山东郯城砂矿金刚石特征及包裹体研究

Study of diamond characteristics and its inclusions in diamonds from the Tancheng placer, Shandong Province

通过对115粒山东郯城砂矿金刚石样品进行矿物学和光谱学特征研究,结果显示郯城金刚石的粒径集中在1.0~4.0mm之间,晶体形态以菱形十二面体为主,其次八面体与菱形十二面体聚形,八面体较少;晶面形貌除倒三角凹坑、塑性变形滑移线、熔蚀沟、生长丘、生长阶梯、叠瓦状蚀象、滴状丘、晕线等原生形貌发育外,小部分发育有次生形貌-绿色色斑,且大多数金刚石的边棱清晰,磨圆程度不高。研究首次测得了郯城金刚石的拉曼特征峰的半高宽数据和金刚石包裹体拉曼谱图,显示郯城砂矿金刚石结晶程度差异较大,暗示其形成的金刚石地质生长条件和环境的复杂性;金刚石包裹体有橄榄石、黄铜矿、针铁矿、石墨矿物,其中橄榄石包裹体占比较高,表明郯城金刚石包裹体类型以橄榄岩型为主,测试结果与华北东部古老克拉通之下的岩石圈地幔大部分由橄榄岩组成的结论一致。对比郯城金刚石与蒙阴金刚石特征的异同,初步探讨了金刚石砂矿的物质来源,为揭示郯城砂矿金刚石的形成及演化提供了金刚石及其包裹体的新的证据。

The results of mineralogical and spectroscopic studies on 115 diamond samples from the Tancheng placer of Shandong Province show that the particle sizes are concentrated in the range of 1.0~4.0 mm. The crystal morphology is mainly rhombic dodecahedron, followed by octahedron and rhombus dodecahedron aggregation;of these the octahedron form is least abundant. In addition to the development of primary morphology, such as the inverted triangle pit, the plastic deformation slip line, the melt erosion ditch, growth hill, the growth ladder, imbricate erosion image, the dropwise hill and the halo line, a small part of the crystal surface has secondary morphology and green spots. Most of the diamonds have clear edges and low roundness. In this study, the half height width data and inclusion Raman spectrum of the Raman characteristic peak of the Tancheng diamonds were measured for the first time. The test results show that the diamond crystallinity of the Tancheng placer is distinctive, indicating the complexity of the geological growth conditions and environment. The diamond inclusions include olivine, chalcopyrite, goethite and graphite minerals, of which olivine inclusions account for a relatively high proportion, indicating that the type of diamond inclusions in the Tancheng is dominated by peridotite. The test results are consistent with the conclusion that most of the lithospheric mantle under the old craton in the east of North China is composed of peridotite. A preliminary discussion on the source of diamond placer is based on the comparison of the characteristics of the Tancheng and the Mengyin diamonds. The study provides new evidence for the formation and evolution of diamond and its inclusions.