STRONTIUM ISOTOPE DIAGENESIS OF BIOGENIC ARAGONITE AND LOW-MG CALCITE

Using petrographic and geochemical criteria, it is possible to identify marine Sr isotopic compositions and diagenetic overprints in carbonate samples. The Sr-87/Sr-86 ratio of originally low-Mg calcite brachiopods and aragonite molluscs from the Boggy, Magoffin, and Brush Creek Formation/Members increases with progressive post-depositional alteration in the presence of diagenetic meteoric waters. In contrast, brachiopods from the Codroy Formation exhibit an increase in non-radiogenic Sr with diagenetic alteration. Based on the seawater Sr-87/Sr-86 curve, this alteration probably occurred in the presence of marine water during the mid-Permian, Triassic, or late-Cretaceous-early-Tertiary. The degree of change in radiogenic Sr probably reflects the physicochemical nature of the diagenetic microsystem. Diffusion-controlled altered material shows the smallest degree of change in Sr isotopic ratios as well as microstructural and other geochemical modifications. In contrast, fluid-controlled altered material shows the greatest shift in isotopic ratios with concomitant microstructural and geochemical changes. Data of diagenetically well-defined biogenic carbonates show that Mississippian seawater Sr-87/Sr-86 was constant with a mean of about 0.70773 and a range of 0.7076-0.7078. The revised high-resolution curve shows significantly higher variations for Pennsylvanian seawater Sr-87/Sr-86 (0.7076-0.7093). A low perturbation marks the mid-Virgilian (0.7076-0.7078), and high perturbations are observed for the early Missourian (0.70846) and latest Morrowan (0.70913). The mean value determined for the latest Morrowan is similar to Sr-87/Sr-86 ratios measured for modern carbonates and seawater. It is further suggested that seawater Sr-87/Sr-86 varies by as much as 0.000154 at any given time.