Behavior Of Lithium Isotopes And Environmental Indications During Granite Weathering

Weathering of rock and soil formation is the key factor for controlling the evolution of earth surface environment,driving element migration and enrichment and maintaining the supply of life resources.Moreover,on geological time scales,the weathering of continental silicates plays an important role in global carbon cycle and regulating of global climate change.Lithium isotope is a new method to study the weathering of continental silicates,but its fractionation mechanisms are still controversial.This is mainly due to the study on the mechanism of lithium isotopic fractionation mainly focusing on basalt weathering,while other silicate-based rocks study is very less.Granite is widely distributed in the continent,and the study of lithium isotopic fractionation during its weathering process is almost blank.Therefore,studying the behavior of lithium isotope during the weathering of granite can further supplement and improve the mechanism of lithium isotopic fractionation during silicate weathering,so that the lithium isotope can be more scientifically applied to the study of continental silicate weathering.In this study,two weathering profiles developed on granites with significant differences in weathering degree are collected,which are located in the eastern edge of the Tibetan Plateau.A method of lithium separated from natural samples is established and lithium isotopic ratios are determined by multi-collector ICP-MS with external precision(2?)less than 0.8‰.The lithium content and isotopic composition of weathering products and individual primary minerals of granite are analyzed.Potentially controlling factors on lithium isotopic fractionation during weathering of granite,such as the temperature and rainfall,weathering degree,formation and dissolution of secondary minerals and differential dissolution of primary minerals,are also discussed.This research has obtained the following important conclusions:(1)Significant differences in lithium content and isotopic composition of the individual primary minerals.Lithium is enriched in biotite while relatively less contents in feldspars and quartz.Quartz has the highest value of?~7Li(+15.6‰~+21.8‰)while other minerals have relatively lower values and with a similar range of variation(-1.6‰~+9.0‰).There is no significant correlation between lithium contents and?~7Li values in these minerals.(2)There are significant differences in the behavior of lithium isotope profiles between the two profiles.At the beginning of granite weathering,significant lithium isotopic fractionation is observed and is in accordance with the Rayleigh fractionation model with?values range from 1.006 to 1.012.Probably,the isotopically lighter Li in the octahedral layer of biotite,while relatively isotopically heavier Li in the interlayer is preferentially released and led to in more ~6Li enrichment in saprolite.As weathering progresses,lithium isotopic fractionation during the formation of secondary minerals and the weathering products are relatively enrichment of ~6Li,while the secondary mineral dissolution process leads to the migration of the relatively enriched ~6Li,resulting in an increase in the?~7Li value in the weathering products.(3)Lithium isotopes can effectively trace the weathering of granite.When exploring the relationship of silicate weathering and climate changing,the significant fractionation of Li isotopes in the early stage of granite weathering must be considered.Uplift mountains may lead to a large number of biotite-containing fresh rocks is exposed and weathering.Weathering of biotite leads to significant fractionation of Li isotopes and this process has a significant effect on the increase of?~7Li in seawater from 60 Ma.In summary,the lithium isotopic fractionation during weathering of granite is related to the weathering degree,dissolution of primary minerals,formation and dissolution of secondary minerals.The main mechanism is:firstly,more ~7Li is released during weathering of biotite,secondly,~6Li tends to be remained into secondary minerals during weathering,and thirdly,more ~6Li is released once again during dissolution of secondary minerals.Then,the concept model of lithium migration and isotopic fractionation is established for weathering of granite.All of the conclusions provide important theoretical basics for the further studies on the continental silicate weathering.