Irish Lower Carboniferous replacement dolomite: isotopic modelling evidence for a diagenetic origin involving low-temperature modified seawater

Abstract

AbstractIrish Lower Carboniferous (Dinantian Subsystem) carbonate rocks are extensively replaced by planar dolomite. This dolomitization is unrestricted in lithology replaced, age of host rock and geographical occurrence. This paper presents geochemical (δ18O, δ13C, 87Sr/86Sr, Sr conc.) modelling evidence, and discussion supporting and extending the theory that replacement planar dolomitization formed in the Waulsortian, and other host rocks under shallow burial conditions via interaction with a low-temperature (c. 50–70 °C) slightly modified seawater. The probable mechanism for transporting the fluid into the carbonates appears to be a variant of Kohout convection, driven by an elevated geothermal gradient. As seawater was drawn inwards, it encountered the units beneath the Waulsortian and scavenged radiogenic Sr. This warmer fluid then migrated upwards and up-slope into overlying Waulsortian and Supra-Waulsortian platform carbonates still undergoing early diagenesis. Calcium in pore fluids, provided by dissolution-precipitation reactions of calcite, was probably incorporated into the modified, and slightly warmer, seawater resulting in the variability noted in the planar replacement dolomite Sr concentrations. The early low-temperature dolomitization of the carbonate host rocks provided a crucial preparation event by creating/redistributing and preserving porosity and permeability. Younger regionally migrating high-temperature fluids directly related to the Zn-Pb mineralization in Ireland exploited these dolomitzed units as aquifers. The models and methodologies presented for understanding dolomite genesis in the Lower Carboniferous rocks of Ireland can be applied to any dolomitized reservoir.