Behaviour of Li isotopes during regolith formation on granite (Massif Central, France): Controls on the dissolved load in water, saprolite, soil and sediment

• Published in: Chemical geology 2019-09, Vol.523, p.121-132
• Main Authors: Négrel, Ph, Millot, R.
• Format: Article
• Language: English
• Subjects: Applied geology; Earth Sciences; Granite; Groundwater; Isotopes; Lithium; Sciences of the Universe; Sediment; Soil; Weathering
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2019.05.037

Lithium (Li) contents and isotopes were studied in all environments of a small river catchment draining granite in the Margeride mountains of the French Massif Central. This covered surface waters, primary and accessory minerals of the granites, the whole rock, and soil and sediment samples developed in the catchment, completed with regional data for mineral waters and rainwater. The integrated investigation aimed at evaluating the potential of Li isotopes as effective tracers of water/rock interaction processes within a granitic environment. The δ7Li values and Li concentrations were measured on sediment- and soil samples, following standard acid-dissolution procedures and chemical purification of Li using the cationic exchange resin protocol in a clean lab. Lithium-isotope compositions were measured with a Neptune MC-ICP-MS and Li concentrations by ICP-MS. The samples represented different stages of granite weathering, including fresh granite, weathered-rock, surface saprolite, and sediments in riverbanks and fields bordering the streams. The extent of Li mobility during granite weathering was first evaluated through determining the percentage change relative to Ti, with a range from −31 to −66% in the collected samples. The weathered-rock was depleted by −47% for Li with negative δ7Li values ranging from −1.9 to −3.4‰. Soil to riverbank sediment samples were characterized by less negative δ7Li values, indicating that Li is enriched in soil with fractionation of Li isotopes and changes of the mineral abundance in the samples. To complement this first view, we i) Modelled the theoretical Li isotope signature of water interacting with granite, using a weathering model based on dissolution; ii) Applied an atmospheric-input correction to surface waters; iii) Applied a Raleigh equation for modelling the Li isotope fractionation when compared with corrected surface water and mineral water; and iv) Compared Li isotopes with Sr-isotope data in a larger weathering framework.