Abstract
Diagenesis of shallow water carbonate sediments in marine and meteoric fluids involves dissolution of metastable Mg-calcites and aragonites and precipitation of low-Mg calcite to produce stable lime-stone. Solid dissolution rates are controlled by mineralogic stability, available surface area (microstructure and grain size), saturation state, and solution chemistry (Mg:Ca ratio and inhibitors such as phosphate). The influence of these variables on dissolution rates of common biogenic aragonites, Mg-calcites, and low-Mg calcites are quantified and a predictive model is offered for the relative rates and sequence of diagenetic reactions.Contrary to the accepted sequence for reactivity, aragonites frequently dissolve more rapidly than Mg-calcites of equivalent grain size in all solution types. The relative dissolution rates are related to the microstructure of the grains. Microstructural complexity of many aragonites appears to overwhelm the difference in mineralogic stability between aragonite and Mg-calcite. Thus, the relative dissolution rates of aragonite and magnesian calcites is a function of both mineralogic stability, saturation state, and microstructure. Dissolved phosphate inhibits both aragonite and Mg-calcite dissolution similarly and so has no effect on the relative reactivity of these phases.The stabilities of Mg-calcites were redetermined using similar methods to those of Plummer and Mackenzie (1974). Free drift dissolution experiments on two biogenic Mg-calcites (12 and 18 mole% MgCO(,3)) at two solution Mg:Ca ratios yield equilibrium constants about three times smaller than those of Plummer and Mackenzie (1974) making aragonite about equal in stability to a 12 mole% Mg-calcite. These values did not vary with solution Mg:Ca ratio, initial pH, or solid:solution ratio. Samples not ultrasonically cleaned and annealed yield values similar to those of Plummer and Mackenzie (1974). The presence of submicron fines and crystal strain may cause more rapid dissolution rates and overestimation of the equilibrium state.The new equilibrium constants for Mg-calcites greatly reduce the solubility offset between aragonite and mg-calcites. This result acts in concert with microstructural effects to allow aragonites to dissolve more rapidly than Mg-calcites containing high mole percents MgCO(,3). The impact of these findings on natural diagenetic systems is evaluated.