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

A variety of carbonates of different geneses, as indicated by petrography and geochemistry, are found throughout 400m of the volcano-sedimentary rocks of the Zaonega Formation of Palaeoproterozoic age in the Onega Basin of Fennoscandia. Following intensive sampling and analysis of varied calcites from drillcore recovered during the ICDP FAR-DEEP program, we report a highly unusual depth distribution of calcite oxygen isotope values (δ¹⁸O_cal). Unprecedentedly for such rocks, the δ¹⁸O_cal values over the full depth interval of 400m are strongly linearly correlated with depth (r² = 0.9015, n=178). We propose that this is the result of major oxygen isotope resetting through water-rock interaction with a fluid of relatively constant oxygen isotopic composition (δ¹⁸O_w). In this model, the observed linear δ¹⁸O_cal-depth relationship is then a consequence of the increase in temperature with depth because of the background geothermal gradient. Minor deviations from the overall linear trend are likely due to bed-scale geological factors including locally high impermeability, and oxygen isotope modification of δ¹⁸O_w by comparatively intense water-rock interaction.
Were the observed δ¹⁸O_cal values to have been reset during the greenschist facies Svecofennian metamorphism which affected the rocks at c. 1800Ma, the implied geothermal gradient of ∼ 560°C km^-1 is geologically unreasonable and, accordingly, this hypothesis is ruled out. Rather, the δ¹⁸O_cal variation of 5‰ over 400m implies a near-surface depth for the rocks during fluid interaction, and this is consistent with a surface-derived origin of the infiltrating fluid (δ¹⁸O_w ∼ -13.6‰ for a surface temperature of 15°C and geothermal gradient of ∼ 52 °C km^-1). It is speculated that the fluid accessed the carbonates from the basin edge by bed-parallel rather than cross-formational flow.
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 ∼258m), 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.