Lithium Isotopes Geochemical Behavior In Typical Silicate Weathering Process

In the past 20 years,with the emergence of multi-acceptance plasma mass spectrometer(MC-ICP-MS)and the advancement of isotope measurement technology,a series of emerging stable isotopes of metals(such as Li,Mg,K,Ba,Zn,Cu)get rapid development.Silicate weathering is an important geological process for controlling the migration of materials on the surface,and the weathering of continental rocks is accompanied by a large amount of leaching and migration of alkali and alkaline earth elements(such as Li,Mg,K,Ba).As a result,significant isotopic fractionation of alkali and alkaline earth elements(Li,Mg,K,Ba)has been observed in various geological reservoirs on the surface.The fractionation behavior of these isotopes during continental weathering has become a frontier topic in isotopic geochemistry.As the lightest alkali metal element,Lithium element has two stable isotopes,6Li and 7Li.The huge quality difference and the unique properties of silicate enrichment make Li isotopes a powerful tool for tracing the weathering process of continental silicates,which has attracted the attention of many researchers.However,many silicate weathered crusts have a significantly higher ?7Li value than unweathered bedrock,which contradicts the results of laboratory adsorption experiments,and cannot explain the Li isotope composition of river water and sea that is significantly heavier than the continental crust.The relationship between the Li and isotope behavior of the silicate weathering process and the important weathering conditions such as precipitation and temperature is lack of relevant studies.There is no report on the comparison of Li isotopic behavior of silicate weathering with different lithologies.Therefore,based on the above scientific issues,this doctoral thesis selected a series of typical silicate weathering profiles for detailed Li isotope research.In this thesis,firstly,Li isotope studies on the weathering crust developed from Leiqiong basalt in Guangdong,local dustfall,and river water are carried out.The weathering crust of Leiqiong basalt is located in the tropical and subtropical monsoon area.The climate is hot and humid.The weathering crust has experienced strong weathering and leaching,and the easy migration elements such as alkali and alkaline earth elements have been lost.The weathering crust is mainly composed of gibbsite and hematite.It is composed of secondary clay minerals and is a typical weathering crust developed from basalt.Research on the Li isotope of the weathered profile,combined with Asian sands,local dust,river water,etc.,shows that there is obvious Li isotope fractionation in the basalt weathering process in tropical areas.The ?7Li value of the weathered basalt crust is lower than that of the unweathered basalt,and the maximum fractionation is The value reaches?10‰.According to leaching experiments,we propose that the desorption and resorption of Li by secondary clay minerals leads to the enrichment of Li at the bottom of the profile,and it has the lightest Li isotope composition.Based on this viewpoint and the conservation of mass,we established a calculation model to simulate the Li content and isotope composition of the weathering crust,which fits the Li isotope composition of the weathering crust very well.Based on the research on the weathering crust of Leiqiong basalt,this doctoral dissertation conducts research on the weathering crust developed by Cenozoic basalt in different climate zones such as Tengchong,Inner Mongolia,Abaga Banner,Wudalianchi,Heilongjiang.The above weathering crust sections are located in different climatic zones,and the weathering conditions such as precipitation and temperature are obviously different,which provides good conditions for studying the Li element and isotope behavior of silicate weathering process in different climatic weathering conditions.Based on the study of the above weathering crust profile and the previous research results,it is found that precipitation and temperature will significantly affect the Li isotope fractionation behavior of the silicate weathering process.When the average annual precipitation is less than 1500 mm,the degree of Li isotope fractionation in the silicate weathering process increases with the increase in the average annual precipitation;and when the average annual precipitation is higher than 1500 mm,the Li isotopes fractionation value in the silicate weathering process decreases with the increase of the average annual precipitation.Studies have shown that temperature will also significantly affect the Li isotope behavior of silicate weathering.When the annual average temperature is lower than 15?,the Li isotope fractionation in the silicate weathering process increases with the increase of temperature;when the annual average temperature is higher than 15?,the Li isotope fractionation in the silicate weathering process will increase with the increase in temperature.In addition,our research found that silicate weathering represented by basalt has obvious differences between land and sea.Weathering crusts with higher ?7Li than unweathered bedrock are all located on ocean islands surrounded by the sea,while the ?7Li of silicate weathering crusts developed on the continent are all lower than that of the unweathered bedrock,indicating marine aerosols,precipitation and other sea-sourced materials The input is the main reason for the high ?7Li value of the oceanic island silicate weathering crust.The contradiction between the high ?7Li value of the weathering crust and the experimental results of the laboratory clay mineral adsorption is solved.Studies have also shown that silicate weathering is the main reason for the increase in the ?7Li value of river water and sea water and the decrease of the Li isotope composition of the continental crust.Tropical areas are the areas with the strongest surface weathering.Therefore,we systematically studied the silicate weathering crusts of different lithologies developed in tropical humid regions,and studied the influence of silicate lithology on the Li isotope fractionation behavior during the weathering process.The Philippines is located in the tropics,adjacent to the South China Sea in the east and the Pacific Ocean in the west,with abundant precipitation and a large number of ophiolite sets.We conducted Li isotope studies on the weathering crust of peridotite(ultrabasic rock)developed in the Surigao region of the Philippines.Studies have shown that weathering crusts developed in peridotite in tropical areas have undergone strong weathering.Alkali metals,alkaline earth metals and silicon have been leached.The chemical components of weathering crusts are mainly alumina and iron oxide,and the main mineral composition is goethite,hematite,illite,etc.The weathering of peridotite(ultrabasicrock)is accompanied by strong Li isotopic fractionation,which can reach?12‰.The adsorption of Al-containing secondary clay minerals(such as illite)is the main cause of Li isotope fractionation.Panama is located in Central America with a narrow land,surrounded by seas,high temperature and humidity,and strong weathering.In this doctoral dissertation,a small-scale section study of two weathered gabbro(basic-ultrabasic)in this area is carried out.Studies have shown that the alkali and alkaline earth elements in the weathering crust of gabbro are leached out in large quantities.The relatively stable elements such as Al and Fe are enriched in the weathering crust,the primary minerals are dissolved,and the secondary minerals are generated in large quantities.The Li in the weathered crust is leached,and the adsorption of secondary clay minerals results in the ?7Li value of the weathered crust being significantly lower than that of unweathered gabbro,with a fractionation of?9‰,indicating that the weathering of gabbro(basic-ultrabasic)is accompanied by weathering Due to the leaching of elements and the formation of secondary minerals,Li isotopes are strongly fractionated.The Yulin City of Guangxi is located on the northern edge of the equatorial region.The climate is tropical and subtropical monsoon climate,with high temperature and rain,and strong weathering.We conducted major,trace and Li isotopic studies on the Darongshan granite weathering crust in this area.Studies have shown that there is no significant difference between the main and trace components of weathered crusts of weathered granite in tropical areas and unweathered granites.There is no significant fractionation of Li isotopes during weathering,and the ?7Li values of weathered crusts are consistent with the ?7Li values of unweathered granites.The above studies show that lithology will significantly affect the Li isotope behavior in the silicate weathering process.The weathering process of basic ultrabasic rocks is accompanied by the loss of a large amount of alkali metals and the formation of secondary clay minerals,resulting in strong Li isotope fractionation.In the process of weathering of acid rocks such as granite,the geochemical composition did not change significantly,and the Li isotopes did not undergo significant fractionation.