Chemical weathering in a tropical watershed,Luquillo Mountains,Puerto Rico III: quartz dissolution rates |
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| Affiliation: | 1. School of Earth and Space Exploration, Arizona State University, Tempe, AZ, United States;2. Department of Geosciences, Pennsylvania State University, University Park, PA, United States;3. Earth and Environmental Sciences Institute, Pennsylvania State University, University Park, PA, United States;4. Department of Geosciences, Idaho State University, Pocatello, ID, United States;5. Department of Geological Sciences, University of Texas at Austin, Austin, TX, United States;1. Curtin University, Miri, Sarawak, Malaysia;2. Universidad Nacional Autónoma de México, México D.F., México;3. Goldbach Geoconsultants O & G, Glattbach, Aschaffenburg, Germany;4. JX Nippon Oil and Gas Exploration (Deepwater Sabah) Limited, Kuala Lumpur, Malaysia;1. Department of Geological Sciences, University of Texas at El Paso, El Paso, TX 79968, USA;2. Isotope Laboratory, Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA;3. State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Science and Mineral Resources, China University of Geosciences, Beijing 100083, China;4. Key Laboratory of the Earth''s Deep Interior, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;5. CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China;6. Earth and Environmental Systems Institute, Pennsylvania State University, University Park, PA 16802, USA |
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| Abstract: | The paucity of weathering rates for quartz in the natural environment stems both from the slow rate at which quartz dissolves and the difficulty in differentiating solute Si contributed by quartz from that derived from other silicate minerals. This study, a first effort in quantifying natural rates of quartz dissolution, takes advantage of extremely rapid tropical weathering, simple regolith mineralogy, and detailed information on hydrologic and chemical transport. Quartz abundances and grain sizes are relatively constant with depth in a thick saprolite. Limited quartz dissolution is indicated by solution rounding of primary angularity and by the formation of etch pits. A low correlation of surface area (0.14 and 0.42 m2 g−1) with grain size indicates that internal microfractures and pitting are the principal contributors to total surface area.Pore water silica concentration increases linearly with depth. On a molar basis, between one and three quarters of pore water silica is derived from quartz with the remainder contributed from biotite weathering. Average solute Si remains thermodynamically undersaturated with respect to recently revised estimates of quartz solubility (<180 μM) but exceeds estimated critical saturation concentrations controlling the initiation of etch pit formation (>17–81 μM). Etch pitting is more abundant on grains in the upper saprolite and is associated with pore waters lower in dissolved silica. Rate constants describing quartz dissolution increase with decreasing depth (from 10−14.5–10−15.1 mol m−2 s−1), which correlate with both greater thermodynamic undersaturation and increasing etch pit densities. Unlike for many aluminosilicates, the calculated natural weathering rates of quartz fall slightly below the rate constants previously reported for experimental studies (10−12.4–10−14.2 mol m−2 s−1). This agreement reflects the structural simplicity of quartz, dilute solutes, and near-hydrologic saturation. |
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