Silicon isotopes in allophane as a proxy for mineral formation in volcanic soils S. Opfergelt a,b,⇑ , R.B. Georg c , K.W. Burton a , R. Guicharnaud d , C. Siebert a , S.R. Gislason e , A.N. Halliday a a Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, United Kingdom b Earth and Life Institute, Université catholique de Louvain, Croix du Sud 2/10, 1348 Louvain-la-Neuve, Belgium c Trent University, Worsfold Water Quality Centre, 1600 West Bank Dr., Peterborough, Ontario, Canada d Agricultural University of Iceland, Keldnaholt 112, Reykjavik, Iceland e Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland article info Article history: Available online 22 March 2011 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 (À0.64 ± 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-sat- uration, 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 for- mation, which is potentially useful in environmental paleo-reconstruction. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Atmospheric CO 2 concentration is controlled over the long term by chemical weathering of Ca and Mg bearing silicate rocks (Berner, 1995). Basaltic bedrock displays a faster weathering rate and CO 2 consumption capacity than other continental silicate rocks (Dessert et al., 2003). In addition, volcanic islands disproportion- ately contribute 45% of the river suspended material to ocean (Milliman and Syvitski, 1992). Therefore, quantifying weathering fluxes from basaltic islands is crucial for an accurate estimate of continental derived input fluxes to the ocean as well as long-term atmospheric C-fluxes. Andosols are volcanic soils typically developing on volcanic ash by chemical weathering. These soils contain mostly short range ordered minerals such as allophane and ferrihydrite (Mizota and van Reeuwijk, 1989; Arnalds, 2004). Allophane is a common short-range-order clay mineral that plays an important role as a precursor mineral to more crystalline minerals, such as halloysite (Ziegler et al., 2003). Depending on the soil pH, andosols will be dominated by allophane above pH 5 and by Al-humus complexes below pH 5 (Mizota and van Reeuwijk, 1989). The rate of mineral formation in soils is very useful in climate and geochemical mod- elling (Godderis et al., 2010). In volcanic environments, Si stable isotopes are promising trac- ers for soil weathering processes (Ziegler et al., 2005; Opfergelt et al., 2010; Bern et al., 2010). Light Si isotopes are preferentially incorporated in secondary weathering products, producing com- panion waters and rivers enriched in heavy Si isotopes (Georg et al., 2007). Allophanes formed experimentally were 0.34‰ lighter than their parental solution (Ziegler et al., 2005). So far, reported Si isotope values for natural allophane are À0.40‰ with no constrain on the parental material (Douthitt, 1982), and À0.37‰ relative to a parental basalt (À0.38‰; Opfergelt et al., 2010). The question is raised whether a constant isotope fractionation factor for allo- phane neoformation can be used in geochemical models, specifi- cally focusing on the time evolution of the mineralogical composition (Godderis et al., 2010). This study investigates Si iso- tope compositions of allophane formed in contrasting conditions in Icelandic volcanic soils, in order to provide a better estimate of the isotopic variability of allophane relative to homogeneous parental basalt. 2. Material and methods Five typical Iceland soil types (Arnalds, 2004) were selected (West Iceland): Histic Andosol (HA), Histisol (H), Gleyic Andosol (GA), Brown Andosol (BA), Vitrisol (V). The parental material is pre- dominantly basalt. Each soil profile was sampled in September 2009 by horizons following description (IUSS, 2006). Major ele- mental content in soils was determined by ICP–AES after borate 0883-2927/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2011.03.044 ⇑ Corresponding author. Tel.: +32 10 47 36 38; fax: +32 10 47 45 25. E-mail address: sophie.opfergelt@uclouvain.be (S. Opfergelt). Applied Geochemistry 26 (2011) S115–S118 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem