Skeletal and nonskeletal components from Mid-Cretaceous limestones have been investigated using coordinated textural, isotopic, and chemical methods to delineate the conditions under which they formed or were altered. The degree of alteration was determined by means of cathodoluminescence, light microscopy, and scanning electron microscopy, along with inferences from isotopic and chemical compositions. Oxygen isotopic values (-2.9 to -0.5 per thousand , PDB) of non-luminescent parts of most bivalves (a pectenid, chondrodontids, and most oysters) and of radiaxial fibrous calcite appear to reflect isotopic equilibrium with contemporaneous Mid-Cretaceous surface seawater. In contrast, oxygen isotopic compositions of non-luminescent, texturally unaltered calcite outer layers of rudists and the calcitic shells of some oysters (-4.0 to -2.5 per thousand ) are significantly lower than luminescent, altered portions of the same specimens or equant calcite cements within skeletal molds. This apparent isotopic "reversal" suggests that vital effects are involved in the secretion of skeletal calcite in rudists and some oysters. All the radiaxial fibrous calcite (RFC) cements analyzed contain less than 1 mole % MgCO 3 . Retention of a primary marine delta 18 O signature and the absence of microdolomite inclusions within well-preserved RFC suggest that this cement precipitated as primary low-Mg calcite cements. Also, micritic envelopes, algal coatings, and internal sediments within the RFC strongly suggest that those cements were formed in shallow marine environments. Isotopic and chemical contents of calcitized skeletons, together with coexisting equant calcite cements indicate that the sediments were mostly stabilized during meteoric-phreatic diagenesis and that effects of deep-burial diagenesis were negligible. The similarity of isotopic and chemical trends in those calcitized skeletons and the equant cements suggests that the skeletons were altered by fluids in a diagenetic system which was open with respect to 18 O, Sr, and Mg, but closed with respect to 13 C, Fe, and Mn.

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