Skeletal aragonite neomorphism — quantitative modelling of a two-water diagenetic system

Abstract

The neomorphism of skeletal aragonite occurs by the simultaneous dissolution of aragonite and precipitation of calcite along thin solution films, which may have an associated zone of partly dissolved aragonite (chalky zone). It has been proposed that pore waters at neomorphic calcite–aragonite contact zones (neomorphic fronts) may be partly isolated from bulk pore waters and thus retain primary geochemical signatures to varying degrees. Iterative computer programs were developed that simulate aragonite neomorphism in two-water diagenetic systems using stable isotope and trace element data. Neomorphism of aragonitic mollusks from the Purbeck Group of southern England (Late Jurassic to Early Cretaceous) was associated with a decrease in strontium concentration from approximately 5100 ppm to 500 ppm. Modelling results indicate a high bulk water/rock ratio and minimal, if any, isolation of the neomorphic front waters for the Purbeck Group mollusks and also for some Pleistocene corals. Partial isolation of neomorphic front waters from bulk pore waters is uncommon, based on published data on the geochemistry of neomorphic calcites. However, partial isolation does occur in some thick, low porosity shells, such as the gastropod Strombus gigas (Pleistocene). Process-based modelling of fluid–rock interaction provides a means for better interpreting trace element and stable isotope data in diagenetic carbonates. Sensitivity analysis results indicate that water/rock ratios cannot be determined at much better than order of magnitude accuracies, because of uncertainties over initial pore water composition, fractionation factors and distribution coefficients.