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

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(13)C and delta(18)O changes through a low- to high-grade greenschist-facies transition within impure, C-13-rich Palaeoproterozoic dolostones. The least-altered dolostones have delta(13)C of +9parts per thousand and delta(18)O of 22parts per thousand. The metamorphic transition is expressed by dolomite + calcite, + quartz K-feldspar - tremolite + calcite(2) dolomite calcite, and defined by C-13 depletion of calcite(2) (c. 3.0parts per thousand), calcite, (1.0-2.0parts per thousand) and dolomite (<1parts per thousand) which is associated with a Rayleigh distillation process. delta(18)O shows a considerable resetting in all carbonate components by around 6parts 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 180 depletion in dolomite (9parts per thousand) and calcite(1) (c. 4parts per thousand) were probably controlled by infiltration into permeable zones of external fluids associated with retrograde alteration; delta(13)C remains largely unaffected.