A crystal growth rate of equation for ancient dolomites; evidence for millimeter-scale flux-limited growth

Abstract

ABSTRACT A linear correlation exists between the width of cathodoluminescent zones (CL) and the size of individual crystals in the Seroe Domi Fm (Miocene, Netherlands Antilles), Burlington-Keokuk Fm (Mississippian), Fort Payne Fm (Mississippian), and Saluda Fm (dovician). If individual zones, correlated by CL character and stratigraphy, are isochronous, then the width of a zone is directly proportional to the rate of crystal growth, Linear correlation coefficients for between-zone thickness and crystal size in individual thin sections range from 0.98 to 0.41, indicating a size dependence governing individual dolomite crystal growth rates. Mononuclear, polynuclear, spiral dislocation, and diffusion-limited crystal growth models operating under homogeneous or inhomogeneous kinetic conditions were considered as possible crystal growth mechanisms. Of the models considered, only mononuclear growth may result in a correlation between zone width and crystal radius in chemically homogeneous systems. However, this relationship is not linear. Any of the growth laws considered may produce a linear size-dependent growth rate when there are temporally constant differences between crystals in the kinetic factors governing growth. Differences in the kinetic factors governing growth could arise from stable variations in the chemistry of pore fluids related to permeability variations in the rock. The radius-rate data give a crystal growth rate equation of the form dr/dr = kr. This equation is consistent with a model wherein the rate of crystal growth is determined by stable variations in the chemistry of pore fluids. This implies that the kinetics of dolomite crystal growth is flux limited and therefore no slower than many diagenetic reactions. The data also support the hypothesis that thin-section-scale variations in dolomite crystal size can result from variations in crystal growth rate.