Abstract

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 δ13C and δ18O changes through a low- to high-grade greenschist-facies transition within impure, 13C-rich Palaeoproterozoic dolostones. The least-altered dolostones have δ13C of +9‰ and δ18O of 22‰. 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‰), calcite1 (1.0–2.0‰) and dolomite (<1‰) which is associated with a Rayleigh distillation process. δ18O shows a considerable resetting in all carbonate components by around 6‰ 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‰) and calcite1 (c. 4‰) were probably controlled by infiltration into permeable zones of external fluids associated with retrograde alteration; δ13C remains largely unaffected.

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