Silicon isotopes in allophane as a proxy for mineral formation in volcanic soils |
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| Authors: | S Opfergelt RB Georg KW Burton R Guicharnaud C Siebert SR Gislason AN Halliday |
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| Institution: | 1. Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, United Kingdom;2. Earth and Life Institute, Université catholique de Louvain, Croix du Sud 2/10, 1348 Louvain-la-Neuve, Belgium;3. Trent University, Worsfold Water Quality Centre, 1600 West Bank Dr., Peterborough, Ontario, Canada;4. Agricultural University of Iceland, Keldnaholt 112, Reykjavik, Iceland;5. Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland |
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| Abstract: | Weathering of basaltic ash in volcanic areas produces andosols, rich in allophane and ferrihydrite. Since the rate of mineral formation is very useful in climate and geochemical modelling, this study investigates Si isotope compositions of allophane as a proxy for mineral formation. Allophane formed in contrasting conditions in five Icelandic soil profiles displays silicon isotope signatures lighter than the basalt in less weathered soils (?0.64 ± 0.15‰), and heavier in more weathered organic-rich soils (+0.23 ± 0.10‰). The fate of the dissolved Si in those volcanic soils strongly depends on Al availability. In organic-rich soils, most of Al is humus-complexed, and the results support that Si precipitates as opaline silica by super-saturation, leaving an isotopically heavier dissolved Si pool to form allophane with uncomplexed Al. This study highlights that Si isotopes can be useful to record successive soil processes involved in mineral formation, which is potentially useful in environmental paleo-reconstruction. |
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