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Diagenesis of Iron in Proglacial Sand Deposits of Late- and Post-Weichselian Age
Dieke Postma, B. Sine Brockenhuus-Schack (2)
Iron diagenesis was studied in Quaternary sand deposits of a temperate climate using an integrated approach of sedimentary petrography and groundwater chemistry. The iron-bearing minerals, amphiboles, pyroxenes, magnetite, and ilmenite are all extensively altered. Quantitatively, amphiboles and pyroxenes are by far the most important. Goethite and lepidocrocite are the most important of the iron oxyhydroxide weathering products, and they coat quartz and feldspars as well as amphiboles and pyroxenes. However, lepidocrocite is absent in the unsaturated zone. Hematite is only present associated with magnetite and ilmenite. Total iron contents of the sediments range from 0.3-0.9 percent, with 30-80 percent as coatings on quartz and feldspar grains, 25-60 percent in the amphibole and pyrox ne fraction, and only 2-15 percent in the magnetite and ilmenite fraction. Therefore, amphiboles and pyroxenes are the most important sources of iron for these diagenetic processes.
The groundwater has a pH between 5.5 and 6.0 and contains 10-30 µM Fe2+ and a low total of dissolved solids. Subtraction of the contribution of the dissolved solids from atmospheric precipitation indicates higher pH and HCO3-, SiO2, Ca2+, Na+m and Fe2+ concentrations due to mineral reactions in the aquifer.
Stability relationships for iron-bearing minerals under the assumption of anoxic conditions show strong undersaturation for iron-bearing silicates, while undersaturation for magnetite and ilmenite is much less. The saturation state for magnetite is strongly dependent on the iron oxide phase formed by incongruent dissolution.
The surfaces of iron silicates and magnetite are highly etch-pitted, indicating that mineral dissolution is controlled by surface reactions rather than by diffusion through alteration products. The rate of mineral alteration is slow in comparison with groundwater flow. Thus, diagenetic changes occur in the deposit as a whole rather than as a diagenetic front advancing through the deposit.
The abundance of FeOOH on all detrital grains suggests that this proglacial deposit could develop into a red bed during further diagenesis. This possibility should be considered in the paleoclimatic interpretation of red beds.
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