Effect of water regime and vegetation on initial granite weathering in a glacier forefield: Evidences from CL, SEM, and Nomarski DIC microscopy

• Granite material exhibited an initial alteration within one-year exposure.
• Suppression of blue luminescence was found in altered K-feldspar grains.
• Plagioclase was sensitive toward redox processes.
• Weathering of biotite required both, constant water and abundant vegetation.

Glacier forefields are characterised by high spatial variability in water availability, solar radiation, and vegetation, as well as weathering processes. The aim of this study is to improve the understanding of initial weathering of granite material with respect to the water regime (availability and residence time of the water) and the vegetation cover. Samples of granite powder were buried in four selected sites at the Damma glacier forefield for one year. A combined application of cathodoluminescence (CL) microscopy and spectroscopy, scanning electron microscopy (SEM), and Nomarski differential interference contrast (Nomarski DIC) microscopy was used to investigate the initial stage of weathering. The examination of the initial weathering stage revealed that the exposed mineral grains were strongly characterised by intra-variations. A clear change in particle shape and surface morphology could not be observed within the relatively short exposure period of one year. However, the grains from the lake sediment exhibited a clearly rounded shape, caused by mechanical abrasion during transport processes. The combination of several microscopic methods revealed (i) a strong influence of vegetation in the case of plagioclase-oligoclase alteration, (ii) a mutual effect of water and vegetation in the case of biotite delamination, and (iii) alterations of potassium feldspar independent of local environments. Investigations with cathodoluminescence microscopy showed the suppression of blue luminescence and developed brown luminescence and confirmed a new insight in potassium feldspar weathering.