TRACKING WHOLE ROCK GEOCHEMICAL CHANGES DRIVEN BY DIAGENETIC RECRYSTALLIZATION OF CARBONATES: AN ANALYSIS OF THE ORDOVICIAN CHICKAMAUGA LIMESTONE (NORTH-CENTRAL ALABAMA)

The growth of massive crystals during interaction with pore fluid stands as one of the more spectacular products of the diagenesis of marine carbonates. However, few researchers have sought to quantify the effects of this process on whole rock geochemistry. The aim of this study is to analyze the influence of diagenesis associated with such massive recrystallization on stable isotopes (δ¹³C and δ¹⁸O), trace metals, and rare earth elements (REEs). Sixteen carbonate samples, collected from eleven stratigraphic horizons along an approximately 150 m. section of the Ordovician Chickamuaga Limestone (Blount County, Alabama), were studied. In some strata, carbonate rocks show significant (>50%) replacement by secondary carbonate crystals, some of which are visible in hand sample, while, in other parts of the section, samples appear pristine and show only minor amounts of visible recrystallization. A petrographic analysis indicates that significantly altered samples show one of two types of recrystallization: either “dolomite” type crystallization, characterized by µm-scale rhombohedral dolomite crystals, or “calcite” type crystallization characterized by mm-scale, polygonal interlocking calcite and some dolomite crystals. High values of trace metals and REEs are mainly concentrated in samples displaying dolomite-type recrystallization, whereas calcite-type recrystallization exerts a much weaker influence on composition. Based on these observations, we conclude that the two types of recrystallization reflect at least two sets of compositionally distinct pore fluids. In terms of stable isotopes, δ¹⁸O values are moderately correlated with the type of recrystallization, with pristine and calcite-type samples generally showing more negative values than dolomite-type samples. However, δ¹³C values show no definitive correlation with the type of recrystallization, and remained relatively constant throughout the section. This result demonstrates that δ¹³C values can be uniquely resistant to certain types of diagenetic changes, even implying that measurements of δ¹³C in heavily altered rocks may potentially yield meaningful results for paleoenvironmental reconstructions.