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The AAPG/Datapages Combined Publications Database
Journal of Sedimentary Research (SEPM)
Abstract
Two-Stage Neomorphism of Jurassic Aragonitic Bivalves: Implications for Early Diagenesis
James P. Hendry (1), Peter W. Ditchfield (2), James D. Marshall (3)
ABSTRACT
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, d18O >= -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 neomorphic 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 neomorphic 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.
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