Anhydrite and the Sr isotope evolution of groundwater in a carbonate aquifer
- Permanent Link:
- Anhydrite and the Sr isotope evolution of groundwater in a carbonate aquifer
- Series Title:
- Chemical Geology
- Jacobson, A.D.
- Publication Date:
- Subjects / Keywords:
- 87Sr/86Sr ( local )
Weathering ( local )
Groundwater ( local )
Anhydrite ( local )
Carbonate ( local )
- serial ( sobekcm )
- Major element concentrations and 87Sr/86Sr ratios were measured in groundwaters and bedrock from the Madison aquifer in western South Dakota. In this region, the Madison aquifer is primarily comprised of dolomite belonging to the Madison Limestone Group. The purpose of the study was to investigate controls on the downgradient evolution of dissolved Sr2+ in a carbonate groundwater system that is recharged by waters with high 87Sr/86Sr ratios draining Precambrian basement rocks and to establish the sources of Sr2+ added to the waters by reaction with the aquifer lithology. A mass-balance model following previous workers was used to calculate amounts and effective rates of mineral dissolution and precipitation during groundwater transport along a âˆ¼240-km flow path. Both the calculated reaction rates and data for the Sr isotope geochemistry of the reacting phases were then used to develop a self-consistent and quantitative description of the concentration and isotopic composition of dissolved Sr2+ in the aquifer waters.
The major ion chemistry of Madison aquifer groundwater is known to evolve according to dolomite dissolution, anhydrite dissolution, calcite precipitation, and ion-exchange with clay minerals. Dissolved 87Sr/86Sr ratios in the Madison aquifer decrease downgradient. Input waters draining the igneous Black Hills have 87Sr/86Sr ratios of âˆ¼0.723, while highly evolved waters in the aquifer have 87Sr/86Sr ratios of âˆ¼0.708. Dissolved Sr2+ concentrations undergo a concurrent increase from âˆ¼1200 to 66,000 nmol/l. Model results indicate that dolomite dissolution exerts a critical control on the major ion chemistry but is not the primary source of nonradiogenic Sr2+, as both the dissolution rate and Sr concentration of dolomite are very low. Anhydrite is readily soluble in water, has a low 87Sr/86Sr ratio (âˆ¼0.708), and a very high Sr concentration (âˆ¼50,000 nmol/g). Anhydrite is also greatly undersaturated in the groundwaters, and the downgradient evolution of Sr2+ accompanies an âˆ¼80-fold increase in dissolved SO
- Original Version:
- Chemical Geology, Vol. 220, no. 3-4 (2005-01-25).
- Source Institution:
- University of South Florida Library
- Holding Location:
- University of South Florida
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- This object is protected by copyright, and is made available here for research and educational purposes. Permission to reuse, publish, or reproduce the object beyond the bounds of Fair Use or other exemptions to copyright law must be obtained from the copyright holder.
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