Halogens (Cl, Br, and I) geochemistry in Middle Triassic carbonates: Implications for salinity and diagenetic alteration of I/(Ca + Mg) ratios

As an important family of elements, halogens (Cl, Br, and I) have been widely used to study geological processes, particularly in high-temperature environments, showing great prospects either in fundamental or in applied studies. However, little work has been conducted for sedimentary and low-temperature diagenetic environments re-construction although these elements have the potential in theory. Here we perform a pilot study on this issue using a case study of Middle Triassic marine dolomitization in South China. Our results show that the average Cl concentrations in limestone, dolomitic limestone, calcitic dolomite and dolomite are 88.3 ppm, 114.7 ppm, 121.3 ppm and 142.9 ppm respectively, and the corresponding Br concentrations are 1.15 ppm, 1.42 ppm, 1.59 ppm and 2.13 ppm, respectively. Both the concentrations of Cl and Br increased with the increasing dolomitization degree. The increasing tendency of Cl and Br is similar to the seawater evaporation trajectory before halite precipitation, indicating that dolomitization occurs in fluids with elevated salinity. Meanwhile, the difference of Cl/Br ratios between our carbonate samples and seawater may be ascribed to the input of terrestrial water, which has variable Cl/Br ratios. As for the concentration of I, the mean values sharply decreased from 0.36 ppm in limestone to 0.13 ppm in dolomitic limestone samples, and it further lowered to 0.09 ppm and 0.08 ppm in calcitic dolomite and dolomite samples, respectively. The calculated I/(Ca + Mg) ratios in limestones vary between 0.05 and 0.58 μmol/mol with an average value of 0.29 μmol/mol, indicating that limestone formed in open ocean oxygen-minimum zone (OMZs) or anoxic basin environment. The average I/(Ca + Mg) ratio decreases from 0.29 μmol/mol in limestone to 0.06 μmol/mol in dolomite, serving as an important case that dolomitization decreases I/(Ca + Mg) ratios of carbonates rocks. Halogens in carbonates therefore have promising potential for investigating the sedimentary and diagenetic environments of carbonate rocks.