Geochemical behavior of riverine magnesium isotopes in the Yarlung Tsangpo River Basin, southern Tibetan Plateau

The riverine magnesium (Mg) isotope composition is generally controlled by sources and fractionation processes. However, it remains unclear in which cases these factors are predominant at the basin scale. In this study, we investigated the major elements, trace elements, and Mg and strontium (Sr) isotope ratios in the Yarlung Tsangpo River (YTR) Basin, southern Tibetan Plateau (TP), to explore the geochemical behavior of Mg isotopes and the dominant controlling factors. Riverine Mg²⁺ was shown to derive primarily from the weathering of silicates and carbonates. Riverine δ²⁶Mg, which is first influenced by the mixing of different lithological sources, was detected ranging from −1.63 ‰ to −0.52 ‰. According to the saturation indexes of common Mg-bearing secondary minerals, the basin was divided into two zones. Above Shannan City (Zone I), where river waters mostly were oversaturated with minerals, positive correlations were observed between δ²⁶Mg and the Ca/Mg and Sr/Mg ratios. This indicated that, after mixing, secondary minerals (talc and chlorite), which tend to preferentially incorporate ²⁴Mg, may be formed in most of waters, leading to an increase in riverine δ²⁶Mg values. The Rayleigh and batch fractionation models were fitted with factors ranging from 0.9975 to 0.9997. Below Shannan City (Zone II), where river waters mostly were unsaturated with minerals, riverine δ²⁶Mg was positively correlated with ⁸⁷Sr/⁸⁶Sr and negatively correlated with the Ca/Mg, Sr/Mg, and HCO₃/Na ratios. This suggested that the riverine δ²⁶Mg were influenced by carbonate dissolution with faster kinetic rates. Our analysis of riverine δ²⁶Mg values in the YTR Basin may be a reference for interpreting the relationships between δ²⁶Mg and δ⁷Li in river systems worldwide. In minerals-oversaturated waters, secondary mineral formation was shown to lead to isotope fractionation, resulting in a positive correlation between the two parameters, whereas in minerals-unsaturated waters, the δ²⁶Mg and δ⁷Li values were negatively correlated, possibly due to the predominance of mineral dissolution.