Isotopic, chemical and textural relations during the experimental alteration of biogenic high-magnesian calcite

Abstract

Dissolution runs with four biogenic high magnesian calcites (HMC) of different magnesium content were conducted in distilled water saturated with 100% CO 2 and 3% CO 2 at 70°C and 35°C in order to study the mechanism of alteration of HMC to low magnesian calcite (LMC). The isotopic composition ( δ 13 C and δ 18 O) of the solutions (−30%. relative to PDB and +40%. relative to SMOW, respectively) was maintained far from isotopic equilibrium with respect to the dissolving carbonate. The isotopic composition and Mg content of the residual solid phase was monitored throughout the approach to stoichiometric saturation. For the coralline algae Goniolithon, heterogeneous and possibly incongruent dissolution occurred which reduced the Mg Ca mole ratio in the solid from 0.44 to 0.26. The alteration was accompanied by an apparent isotopic equilibration of up to 20% of the solid. Runs with two littoral foraminifera, Archaias and Sorites showed a decrease in the Mg Ca mole ratio from 0.18 to 0.14 in the solid and an apparent isotopic equilibration of up to 10%. These changes began to occur before the bulk solution became saturated with respect to LMC. On the other hand the echinoid Clypeaster dissolved homogeneously and congruently with much smaller changes in magnesium and stable isotope composition: from 0.16 to 0.14 in Mg Ca mole ratio and less than 5% apparent isotopic equilibration. All isotopic shifts were in the direction of equilibrium with the solution isotopic composition. No isotopic shifts were observed in runs with Iceland spar under identical conditions. Textural differences in microstructure discovered by SEM examination of the HMC materials used suggest the possibility of an overall textural control over the observed differences in isotopic and chemical behaviour during the dissolution runs.