evidence of soil-derived CO2 which commonly shifts δ13C to more negative values. δ18O values show 2–6‰ fluctuations in distinct textural components from the same parental dolostones which were caused by syn-depositional/early diagenetic processes. Genetically different dolomites largely retain their syn-depositional/early diagenetic δ18O values ranging from 9‰ (mantle-influenced) to 30‰ (evaporation-induced).The high-grade greenschist facies metamorphism, marked by the reaction dolomite+quartz+H2O→tremolite+calcite2+CO2, significantly reset δ13C (by 3‰) in the metamorphic calcite2 and δ18O (by 6‰) in all carbonate components. In such cases, the best proxy for depositional δ13C values of impure dolostones can only be provided by microcored samples of the dolomite phase.