Simulation of limestone diagenesis — a model based on strontium depletion

Abstract

Limestone diagenesis has been simulated by a computer model based on strontium and zinc depletion in meteoric groundwater. This has enabled estimates to be made of the minimum number of pore volumes of groundwater that must have infiltrated many limestones and the accompanying degree of recrystallization that must have occurred in order to produce the observed reductions in the trace element concentrations. The amount of strontium lost by Barbados and Barbuda aragonite during early diagenesis indicates that the 0.05 value of Katz et al. (1972) for the distribution coefficient of strontium during the aragonite-to-calcite transformation is more accurate than the 0.14 value of Kinsman and Holland (1969). About 3000–6000 pore volumes of meteoric groundwater are required to transform aragonitic limestones on Barbados and Barbuda to low-magnesian calcite with average strontium contents of 1700 and 500 ppm, respectively. Generally, the model suggests that early diagenesis will not decrease the strontium contents of limestones below about 400 ppm.Extrapolation to late diagenesis indicates that during the recrystallization of low-magnesian calcite, tens of thousands to hundreds of thousands of pore volumes of meteoric solution must pass through limestones to reduce their strontium contents to between 100 and 200 ppm. Variation in the degree of solution and reprecipitation of CaCO3 that characterizes the residence periods of pore volumes of solution infiltrating low-magnesian calcite limestones affects the number of bulk recrystallizations, but has little influence on the total volume that passes through these limestones during late diagenesis.Porosity differences and variations in groundwater acidity have more effect on limestone strontium contents during late diagenesis than do the initial mineralogical differences. Facies-controlled porosity variations may have partly produced the range of strontium contents exhibited by ancient limestones.The directional trends of strontium concentrations and the low average strontium content (143 ppm) in the limestones of the Dunedin Formation in British Columbia indicate that it underwent late diagenesis by solutions that, during their passage, had a residence period of many solution–reprecipitation events.Zinc in solution does not undergo a significant increase in concentration during meteoric diagenesis because its distribution coefficient is greater than 1. The observed absence of directional trends of zinc within the Dunedin Formation supports this model prediction.Abnormally high concentrations of strontium are retained in hydrocarbon-bearing limestones because the solution–reprecipitation process is inhibited by the presence of hydrocarbons. This suggests that strontium anomaly surveying would provide a useful adjunct to present hydrocarbon exploration techniques.