A STABLE ISOTOPE TEST FOR DIAGENESIS COMPARING BRACHIOPODS, BRYOZOANS, AND CEMENTS IN THE ORDOVICIAN DUNCANNON GROUP, SOUTHEASTERN IRELAND

The use of lower Paleozoic carbonate fossils for paleothermometry as derived from stable isotope values is increasingly common. The degree of preservation of primary environmental information in skeletal carbonate is a primary concern given the pervasive diagenesis in many sequences. In this study, we compare stable isotope values of calcitic brachiopod and bryozoan allochems with secondary cements to evaluate skeletal preservation. This study was conducted on the Courtown and Tramore Limestone Formations of the Ordovician (Llanvirn-Caradoc) Duncannon Group which outcrops in southeastern 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. Previous work by Diskin (1996) suggested the orogenies exposed the rocks to sub-greenschist facies heat flow. Was this enough to diagenetically alter the allochems' geochemistry and render them unusable for paleothermometry? Eight 2"x3"x100 mm thin sections were made. Sixty ³20 mg samples of carbonate from bryozoans, brachiopods, and cements (matrix and veins) were collected with 10mm spatial precision using a computer-driven micromilling device to minimize mixing of allochems and cements. Results indicate d¹³C values of 1.36 - 7.07 ‰VPDB, and d¹⁸O values of 1.49 - 7.81 ‰VPDB. The brachiopod and bryozoan d¹⁸O values were consistently more similar to each other than either was to the cements. This suggests minimal diagenetic alteration of the geochemistry of the allochems. Another possibility is that the allochems were differentially altered by diagenesis due to initial differences in permeability and/or composition. The latter possibility was rejected qualitatively by cathodoluminescence.