Previous models of early carbonate diagenesis assume mineralogy controls alteration sequence, with magnesian calcites dissolving more rapidly than aragonites. Results of this study indicate that: (1) mineralogic effects can be overridden by microstructure; and (2) dissolved magnesium enchances dissolution rates.
The study determined laboratory dissolution rates of biogenic grains found in modern carbonate environments and evaluated the relative importance of grain mineralogy, microstructure, and solution chemistry by determining dissolution rates at various undersaturations in seawater and in freshwater solutions containing different amounts of dissolved magnesium.
Although aragonitic grains dissolved more rapidly than low-magnesian calcites of the same grain size, most aragonites also dissolved as fast, or faster than, magnesian calcites containing 12 to 17 mole % MgCO3. Mineralogy alone, then, is not the sole control on reactivity.
Dissolution rate is also affected by microstructure. Microstructure determines the amount of surface area available for dissolution and may exert greater control over reactivity than mineralogy. For example, the porous but smooth surface of an echinoid (magnesian calcite) dissolves much more slowly than aragonitic coral and gastropod grains, which have more complex microstructures.
The presence of dissolved magnesium enhances rates of dissolution, but does not strongly affect the relative reactivity between different grain types. The absolute dissolution rates show a strong progressive decrease in magnesium-depleted solutions.
Thus, assumption of mineralogic control over grain reactivity during early diagenesis is an oversimplification. Microstructure and solution chemistry emerge as important variables with predictive power for modeling both porosity development and diagenetic evolution within carbonate sequences.