Fluid-rock interaction effects on Li isotope behavior in continental geothermal systems

Lithium (Li) isotopes have been used as a sensitive proxy to trace water-rock interaction, but its behavior during this process in geothermal system is still unclear. In this study, we use Rehai geothermal system in Tengchong, China, as a natural lab to investigate the behavior of Li isotopes in different pH conditions (acid and alkaline fluids). Alkaline fluids are characterized by high Li contents of 5–10 ppm compared to 0.3–1.5 ppm in acid fluids. Mineral formation path was modeled by PHREEQC and WATCH program. Acid fluids are far-from equilibrium that inhibits the formation of secondary minerals, and the δ7Li data are from −7.8‰ to −0.8‰, lower than the host rock (−0.4‰). In the absence of evidence for secondary mineral formation along the flow path, the negative correlation of δ7Li versus Li/Na ratio is explained by different degrees of dissolution, i.e., incongruent dissolution, resulting in a wide variable δ7Li. For alkaline fluids, clay minerals such as chlorite and smectite are formed during water-rock interaction and fluid boiling process. These samples have uniform δ7Li value of 0.4–0.9 ‰ (with a mean of +0.6‰), slightly higher than the host rock (−0.4‰). Intensive water-rock interaction drives the system to reach equilibrium with host rock, causing the congruent dissolution of host rock. Thus, the fluids show the geochemical characteristics of host rock, and conceal the role of clay minerals. This study enhances our understanding for the controlling factors of Li isotopes in hydrosphere, which highly depends on system characteristic (pH, temperature, water-rock ratio and duration time etc.), the fluxes of dissolution and precipitation, and the critical role of these parameters needs to be considered carefully when interpreting δ7Li values, especially judging whether the system is in equilibrium with surrounding rock.