Increased Mg release rates and related Mg isotopic signatures during bacteria-phlogopite interactions

Abstract Phlogopite weathering experiments were carried out under aerobic conditions in a closed system to study Mg isotopic fractionation during the dissolution and processes involved in the presence of bacterial strains. Four different bacterial strains with different metabolisms were chosen. Biotic experiments were performed in batch reactors at 24 °C for 12 days. In parallel, abiotic phlogopite weathering experiments were also performed using organic and nitric acids. Citric and gluconic acids were used to model the effects of chelating agents produced by bacteria, and nitric acid was used to model the effects of acidifying agents. The results indicate that both decreases in pH and the production of metabolites during heterotrophic bacterial activities significantly accelerate the release of Mg and Si in solution. Also, at a given pH, the fraction of elements released in solution is greater in the presence of bacteria or citric acid compared to that released in the presence of nitric and gluconic acids. Magnesium isotopic analyses indicate that the solutions obtained using nitric acid and acidifying bacterial strains display, on average, δ 26 Mg values that are close to or slightly heavier (by 0.4‰) than those of fresh phlogopite (−1.2‰ ± 0.08). In contrast, in experiments performed with citric acid, the Mg leached into solution is slightly enriched in light isotopes by −0.3‰ relative to the initial phlogopite, while its binding to organics is expected to be related to its preferential enrichment in heavy isotopes. Despite its small range of variations, the δ 26 Mg values of solutions vary inversely with pH, thus suggesting that secondary phases preferentially enriched in 26 Mg at the highest pH values may play a key role.