The role of disseminated calcite in the chemical weathering of granitoid rocks |
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| Institution: | 1. Department of Geoscience and Natural Resource Management, Geology Section, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark;2. Nordic Center for Earth Evolution (NordCEE), University of Copenhagen, Denmark;3. Centro de Investigaciones Geológicas, Universidad Nacional de La Plata, CONICET, La Plata, Argentina;4. National Museum of Denmark, Department of Conservation and Natural Sciences, Prinsens Palae, Frederiksholms Kanal 12, DK-1220 Copenhagen K, Denmark;5. Danish National Research Foundation''s Center for Textile Research, SAXO Institute, University of Copenhagen, Karen Blixens Vej 4, DK-2300 Copenhagen S, Denmark;1. Department of Environment and Natural Resources, Payame Noor University, PO Box 19395-3697 Tehran, Iran;2. Department of Soil Science, College of Agriculture, Isfahan University of Technology, 84156-83111 Isfahan, Iran;3. Departament de Medi Ambient i CiènciesdelSòl, Universitat de Lleida, Av. Alcalde Rovira Roure 191, 25198 Lleida, Catalunya, Spain;4. Department of Soil Science, College of Agriculture, Islamic Azad University, Khorasgan Branch, Isfahan, Iran;5. Department of Soil Science, College of Agriculture, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran;1. Laboratoire de Planétologie et Géodynamique de Nantes, CNRS, UMR6112, Université de Nantes, 44322 Nantes, France;2. Institut de Recherche en Astrophysique et Planétologie, UPS-OMP, Université de Toulouse, Toulouse, France;3. University of Tennessee, Department of Earth and Planetary Sciences, Knoxville, TN 3799, USA;4. Los Alamos National Laboratory, Los Alamos, NM 87545, USA;5. U.S. Geological Survey Astrogeology Science Center, Flagstaff, AZ 86001, USA;6. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91125, USA;7. NASA Ames Research Center, Moffett Field CA USA;8. GeoRessources, Université de Lorraine, Nancy, France;9. College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA;10. Division of Geologic and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA;11. Astromaterials Research and Exploration Science Division, NASA Johnson Space Center, Houston, TX 77058, USA;12. German Aerospace Center (DLR), Institut für Optische Sensorsysteme, Berlin-Adlershof, Germany;13. Earth and Planetary Sciences, University of California, Davis, USA;1. Key Laboratory of Mountain Surface Processes and Ecological Regulation, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences and Ministry of Water Resources, Chengdu 610041, China;2. University of Chinese Academy of Sciences, Beijing 100049, China;3. Dagu Glacier Administration Bureau, Heishui County 623500, Sichuan Province, China |
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| Abstract: | Accessory calcite, present at concentrations between 300 and 3000 mg kg?1, occurs in fresh granitoid rocks sampled from the Merced watershed in Yosemite National Park, CA, USA; Loch Vale in Rocky Mountain National Park CO USA; the Panola watershed, GA USA; and the Rio Icacos, Puerto Rico. Calcite occurs as fillings in microfractures, as disseminated grains within the silicate matrix, and as replacement of calcic cores in plagioclase. Flow-through column experiments, using de-ionized water saturated with 0.05 atm. CO2, produced effluents from the fresh granitoid rocks that were dominated by Ca and bicarbonate and thermodynamically saturated with calcite. During reactions up to 1.7 yr, calcite dissolution progressively decreased and was superceded by steady state dissolution of silicates, principally biotite. Mass balance calculations indicate that most calcite had been removed during this time and accounted for 57–98% of the total Ca released from these rocks. Experimental effluents from surfically weathered granitoids from the same watersheds were consistently dominated by silicate dissolution. The lack of excess Ca and alkalinity indicated that calcite had been previously removed by natural weathering.The extent of Ca enrichment in watershed discharge fluxes corresponds to the amounts of calcite exposed in granitoid rocks. High Ca/Na ratios relative to plagioclase stoichiometries indicate excess Ca in the Yosemite, Loch Vale, and other alpine watersheds in the Sierra Nevada and Rocky Mountains of the western United States. This Ca enrichment correlates with strong preferential weathering of calcite relative to plagioclase in exfoliated granitoids in glaciated terrains. In contrast, Ca/Na flux ratios are comparable to or less than the Ca/Na ratios for plagioclase in the subtropical Panola and tropical Rio Icacos watersheds, in which deeply weathered regoliths exhibit concurrent losses of calcite and much larger masses of plagioclase during transport-limited weathering. These results indicate that the weathering of accessory calcite may strongly influence Ca and alkalinity fluxes from silicate rocks during and following periods of glaciation and tectonism but is much less important for older stable geomorphic surfaces. |
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