Mechanisms of magnesium isotope fractionation in volcanic soil weathering sequences, Guadeloupe

Magnesium (Mg) stable isotopes are increasingly used as a weathering proxy in soils and rivers, but the impact of the mineralogy of secondary phases on isotopefractionation remains obscure. A better understanding of the behaviour of Mg isotopes during weathering processes is a mandatory step toward deployment of this new tracer for understanding chemical fluxes exported from the critical zone. Here we investigate isotopic variations in δ26Mg in bulk soils and clay fractions relative to their parent andesite in three soilweatheringsequences from Guadeloupe formed under contrasting climatic conditions. Soils formed in drier conditions (low precipitation) contain smectite, whereas soils formed under wet conditions (high rainfall) are characterized by halloysite and Fe-oxides or kaolinite. All clay fractions have Mg isotopic compositions (δ26Mg −0.41‰ to −0.10‰) similar to or heavier than their parent andesite (δ26Mg −0.47‰) supporting the preferential incorporation of heavy Mg isotopes in secondary Mg-bearing clay minerals with the first direct measurements on clay fractions. Soils with lighter Mg isotope compositions have greater quantities of exchangeable Mg. The data support a contribution from sea spray to the exchangeable Mg pool correlated to the soilweathering degree. This study highlights for the first time that the soil δ26Mg not only depend on δ26Mg of the parent rock, and on any fractionation that might occur, but also on the Mg retention on the exchange complex, which could in turn be controlled by external inputs such as sea spray.