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

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