扬子克拉通西部砂矿型刻面状金刚石多晶的显微红外光谱研究及意义

Study on the Micro-FTIR Spectra of the Euhedral Faceted Polycrystalline Diamonds(EFPCDs)from Western Yangtze Craton and Its Geological Significance

扬子克拉通西部刻面状金刚石多晶的微区显微红外光谱研究结果表明,多晶以IaAB型为主,其中的氮含量变化较大,介于25.70-358.35μg·g-1之间,且同一多晶的不同晶粒中的氮含量有明显差异。金刚石中的＂A氮心→B氮心＂聚集转变不完全,且B%集中在40%左右,未见C氮心;多晶不是在金刚石的成核阶段所形成的,而是在各个金刚石晶粒形成后在地幔储藏期间聚集在一起的。其形成环境较华北克拉通东部的山东蒙阴刻面状金刚石多晶更为复杂;多晶极可能形成于地幔深部160-180km的范围内,达到扬子克拉通的核部深度,接近于岩石圈底部,为地幔深源成因;多晶中的sp2杂化C—H键的存在有利于片晶氮的形成,其浓度一般要高于sp3杂化C—H键的浓度。

The results of Micro-FTIR spectra analysis of the euhedral faceted polycrystalline diamonds（EFPCDs）from the Western Yangtze Craton show that the EFPCDs are mostly IaAB type,the concentration of nitrogen varies greatly from 25.70-358.35μg·g-1.Different nitrogen content distributes in different diamond grains or position in same sample.The C Center was not found in the samples and the conversion from A center to B center is incomplete,in the meanwhile,B% value concentrated in40%.Thus,polycrystalline diamonds are not formed in the stage of nucleation but gathered together after formation of the individual diamond grains during the residence time in the mantle.And its formation environment is more complex than the euhedral faceted polycrystalline diamonds from Mengyin kimberlite,the Eastern of North China Craton.The diamonds extremely possibly originated in the deep mantle from 160 to 180km,reaching the depth of the core of the Yangtze Craton,at the same time it is close to the bottom of the lithosphere.The C—H bond of sp2 hybridization are conducive to the formation of platelets in diamonds.Meanwhile,its concentrations are generally higher than the C—H bond of sp3 hybridization in the samples.