Detrital record of the denudation of volcanic islands under sub-tropical climate (Cape Verde) |
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| Institution: | 1. Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, United States of America;2. School of Earth Sciences, University of Bristol, Clifton, BS8 1RJ Bristol, UK;3. School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK;4. NERC Argon Isotope Facility, SUERC, Scottish Ent. Tech. Park, Rankine Ave., East Kilbride G750QF, UK;5. Montserrat Volcano Observatory, PO Box 316, Flemmings, MSR, 1110, Montserrat;6. British Geological Survey, Environmental Science Centre, Keyworth, Nottigham NG12 5GG, UK;7. Ocean Exploration Trust, Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, United States of America;1. Institute of Precambrian Geology and Geochronology, nab. Makarova 2, St. Petersburg, 199034, Russia;2. St. Petersburg State University, Institute of Earth Sciences, Universitetskaya nab. 7/9, St. Petersburg, 199034, Russia;3. A.P. Karpinsky Russian Geological Research Institute, Srednii prosp. 74, St. Petersburg, 199106, Russia;4. Department of Geology, Catoca Geological-Mining Community, Luanda, Republic of Angola |
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| Abstract: | This research is focused on the composition of the sediments produced in volcanic islands when the climate does not favour weathering. The XRD mineralogy (bulk sample and fraction finer than 63 μm), petrography and geochemistry of a set of bedload stream and beach samples collected in the “old” Maio and the “young” Fogo islands of Cape Verde archipelago are used to investigate the compositional transformations promoted by exogenous processes during island denudation. The main factor responsible for the variability in sediment composition is the incorporation of biogenic material derived from the evolving shelves; it largely exceeds the effects of the exhumation of different volcanic and basement units. Given the arid climate (and steep land surface in Fogo), only the most labile components of basaltic rocks, such as volcanic glass, are decomposed. The incipient weathering and sorting processes are responsible for the depletion of Al in bedload deposits. The same happens with other elements usually regarded as non-mobile (namely, Nb, Th, REE, etc.), while Mg is concentrated. Thus, weathering indices grounded on the premise that “mobile” elements are lost and “non-mobile” elements are enriched via weathering are useless in Cape Verde bedload sediments. With time, weathering is able to promote Na leaching and the formation of secondary minerals, which tend to retain non-mobile elements released in the earlier stages of alteration (e.g., LREE, Th, Y, Nb, Ta etc.). Sorting processes are responsible for the selective removal of less-dense grains, explaining local differences between beach and stream deposits. Beach placers are enriched in augitic clinopyroxene (occasionally also in olivine in the Fogo island), and Sc, Cr and Co. Niobium and Ta must be hosted in fine-grained particles that are easily windblown and their abundances in dusts may reveal Cape Verde as a source of airborne material crossing the Atlantic Ocean. |
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| Keywords: | Geochemistry Mineralogy Weathering Sorting Cape Verde Volcanic islands |
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