Adsorption- and desorption-controlled magnesium isotope fractionation during extreme weathering of basalt in Hainan Island, China

Abstract

Magnesium isotopic compositions of a set of clay-rich saprolites developed on the Neogene tholeiitic basalt from Hainan Island in southern China have been measured in order to document the behavior of Mg isotopes during continental weathering. Compared with unaltered basalts (δ26Mg=−0.36‰), the overlying saprolites are strongly depleted in Mg (i.e., τTh,Mg=−99.1% to −92.9%), and display highly variable δ26Mg, ranging from −0.49‰ to +0.40‰. Magnesium concentration and δ26Mg value of the saprolites display a general increasing trend upwards in the lower part of the profile, but a decreasing trend towards the surface in the upper part. The variations of Mg concentration and isotopic composition in this weathering profile can be explained through adsorption and desorption processes: (1) adsorption of Mg to kaolin minerals (kaolinite and halloysite), with preferential uptake of heavy Mg isotopes onto kaolin minerals; and (2) desorption of Mg through cation exchange of Mg with the relatively lower hydration energy cations in the upper profile. Evidence for adsorption is supported by the positive correlation between δ26Mg and the modal abundance of kaolin minerals in saprolite of the lower profile, while negative correlations between δ26Mg and concentrations of lower hydration energy cations (e.g., Sr and Cs) in the upper profile support the desorption process. Our results highlight that adsorption and desorption of Mg on clay minerals play an important role in behavior of Mg isotopes during extreme weathering, which may help to explain the large variation in Mg isotopic composition of river waters.