Silicon isotopes and the tracing of desilication in volcanic soil weathering sequences, Guadeloupe

Silicon (Si) stable isotopes have the potential to become a useful weathering proxy, given that light Si isotopes are preferentially incorporated into secondary clay minerals. Here we investigate how Si depletion in soils and associated clay mineralogy influence the Si isotope fractionation associated with clay mineral formation. We report δ30Si compositions in bulk soils and clay fractions relative to their parent andesite in three soil weathering sequences from Guadeloupe that were formed under contrasting climatic conditions. Strongly desilicated soils containing kaolinite that formed in wet areas (high precipitation) are compared with less desilicated soils containing smectite formed in drier conditions (low precipitation). Clay fractions are isotopically lighter than the parent andesite (δ30Si-0.23‰), and increasingly lighter with Si depletion in soils, which supports the view that the Si isotope composition in secondary clay fractions is controlled by the degree of soil desilication. It is shown that the Si isotope fractionation factor between the parent silicate material and the secondary clay minerals is smaller for Si-rich secondary clay minerals such as smectite and larger for Si-poor secondary clay minerals such as kaolinite. This study provides new insights to better define Si isotopes as a proxy for environmental conditions for clay neoformation.

► The δ30Si ratio in the clay fraction depends on the degree of soil desilication.
► Lighter Si isotope compositions in kaolinite than in smectite relative to andesite
► Insights to define δ30Si as a proxy for environmental conditions for clay formation