ALBERT

Description: Within the framework of this thesis investigations on formation and dissolution processes of Mg-calcites have been carried out. Based on the results of mineralogical and isotope geochemical investigations it was possible to reconstruct the formation processes of authigenic high Mg-calcite concretions sampled from mud mounds of the Costa Rica forearc. Furthermore, based on laboratory work, the oxygen isotope fractionation of high Mg-calcite in aqueous media was examined. Finally, dissolution experiments on CaMg-carbonates were performed and theoretic kinetic parameters could be defined for the dissolution processes. Chapter I presents a short introduction describing the natural occurrence of Mg-calcites and their formation processes. Furthermore, different experimental techniques, which are applied to investigate precipitation- and dissolution processes of carbonates, are introduced. Formation processes of authigenic carbonates in Mounds 11 and 12 (Costa Rica forearc) are discussed in Chapter II. The pore water geochemical profiles from sediment cores collected in the study area show the active diagenetic processes leading to carbonate mineral precipitation. Investigation of the stable carbon and oxygen isotope composition of the collected authigenic concretions indicates a complex formation scenario, possibly controlled by sediment erosion, varying fluxes of methane-rich fluid, and thus migration zones of carbonate formation. Based on the stable isotope composition of the authigenic concretions and the respected geochemical pore fluid composition a common deep fluid source is postulated for the two mud mounds. The third chapter investigates the effect of Mg2+ incorporation on the oxygen isotope fractionation of high Mg-calcite in the temperature range between 25 and 80oC. Synthetic high Mg-calcites with various MgCO3 content are prepared in the laboratory from oversaturated solutions with respect to calcite. A new technique for the separation of crystalline from amorphous carbonate was developed. The results, including a new expression for the Mg-calcite-water oxygen isotope fractionation, imply that the incorporation of Mg2+ in the calcite crystal lattice has a much bigger effect on the Mg-calcite - water oxygen isotope fractionation, than it was previously defined by laboratory precipitation experiments at 25°C. The new isotope fractionation factor (this study) rather fits to recently published thermodynamic calculations, and will help to improve our knowledge about the environmental conditions during natural Mg-calcite formation. The investigation of CaMg-carbonate dissolution in aqueous media is presented in the fourth chapter. The run of the concentration vs. time curves could be fitted by a simple kinetic equation. Kinetic constants controlling the equation were determined and are comparable to previously published data. Furthermore, experiments performed in distilled water showed that dissolution rates of authigenic (13%)Mg-calcite are comparable to dissolution rates of calcite, when the geometric surface is used in the kinetic equation. In the case of dissolution experiments in NaCl solutions (0.6M), with ionic strength similar to seawater, the solvent obviously strongly determines the reaction order and dissolution rate of Mg-calcite. It is believed that the presented results could be used to improve the existing diagenetic models describing diagenetic dissolution/precipitation processes of CaMg-carbonates in e.g. cold seep areas.