Stable Isotope Evidence for Diagenesis of the Ordovician Courtown and Tramore Limestones, South-Eastern Ireland

Abstract:Use of Lower Palaeozoic carbonate fossils for palaeothermometry as derived from stable isotope geochemistry is increasing. With all these studies, there is the concern of how pristine the skeletal carbonate is in relation to diagenesis. In this study, we compared the stable isotopes of calcitic brachiopod and bryozoan allochems with secondary cements to determine if the skeletal isotopes were reset during diagenesis. The study was conducted on the Courtown and Tramore Limestone Formations of the Ordovician (Llanvirn-Caradoc) Duncannon Group, which outcrops in south-eastern Ireland. These formations consist of calcareous mudrocks and argillaceous limestones. Bryozoans and brachiopods are common allochems in both formations. The Duncannon sediments were cleaved, folded and faulted by compression associated with the Caledonian and Hercynian orogenies. Bulk rock samples were slabbed and thick-sectioned. Fifty three $\geq20\mu$g samples of carbonate from bryozoans, brachiopods and matrix (cements and veins) were collected with 1μm spatial precision using a computer-driven micromilling device to minimise mixing of allochems and matrix. Results indicate $\delta^{13}C$ values of -8.8% to +0.3% VPDB and $\delta^{18}C$ values of -17.8% to -11.5% VPDB. In the Courtown samples, the brachiopod and bryozoan $\delta^{18}C$values were consistently more similar to each other than either was to the matrix, but this was not true for the Tramore samples. This suggests different alteration histories (more likely) or a different source of carbonate for the Courtown matrix (i.e. sparry cement) as opposed to that of the Tramore matrix (i.e. abraded skeletal material) (the less likely explanation). Palaeotemperature calculations from the oxygen isotopes ($78\degreeC to 128\degreeC$) indicate exposure to isotopically light groundwater or meteoric fluids following the previous and hotter Caledonian and Hercynian orogenies. All three phases of alteration would have overprinted any original palaeoenvironmental signal that might once have existed.