Weathering of quartz as an Archean climatic indicator |
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| Institution: | 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;1. Université catholique de Louvain, Earth and Life Institute, ELIe, L7.05.10 1348 Louvain-la-Neuve, Belgium;2. Université catholique de Louvain, Earth and Life Institute, George Lemaître Center for Earth and Climate Research, L4.03.08 1348 Louvain-la-Neuve, Belgium;3. Université Libre de Bruxelles, Department of Earth and Environmental Sciences, CP160/02. Avenue F.D. Roosevelt 50, 1050 Brussels, Belgium;1. Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA;2. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA |
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| Abstract: | Chert and other hard monomineralic quartz grains weather mostly by mechanical processes in modern environments. Their clasts are overrepresented in conglomerates and sands relative to their sources regions. Conversely, macroscopic dissolution features, including quartzite karst, are rare but not nonexistent. The similar rarity of quartz dissolution in Archean deposits provides a paleothermometer for climate on the early Earth. For example, chert is overrepresented in conglomerates and sands of the ～3.2 Ga Moodies Group (South Africa) relative to the source region. Features related to the far-from-equilibrium dissolution rate are particularly diagnostic as it increases an order of magnitude over 25 °C, much more than solubility. Extrapolating from observed dissolution rates in modern environments that weather at ～25 °C, we expect obvious dissolution features in ancient climates above ～50 °C. Polycrystalline quartz and chert would readily disaggregate by solution along grain boundaries, yielding silt and clay. Quartz grains within slowly weathering granite would become friable, yielding silt and clay, rather than sand. At still higher temperatures, Al2O3-rich clays from weathered granite would stand above solution-weathered chert on low-relief surfaces. The observed lack of these features is evidence that the Archean climate was not especially hot. |
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