Calculation of simultaneous isotopic and trace element variations during water-rock interaction with applications to carbonate diagenesis

Abstract

Equations are developed for the calculation of the simultaneous variations in trace element concentrations and radiogenic and stable isotopic compositions that occur during water-rock interaction. The equations are of general use for modeling chemical and isotopic variations in porous media and are applied here to the diagenesis of carbonate rocks and sediments. The variables which control the ultimate geochemical composition of diagenetic carbonates include the composition of the original sediment and fluid, water:rock ratio, fractionation factors, distribution coefficients, open vs. closed system behavior, and porosity. Owing to the extreme differences in the concentrations of oxygen and carbon in diagenetic fluids, carbonate minerals equilibrate with fluid δ 18O values at three orders of magnitude lower water:rock ratios (<10) than the water:rock ratios at which they equilibrate with fluid δ 13C values ( 10 3). 87Sr 86 Sr ratios are affected at variable rates. In order to reset the 87Sr 86Sr value of a marine limestone during freshwater diagenesis, water:rock ratios similar to those calculated for equilibration of δ 13C values are required, while Sr-Ca-rich brines can reset the 87Sr 86Sr value of a limestone at low water:rock ratios, similar to those calculated for equilibration of δ 18O values. Water:rock ratios exceeding 10 3 are required to affect the rare earth element (REE) patterns and Nd isotopic compositions of carbonate sediments during diagenesis. These large, relative differences in the response of different isotopic systems to water-rock interaction translate into characteristic trends on isotope and trace element covariation diagrams that can be used to distinguish between ( 1 ) different models for water-rock interaction and (2) different processes such as water-rock interaction, mixing of fluids, and mixing of mineral endmembers. An example is the use of simultaneous variations of O and Sr isotopes in modeling the freshwater diagenesis of limestones. Fresh-water-limestone interaction pathways are independent of Sr-Ca exchange distribution coefficient (K Sr-Ca D) values over an order of magnitude range of 0.01 to 0.15, because of the large differences in the water:rock ratios necessary to equilibrate the two isotopic systems. Carbon vs. oxygen isotopic variations in the same system can be used to distinguish between water-rock interaction, mixing of fluids, and mixing of end-member calcites. These modeling approaches are applied to the regionally extensive dolomites of the Mississippian Burlington-Keokuk Formation. Compared to its early dolomite precursor (dolomite I), second-generation replacement dolomite (II) in the Burlington-Keokuk Formation has higher 87Sr 86Sr ratios, lower δ 18O values and Sr concentrations, and similar δ 13C values, Nd isotopic compositions, and REE patterns. A multistage model calculation can account for the dolomite II data via recrystallization of dolomite I, whereby the δ 18O values of dolomite II record a relatively minor and late portion of the water-rock interaction history of the samples while Sr in the dolomites preserves an earlier and larger segment of the same history. The recrystallization process was effected by extraformational brines in a relatively open system with respect to Sr, while C and the REE were unaffected.