Lithium Isotopic Signatures Of Meso And Neoarchean Carbonate Rocks And Their Geochemical Significance

The Archean is a key period of the earth's evolution.However,due to the long time geological evolution,most records of information has been removed or damaged due to later geological processes,which causes great difficulties for understand the history of earth's evolution during the period.This study attempts to use the Li isotope geochemistry to preliminarily limit the surficial environmental processes associated with Archean seawater.The Kaapvaal craton in South Africa is a representative area for studying the early earth's crustal evolution and surface processes.We selected marine carbonate samples deposited in the Meso-and Neoarchean,after summing up the previous sample dissolution experience,we designed and verified a new method of sample dissolution to accurately extract the lithium isotopic infromation which recorded in the carbonate minerals,and found that 0.5 N acetic acid can effectively avoid the interference of silicate components in carbonate rocks.We performed Li isotope analysis on carbonate rocks and found that the carbonate show light Li compositions of??1‰ during the period of 3.0?2.9Ga,and increased of+7 ‰ to+10‰ during the period of 2.6?2.5Ga.Through inversion calculation,the Li isotope composition of seawater in the two periods was?+12‰and?+20‰,respectively,which are significantly lower than modern seawater(?+ 31‰),however,the ?7Li value of seawater during 2.6?2.5Ga is more than 8‰ higher than that at 3.0?2.9Ga.As an effective tracer for continental weathering of silicate rocks,Meso and Neoarchean seawater shows relatively low ?7Li values which indicates the continental weathering at that age tended to be consistent.Under the environmental conditions where the surface temperature and atmospheric CO2 content of Archean are quite high,the silicate rocks were rapidly eroded and thus resulted in the ?7Li value of river water extremely closed to the upper continental crust(0?+2‰),which ultimately affected the Li isotopic composition of Archean water.During 3.0?2.5Ga,the decrease in the overall temperature of the seawater and the enhancement of inconsistent continental weathering may jointly lead to an increase in the seawater ?7Li value during late Archean.The study of Li isotopes of Archean carbonate can effectively invert the Li isotopic composition of paleoseawater,and provide new information for understanding the surficial environmental processes related to the Archean seawater.