Silicon isotopes in allophane as a proxy for mineral formation in volcanic soils

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.

► The fate of dissolved Si in volcanic soils strongly depends on Al availability. ► Lighter δ30Si ratios in allophane relative to the basalt reflect weathering processes. ► Heavier δ30Si ratios in allophane indicate Si precipitation as Al is humus-complexed. ► Allophane δ30Si ratios could be useful in paleo-environmental reconstruction.