Study On The Mineralogical Characteristic Of Olivine In Jilin Dunhua

Peridotite xenoliths are directly from the upper mantle.Olivine is the main mineral of peridotite xenoliths and chromium spinel is its accessory mineral,through which we can obtain the evolution information of the deep earth.The thesis takes the basalt containing peridotite xenoliths from Dunhua Yiqisong and Dashihe deposits in Jilin province as the research object.The mineralogical characteristics of peridotite were studied by means of visual macroscope observation and polarizing microscope observation,electron microprobe analysis and laser Raman spectroscopy test etc.And the composition of olivine in the two deposits and even all over the world were collected,compared and analyzed.Finally,we find that the composition of olivine and spinel has an indicative effect on the mantle environment.The spinel lherzolites from two deposits are the complex rock composed by the forsterite(50%-75%),enstatite(15%-30%),diopside(10%-15%)and chromium spinel(2%-5%).Olivine commonly has kink band and triple-point structures.The Yiqisong lherzolites are yellowish in color,medium-grained,and weathered;the Dashihe lherzolites are greenish in color,coarse-grained,and fresh.The CaO content of olivine xenocrysts in both deposits was lower than 0.1wt%,the Ni O content was higher than 0.29wt%,and the Fo value was between 89.5-92,which means the degree of primary mantle melting is high.However,the Ca O content of olivine phenocrysts was higher than 0.1wt%,the Ni O was lower than 0.26wt%,and Fo was lower than 80,indicating that olivine phenocrysts were crystallized from magma.The common components of xenocrysts are gradient bands,and the marginal components of xenocrysts are similar to those of olivine phenocrysts,indicating that there is a fast elements exchange reaction between magma and phenocrysts,which may be one of the mantle evolution modes.From olivine phenocryst,olivine xenocryst to olivine of peridotite xenoliths in both deposits,their Ca O,Mn O and Fe O contents decrease with the increase of Fo,while the Ni O content increases with it.The colors of spinel in different peridotite xenoliths in the two mining areas are different.The darker the color,the larger the Cr#,which means the degree of lithospheric mantle melting is higher.The Raman spectra of olivine in the Yiqisong lherzolites are similar to those in the Dashihe lherzolites.The olivine at the center of the peridotite xenoliths has a stronger Raman peak at 823cm-1 and 855cm-1 than at the edge,and the silicon-oxygen tetrahedral bonding force in its mineral structure is stronger than that at the edge.The Raman spectra of spinel in different mining areas and peridotite xenoliths are quite different,but they all have the Raman characteristic peaks at 735cm-1 and 610cm-1.Myanmar,Pakistan,the United States,China,Vietnam and Egypt are the major producers of olivine gemstones today.The composition of olivine is similar in 12 other countries except Pakistan,South Africa and Albania,with Fo 89.3-91.5.The Fo values of olivine in Dunhua,Zhangjiakou,Jining,Datong and Donghai regions of China are between 90 and 92,and their chemical compositions are similar.Their peridotite xenoliths are all derived from the transitional type mantle,and the degree of the mantle melting is relatively high.