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Three categories of fibrous calcite from early to middle Caradoc platform‐marginal buildups in east Tennessee can be delineated using cathodoluminescent microscopy, minor element chemistry and stable C‐O isotopic composition. Bright luminescent fibrous cement has elevated Mn (>1000 p.p.m.), negative δ13C and intermediate δ18O values relative to other types of fibrous calcite. This cement reflects fibrous calcite that interacted with reducing Mn‐rich fluids. Dully luminescent fibrous cement has elevated Fe (>400 p.p.m.), positive δ13C and negative δ18O values relative to other fibrous cements. This cement was stabilized by burial fluids. Nonluminescent fibrous cement has low Mn and Fe (generally below 400 p.p.m.) and positive δ13C and δ18O values relative to other types of fibrous calcite. The latter cement is interpreted to be the best material for determining the isotopic composition of calcite precipitated in equilibrium with early to middle Caradoc seawater, which is δ13C=1% PDB and δ18O=−4 to −5‰ PDB. Results from this study and Ashgillian brachiopods indicate that the average δ18O composition of the Ordovician ocean, during nonglacial periods, was probably never more negative than −3‰ SMOW. Assuming an Ordovician seawater δ18O value of −1‰ SMOW, Holston Formation fibrous cements would have precipitated at temperatures between 27 and 36 °C, which is near the upper temperature limit for metazoans. A seawater δ18O value of −2‰ SMOW yields temperatures ranging from 23 to 31 °C, while a −3‰ SMOW value yields temperatures of 18–26 °C.

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