H071-07
Flow-path Influences Chlorite Oxidation and Dissolution in a Watershed

Wednesday, 9 December 2020: 16:24
Virtual
Ruxue Liao, Lanzhou University, Lanzhou, China; Pennsylvania State University, State College, United States, Xin Gu, Penn state university, Department of Geosciences, State College, PA, United States and Susan Brantley, Pennsylvania State University, Earth and Environmental Systems Institute, University Park, PA, United States
Recorded Presentation
Abstract:
The relationship between oxidation and dissolution in vermiculitization of chlorite has never been completely understood, especially under different hydrological and geochemical conditions. In this work, weathering of chlorite is studied in several boreholes and soils across a small watershed (Shale Hills) from grain to watershed scale. At watershed scale, the oxidation, dissolution and vermiculitization degree of chlorite are quantified by parameters from different measurements and correlated with each other. Chlorite dissolution is found to lag behind chlorite oxidation under the valley. Both occur simultaneously under the ridge. This is likely caused by the differences in water table position, flow rates, concentrations of dissolved oxygen (DO), and pH between ridge and valley. The oxidation of chlorite and pyrite commence together near the water table under the ridge but commence below the water table under the valley. This is likely because of larger flow rates and O2 transport rates under the valley. Additionally, carbonate is abundant under the valley and its dissolution increases the pH of water, thus preventing the interlayer hydroxide sheet of chlorite from dissolving in carbonate-rich depth intervals. Vermiculitization of chlorite in Shale Hills is a process where the interlayer filling degree decreases through dehydroxylation and dissolution via intermediate hydroxy-interlayered vermiculite (HIV). The process can be described as a stepwise chemical reaction: Chlorite => partially oxidized chlorite => HIV => vermiculite => Fe(hydr)oxides + kaolinite. In terms of grain scale, the chlorite Si/Al ratio increases and Mg/Al ratio decreases in transforming to vermiculite. Importantly, the FeLĪ²/FeLĪ± intensity ratio measured by Electron Probe Micro Analyzer (EPMA) demonstrates that even chlorite grains from the same depth can be differentially oxidized and dissolved.

Key words: deep weathering, chlorite oxidation, dissolution, pyrite, ridge and valley

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