Hydrothermal dedolomitisation of carbonate rocks of the Paleoproterozoic Zaonega Formation, NW Russia — Implications for the preservation of primary C isotope signals

• Published in: Chemical Geology
• Main Authors: Kreitsmann, T., Külaviir, M., Lepland, A., Paiste, K., Paiste, P., Prave, A. R., Sepp, H., Romashkin, A.E., Rychanchik, D.V., Kirsimäe, K.
• Other Authors: University of St Andrews.School of Earth & Environmental Sciences, University of St Andrews.Marine Alliance for Science & Technology Scotland, University of St Andrews.Scottish Oceans Institute, University of St Andrews.St Andrews Sustainability Institute, University of St Andrews.St Andrews Isotope Geochemistry
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2020
• Subjects: Dedolomitisation; Carbonate geochemistry; Carbonate stable isotopes; Shunga Event; QE Geology; DAS; SDG 14 - Life Below Water; QE; Norway; karelia*
• Online Access: https://hdl.handle.net/10023/19582; https://doi.org/10.1016/j.chemgeo.2019.03.002

This study was supported by Estonian Science Agency project PUT696 and PRG447, and Estonian Centre of Analytical Chemistry. K.P. and A.L. were supported by the Research Council of Norway through its Centres of Excellence funding scheme grant No. 223259. The Paleoproterozoic Zaonega Formation in Karelia, NW Russia, has played a key role in understanding the environmental conditions postdating the Great Oxidation and Lomagundi-Jatuli Events. Its carbonate- and organic-rich rocks (shungite) define the postulated Shunga Event representing an accumulation of very organic-rich sediments at c. 2 Ga and are central in ideas about changing ocean-atmosphere composition in the wake of those worldwide biogeochemical phenomena. Our work focussed on a key interval of carbonate rocks in the upper part of the Formation to: (i) obtain new high-resolution carbon, oxygen and strontium isotope data complemented by detailed petrography and mineralogical characterisation and (ii) expand upon previous studies by using our data to constrain geochemical modelling and show in greater detail how magmatic hydrothermal fluids induced dedolomitisation and altered geochemical signals. Our findings show that the δ13Ccarb of calcite-rich intervals are the most altered, with values between −16.9 to 0.6‰, whereas the dolomite-dominated parts retain the best-preserved (i.e. most original) values. Those define a trend of steadily increasing δ13Ccarb, from −6 to +0.5‰, which we interpret as a return to normal marine conditions and carbonate‑carbon values following the Lomagundi-Jatuli Event. Peer reviewed