Unusual, basin-scale, fluid-rock interaction in the Palaeoproterozoic Onega basin from Fennoscandia: Preservation in calcite δ18O of an ancient high geothermal gradient

There is an intriguing distribution of Na in the sedimentary rocks of the Zaonega Formation. Sodium is relatively abundant in rocks below a certain depth (the lowermost dolostone at ∼258 m), but rare in shallower sequences. It is argued that this distribution did not originate with the basin-scale fluid-rock interaction documented above, but may rather be the result of evaporite dissolution, and subsequent redistribution of soluble elements during fluid flow associated with the syndepositional emplacement of basin-wide igneous rocks.