Two-Stage Neomorphism of Jurassic Aragonitic Bivalves: Implications for Early Diagenesis

Abstract

ABSTRACT Aragonitic bivalves in Middle Jurassic paralic limestones from Lincolnshire (Eastern England) have been neomorphically replaced by inclusion-rich calcite spar. Textural features of spar are analogous to examples from other calcitized Jurassic molluscs, but Lincolnshire bivalves show evidence for two distinct stages of replacement. Staining, cathodoluminescence, and backscatter scanning electron microscopy reveal an initial volumetrically minor and frequently fabric-selective replacement by nonferroan calcite, with most of the remaining skeletal aragonite subsequently replaced by ferroan calcite spar. Stable-isotope data from individual bivalves define a negative covariant mixing trend ranging from early, relatively 13C-depleted and 18O-enriched nonferroan calcite (d13C = -2 PDB), to ferroan calcite with composition similar to associated late pore-filling cements (d13C » +2 PDB, d1 O » -7 PDB). The early diagenetic end member precipitated from depositional pore fluids, with bicarbonate supplied both from dissolution of skeletal aragonite and bacterial oxidation of organic material. In contrast, ferroan spar precipitated during burial diagenesis and dewatering of shales surrounding the limestone. Bulk trace-element signatures of the bivalves are dominated by late ferroan calcite, but electron probe microanalysis and intrinsic blue cathodoluminescence shows that early calcite contains negligible amounts of Fe, Mn, and Mg. Together with isotopic and fabric data this indicates that early neomorphism occurred at shallow depths and may even have begun at the sediment-water interface. In the absence of through-flowing meteoric waters, early replacement did not proceed via an inward-migrating neomorphic front. Instead, calcitization was initiated at numerous individual reaction centers throughout the shells and was principally controlled by the distribution of the skeletal organic matrix. This contrasts strongly with most previous reports of aragonite neomorphism during arly diagenesis.