Other
Scientific paper
Feb 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991gecoa..55..505b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 55, Issue 2, pp.505-513
Other
2
Scientific paper
Using petrographie and geochemical criteria, it is possible to identify marine Sr isotopic compositions and diagenetic overprints in carbonate samples. The 87 Sr / 86 Sr 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 87 Sr / 86 Sr 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 87 Sr / 86 Sr 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 87 Sr / 86 Sr (0.7076-0.7093). A low perturbation marks the midVirgilian (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 87 Sr / 86 Sr ratios measured for modern carbonates and seawater. It is further suggested that seawater 87 Sr / 86 Sr varies by as much as 0.000154 at any given time.
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