Chemical weathering and lithium isotope fractionation of biotite in ice

Abstract

Chemical weathering is one of the main causes of the physicochemical change of the Earth's surfaces. However, the effect of the ice and its properties on chemical weathering needs to be researched. This study aims to investigate the chemical weathering of biotite in the ice as an experimental approach. To investigate the chemical weathering of biotite in ice, the dissolution of biotite was carried out under the environments modifying the subglacial environments (-21 ℃, 'I' groups), and compared with the room temperature (25 ℃, 'W' groups). The relative abundance of dissolved species and lithium isotopes were applied as indicators of the congruency and intensity of weathering processes. The mixtures of the powdered biotite with a particle size of 64-150 μm and nitric acid at pH 3 were prepared for samples. The groups of samples under both temperatures were reacted to for 13 minutes to 3146 hours. The dissolution of biotite during experiments was presented similarly to the chemical signature of the early stage of weathering for biotite. The concentration of dissolved species demonstrates that the dissolution of biotite is clearly observed in both groups, but they proceed with different aspects. The dissolved concentrations of both groups indicate the behavior of elements dependent on the kinetics of dissolution between crystallographic sites of the biotite. The residues of long-term reacted samples (W10, I10) were analyzed by XRD and FE-SEM to observe the altered structures of biotite and find the secondary mineral precipitate. The agglomerated structures of weathering products were observed from SEM scanning images of long-term reacted samples. However, weathered structures of ice sample were only visible on the limited area of biotite. XRD results of long-term reacted samples of both groups and variation of Si concentration suggest potential aggregation of amorphous silica or precipitation of non-identified clay minerals occur during the ice weathering processes that were reported in previous studies on polar regions, but the possibility of secondary precipitation of oxide still needs further discussion. The concentrations and δ7Li of dissolved lithium of long-term reacted samples were measured with the values of 5.74 μM and 3.84‰ for 25 ℃ and 2.50 μM and 6.71‰ for -21 ℃. Their fractionated behaviors of lithium were the results of the different congruency and intensity between the dissolution of biotite and incorporation in secondary minerals. Isotope analysis for lithium is quite helpful for detecting the chemical signature of the early stages of weathering and inferring the weathering aspects. This research focuses on silicate weathering in ice and the role of ice as a reaction medium, which has received little attention in previous studies, and it implies expanding our understanding of chemical weathering in subglacial and supraglacial regions, as well as the effects of meltwater discharge in the polar regions.