The Relationship between Rate, Mechanism, and Solution Composition in Diagenetic Reactions: Insight from Observation of Mineral Surfaces
Rolf S. Arvidson
Rice University, Houston, TX
Despite the basic role of thermodynamics as a driving force in the diagenesis of carbonate minerals, both the actual reaction path and its time scale are a function of chemical kinetics. This constraint is the rule for early diagenetic reactions in low temperature environments, and applies to higher temperature burial regimes as well. It has become increasingly evident that our understanding of many classic problems in diagenesis is limited by knowledge of how mineral surfaces participate in these reactions. The kinetics of mineral surface reactions are the fundamental constraint in many diagenetic processes in which metastable marine phases undergo dissolution and replacement by stable secondary phases. Although much information has been provided by measurement of reaction rates in bulk experiments, key insights are now available from direct examination of mineral surfaces, often available in situ and in real time.
These insights include site-specific control of dissolution and growth rate and the relationship between free energy and reaction mechanism. Here we examine how direct observations made by vertical scanning interferometry, atomic force microscopy, and related instruments can be used to resolve the role of impurity components in crystal dissolution and growth processes, provide insight into the problem of reactive surface area, and understand how mechanism and integrated reaction rate may depend on the ratio, and not simply the product, of framework components. These observations also highlight the severe limitations of simple, empirical rate laws, particularly in brine environments where ratios of solution components may approach extreme values.