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Journal of the Geological Society; 2003; v. 160; issue.1; p. 71-82;
DOI: 10.1144/0016-764902-008
© 2003 Geological Society of London

Original Article

Fractionation of carbon and oxygen isotopes in 13C-rich Palaeoproterozoic dolostones in the transition from medium-grade to high-grade greenschist facies: a case study from the Kola Superdeep Drillhole

Victor A. Melezhik1, Anthony E. Fallick2, Yury P. Smirnov3 & Yury N. Yakovlev3

1 1Geological Survey of Norway, Leiv Eirikssons vei 39, N-7491 Trondheim, Norway (e-mail: victor.melezhik@ngu.no)
2 2Scottish Universities Environmental Research Centre, East Kilbride, Glasgow G75 0QF, UK
3 3Scientific Industrial Centre ‘Kola Superdeep’, 184415 Zapolyarny, Russia

Samples from the Kola Superdeep Drillhole (12 262 m), a deep drillhole (1060 m), and from the surface, seaprated by only around 10 km, provided a unique opportunity for direct tracing of {delta}13C and {delta}18O changes through a low- to high-grade greenschist-facies transition within impure, 13C-rich Palaeoproterozoic dolostones. The least-altered dolostones have {delta}13C of +9{per thousand} and {delta}18O of 22{per thousand}. The metamorphic transition is expressed by dolomite + calcite1 + quartz ± K-feldspar <- tremolite + calcite2 ± dolomite ± calcite1 and defined by 13C depletion of calcite2 (c. 3.0{per thousand}), calcite1 (1.0–2.0{per thousand}) and dolomite (<1{per thousand}) which is associated with a Rayleigh distillation process. {delta}18O shows a considerable resetting in all carbonate components by around 6{per thousand} caused by a Rayleigh distillation process coupled with isotopic exchange between the carbonates and fluids with an external source of oxygen. The retrograde alteration is expressed by the formation of quartz–chlorite veinlets within tectonically bound zones of brecciated and sheared dolostones. The maximum 18O depletion in dolomite (9{per thousand}) and calcite1 (c. 4{per thousand}) were probably controlled by infiltration into permeable zones of external fluids associated with retrograde alteration; {delta}13C remains largely unaffected.

Key Words: Kola Superdeep Drillhole • Palaeoproterozoic • dolomite • calcite • stable isotopes