Interpreting crystal growth kinetics of ancient dolomites

Crystal growth has been described by the following models; (1) mononuclear growth model, (2) birth and spread growth model, (3) polynuclear growth model, and (4) spiral growth model (Christian 1972, Nielson 1964, Ohara and Reid 1959, Burton et al. 1951). In order to apply these growth models to ancient dolomite reservoirs, an estimate of the growth rate is necessary. The zone width in concentrically-zoned, luminescent dolomites can be used to estimate growth rate (Nordeng and Sibley 1996, Carlson 1989, Kretz 1974, 1973). By rearranging the equations for the known growth models, crystal population trends on a zone-width vs. crystal-size plot can be constructed to distinguish between groups of crystals growing by mononuclear growth versus the following mechanisms: (1) polynuclear growth, (2) birth and spread growth, (3) diffusion-limited and (4) spiral growth mechanisms. To determine the extent that any group of crystals within a dolomite unit grew by the same crystal growth mechanism, samples were examined at regular intervals from three luminescent dolomites: (1) Saluda and Whitewater Formations (Ordovician), Indiana, U.S., (2) Burlington-Keokuk Formation (Mississippian) Missouri, U.S., and (3) Seroe Domi Formation (Miocene), Curacao, Netherland Antilles. Plots of zonewidth vs. crystal-size for crystals in an individual sample were constructed. Crystal growth equations fitted to each sample's zonewidth vs. crystal-size plot were compared among samples from: (1) the same lithology and formation, (2) different lithologies and the same formation, (3) the same lithology and different formations and (4) different lithologies and different formations. Analysis of crystal growth equations from all three dolomites indicate a linear relationship, dr/dt = k′r, where growth rate increases with crystal size (r) indicating crystal growth mechanism does not vary on the outcrop scale with lithology, δ 13C, δ18O, or age of formation. The average slope (k′) of all three dolomites was 0.181. Correlation coefficients for the equation dr/dt = kr ranged from 23% to 81%. Zone-width vs. crystal-size plots constructed for pairs of crystals adjacent to each other in the Saluda and Burlington-Keokuk formations revealed the same growth mechanism dr/dt = k′r, indicating growth was controlled by a crystal surface process instead of solution variability.