Noble Gas And Internal Characters Of Diamond From Mengyin Area,Shandong Province And The Geologic Genesis

We presented a comprehensive investigation of noble gases in diamond from Mengyin,Shandong province,and the genetic mechanism of them.try to trace the destruction evolution of North China Craton lithospere mantle.First of all,we did research by CL,DiamondView,BSE.Majority diamonds from Mengyin present a melted appearance,rounded crystals,and many crystals have melted fracture,This is a speaking witness for diamonds had experienced strong melting.CL,DiamondView,BSE techniques have been used to study the characteristics of intergrowth texture and inclusions in diamond from Mengyin area.The growth structure,period and process of diamond can be revealed by the cathodluminescence(CL)technique.CL results show that most of diamond samples have multi-stage of complex structure and discontinuous growth histories.The rare agate-like structure of diamond is discovered in Mengyin area,means that the melting has happended in every period in their growth history melting.DiamondView results show that parts of the samples'annules present different colores,the blue color means N3 concentration and the green color means H3 concentration,so we know that mantle fluid composition changed periodically in different growth annule.It indicate that mantle fluids here related with diamonds was inhomogeneous.The technique we apply in the process is single vacuum crushing,which can selectively extract noble gases from inclusions with less noble gas extraction from the chromite lattice.We got 8 groups of noble gas data of diamonds from Mengyin(mainly about He,Ne,Ar).The quantity of the noble gas is very low.Argon and Neon isotopic composition are very similar to air ratio,indicate that they were effected by air composition.After analysising BSE and noble gas result together,we find that there is some relevance with solid inclusions composition in the diamonds and helium isotopes ratio.Parts of the diamonds'solid inclusions composition is mainly pyroxene,beyond that they also has some shallow oringin minerals,their ~3He/~4He are very low,closed to 1Ra,two data even lower than 1Ra;Diamonds whose solid inclusions are typical mantel peridotite composition(Olivine,spinel,garnet,chromite and so on),their ~3He/~4He data are higher than the formers,highest datum is 5.17Ra,these indicate that genetic mechanism of diamonds from Menyin are very complicate.But even the highest datum is also lower than MORB value(8±1Ra),Ne and Ar isotopic characteristics are close to air's,while against to the typical mantle's,which indicates that the formation environment of diamonds in Mengyin area contains crust source composition,related with supra-subduction zone setting,and also bring a large deal of air composition in the meantime.Indicating that old North China Craton(NCC)lithospheric mantle contains some crust source.