FIBROUS CALCITE FROM THE ORDOVICIAN OF TENNESSEE - PRESERVATION OF MARINE OXYGEN ISOTOPIC COMPOSITION AND ITS IMPLICATIONS

Three categories of fibrous calcite from early to middle Caradoc platf orm-marginal buildups in east Tennessee can be delineated using cathod oluminescent microscopy, minor element chemistry and stable C-O isotop ic composition. Bright luminescent fibrous cement has elevated Mn (>10 00 p.p.m.), negative delta(13)C and intermediate delta(18)O values rel ative to other types of fibrous calcite. This cement reflects fibrous calcite that interacted with reducing Mn-rich fluids. Dully luminescen t fibrous cement has elevated Fe (>400 p.p.m., positive delta(13)C and negative delta(18)O values relative to other fibrous cements. This ce ment was stabilized by burial fluids. Nonluminescent fibrous cement ha s low Mn and Fe (generally below 400.p.p.m.) and positive delta(13)C a nd delta(18)O values relative to other types of fibrous calcite. The l atter cement is interpreted to be the best material for determining th e isotopic composition of calcite precipitated in equilibrium with ear ly to middle Caradoc seawater, which is delta(13)C=1 parts per thousan d PDB and delta(18)O=-4 to -5 parts per thousand PDB. Results from thi s study and Ashgillian brachiopods indicate that the average delta(18) O composition of the Ordovician ocean, during nonglacial periods, was probably never more negative than -3 parts per thousand SMOW. Assuming an Ordovician seawater parts per thousand(18)O value of -1 parts per thousand SMOW, Holston Formation fibrous cements would have precipitat ed at temperatures between 27 and 36 degrees C, which is near the uppe r temperature limit for metazoans. A seawater delta(18)O value of -2 p arts per thousand SMOW yields temperatures ranging from 23 to 31 degre es C, while a -3 parts per thousand SMOW value yields temperatures of 18-26 degrees C.