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Early diagenesis of metastable marine cements occurs through a phase of dissolution along intercrystalline boundaries which is accompanied by precipitation of low magnesium calcite (LMC) within enlarged intercrystalline pores. This LMC cement is a luminescent phase complexly intergrown with non-luminescent, corroded crystallites of the precursor fibrous marine cement. This intergrowth results in early coalescence of the multicrystalline cement, which effectively isolates metastable phases from open chemical exchange with ambient pore waters during subsequent diagenesis.
Closure of the diagenetic system during subsequent stabilization is indicated by the preservation of chemical signatures retained within final calcitized products. Multiple carbon and oxygen isotopic analyses of a single generation of marine cement, for example, define strongly covariate compositional trends that reflect varying mixtures of the luminescent and non-luminescent calcites which presently comprise the stabilized marine precursor. End-member compositions of such trends reflect the compositions of intergrown LMC and precursor marine cement, respectively. Although early coalescence provides for closure of the chemical system, it does not prevent ultimate stabilization of metastable phases to LMC. Importantly, metastable relics are not preserved in ancient marine cements.
From all available data on solid-state processes, we infer that, at diagenetic temperatures, water is a required diagenetic medium to effect transformations of aragonite and high magnesium calcite phases to LMC. If, however, water is involved in this stabilization process, how is it possible to maintain a chemically closed system? An abundance of associated fluid inclusions is characteristic of fibrous cement mosaics. Such fluids, trapped along intercrystalline boundaries during early coalescence, migrate through the metastable host. As metastable phases dissolve, driven by their solubility difference with LMC, they concomitantly precipitate LMC, which paramorphically replaces the precursor cement. Such a mechanism not only provides for the retention of overall crystal fabric, via a s bmicron dissolution-precipitation process, but also provides for the maintenance of chemical signatures of the dissolving, metastable precursor cements.
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