Abstract:

Magnesium (Mg) is a major constituent of terrestrial water, and significant mass-dependent fractionation of Mg isotopes can occur during various water-rock interactions. Consequently, Mg isotopes have considerable potential for tracing these processes. This paper systematically summarizes the Mg isotopic compositions of the key reservoirs that define the initial Mg isotopic composition of natural waters, including silicates, carbonates, evaporites, and rainwater. Silicates typically exhibit high and variable δ²⁶Mg values, whereas carbonates typically have low and relatively uniform δ²⁶Mg values. Evaporites display a wide range of δ²⁶Mg values depending on their mineralogy, and the δ²⁶Mg values of rainwater are often shaped by local environmental conditions. Subsequently, geochemical processes that remove Mg from water, such as clay formation, carbonate precipitation, adsorption, cation exchange, and plant uptake, fractionate Mg isotopes. Specifically, the formation of secondary minerals like montmorillonite, adsorption onto solid surfaces, and plant uptake preferentially incorporate heavy Mg isotopes (e.g., ²⁶Mg) into the solid/biological phase, leaving the surrounding water enriched in the light ²⁴Mg. Conversely, the formation of chlorite, cation exchange, and carbonate precipitation can preferentially incorporate lighter Mg isotopes, leaving the water heavier. The Mg isotopic signatures of rivers and groundwater are further influenced by hydrological setting and water-rock interaction timescales. In river water, which has high renewal rates, the Mg isotopic signature is primarily shaped by silicate and carbonate dissolution, clay formation, and cation exchange. In contrast, groundwater, which typically involves longer water-rock interaction timescales, can also be significantly influenced by adsorption. Furthermore, in regional groundwater systems with long flow paths, Mg isotopes serve as effective tracers for identifying multiple water-rock interaction processes in both carbonate- and silicate-dominated aquifers; the nature of these reactions is primarily controlled by aquifer mineralogy. Future research in Mg isotope hydrogeochemistry should prioritize: (1) integrating Mg with K isotopes to better identify adsorption processes in river systems, and (2) characterizing Mg isotopic variations along flow paths in aquifers of different lithologies and scales.

Key words: Mg isotopes, water-rock interactions, river water, groundwater, isotope fractionation, adsorption