Raman Spectroscopic Characteristics Of Columbite And The Implications For Columbite U-Pb Age Concordance

In this study,Raman spectroscopic analysis was performed on columbite-（Mn）samples from a variety of previously studied rare-metal pegmatites in Xinjiang,China（including the Jing’erquan No.1 spodumene-subtype,Dakalasu No.1 beryl–columbite-subtype,and Kalu’an spodumene-subtype pegmatites）to quantify the relationship between the degree of metamictization of columbite and Raman spectra.For all of the analyzed columbites-（Mn）,the position（p）and the full width at half maximum（FWHM）of the strongest band(A_1gvibration mode related to the Nb/Ta–O bond)in the Raman spectra are negatively correlated.Combined with previously determined U–Pb isotopic data and major–minor-element data of the columbites-（Mn）,their degree of metamictization was quantified usingα-dose and dpa,both of which were corrected for effects caused by annealing.The results demonstrate that the columbites-（Mn）from Jing’erquan and Kalu’an are well crystalline,whereas those from Dakalasu are in a stage that is transitional between crystalline and amorphous.The main factor influencing the key parameters（i.e.band position and FWHM）of the strongest Raman band of columbite-（Mn）is metamictization caused by radiation damage,whereas composition and crystal orientation show limited influence.A set of equations is established to quantify the degree of metamictization of columbite using the band position and the full width at half maximum:FWHM=8.309×ln（^aD）+30.11（R²=0.9861）;p=-5.187×ln（^aD）+867.09（R²=0.966）;FWHM=8.1453×ln（^adpa）+48.425（R²=0.9822）;and p=-5.078×ln（^adpa）+855.67（R²=0.9594）.In addition,a correlation study was conducted between the key parameters of the strongest Raman band of columbite and the U-Pb age concordance.The results showed that the FWHM had a negative correlation with the U-Pb isotope age concordance,while the band position（p）had a positive correlation with the U-Pb isotope age concordance.Some trace elements with higher contents in the sample were counted and analyzed in correspondence with the band positions of the secondary Raman bands in the Raman spectrum of the sample.The analytical results indicate that the contents of U,Pb,Zr,Hf,and Sn are negatively correlated with the band positions of the minor Raman bands(e.g.,～133cm^-1),that is,as the element content increases,the band positions of the minor Raman bands gradually shift towards lower wavenumbers;in contrast,the contents of Th and W are positively correlated with the band positions of the minor Raman bands,which means that the band positions of the minor Raman bands gradually increase with the increasing Th and W contents.