扬子克拉通深部作用过程：沅水流域砂矿大金刚石及其包裹体记录

Macrocrystalline diamonds (MCDs) are exclusive probes from deep mantle for studying the evolution of ancient lithospheric mantle and recycling of carbon. Our previous researche shows that MCDs from the Ruan river basin, Hunan are characterized by distinct carbon isotopes, nitrogen abundances, proportions of peridotite/ecologite parageneses of inclusions, and compositions of inclusion minerals such as the occurrence of kyanite, obviously different from those of MCDs from the North China craton, indicating some features related to ancient crustal substances, and hence implying there probably occurred subduction of ancient ocean crust slab into lithospheric mantle beneath the Yangtze craton and resulted in the formation of ecologite mantle and recycling of crustal carbon into lithospheric mantle. This project plans to perform analyses upon directionally-cut diamond plates on carbon isotopes, nitrogen and hydrogen abundances of diamonds along various growth layers from core to rim, as well as the inclusion mineral assemblages and their major & minor element composition, by using in-situ analyzing techniques such as cathodoluminescence microscopy, FTIR(Mapping), LA-(MC)-ICPMS and SIMS, etc.. The results will be applied to approach to the mantle environment and physics-chemistry conditions for diamonds' forming such as sources and their variation of carbon and nitrogen, the temperature and pressure and the oxygen fugacity, and finally to explore the relationship between diamonds' formation and the evolution of deep lithospheric mantle beneath the Yangtze craton, and the carbon cycling event between the surface and mantle. Furthermore, we hope to provide some geochemistry clues for prospecting primary mine for Hunan alluvial diamonds.

大金刚石(MCDs)是记录古岩石圈地幔演化过程及深部碳循环独特的"探针"。已开展的预研究发现，湖南沅水流域大金刚石不同生长层碳同位素组成及氮含量变化、包裹体矿物组合、矿物组成及化学成分等显示金刚石形成过程中可能有古老地壳物质的参与。本项目拟通过激光定向切片，采用阴极发光、FTIR(Mapping)等方法，结合电子探针，LA-(MC)-ICPMS及SIMS等微区微束分析技术，对沅水流域大金刚石内部不同生长层碳同位素组成，氮和氢杂质元素含量与分布，包裹体矿物组成（特别是E型包裹体）和化学组成，同位素组成等开展原位 (in-situ)分析，进一步示踪沅水流域大金刚石生长过程中地幔深部物质组成特征及演化，限定金刚石生长过程温度、压力和氧逸度等物理化学参数的变化，进而探讨沅水流域金刚石形成与扬子克拉通地幔深部地质过程及壳-幔碳循环事件的内在成因联系，为湖南寻找原生金刚石矿提供科学依据。

来自岩石圈地幔的大金刚石(MCDs)是记录岩石圈地幔演化过程及深部碳循环独特的“探针”。本项目通过对贵州镇远新发现钾镁煌斑岩及贵州215颗原生及次生金刚石、湖南297颗、澳大利亚阿盖尔及津巴布韦200多颗金刚石进行激光定向切片、阴极发光CL、FTIR(Mapping)测试、结合电子探针、LA-(MC)-ICPMS及SIMS等原位分析测试研究，获得如下认识：.1）新发现菖蒲塘钾镁煌斑岩与 “东风一号”钾镁煌斑岩( D1)均属于岩脉（体）浅部相，其铬尖晶石捕虏晶具有高Cr(Cr#>0.6)、Ni和低Ti、Ga特征，显示该地铬尖晶石与受俯冲作用改造的地幔橄榄岩有关，为幔源捕掳晶，显示贵州地区寻找隐伏钾镁煌斑岩仍然有较好的前景；.2）贵州原生金刚石粒度普遍较小、破碎度大、近95%的样品表面有熔蚀现象，熔蚀坑、熔蚀孔洞、熔蚀沟槽等蚀像极为发育，双晶比例高，以极低氮含量Ⅱa型金刚石为主（约75%），而湖南现代砂矿金刚石样品颗粒完整，以ⅠaAB型为主（93%）； .3）苍蒲塘钾镁煌斑岩U-Pb 年龄跨度大，207Pb/206Pb(>1000 Ma)或206Pb/238U(<1000 Ma)年龄变化范围在1849±17 Ma 到451±7 Ma 之间,主要集中在855 Ma 左右，εHf(t)值变化在 -16.1~+ 6.5，有老至3498Ma的两阶段Hf模式年龄，暗示了岩石源区存在古太古代-古元古代地壳物质，晚古元古代到早新元古代扬子克拉通原生金刚石岩区存在多期次地壳再造热事件；.4）湖南沅水砂矿大金刚石(22颗)原位δ13C值变化在-22.36 ~ -2.41‰，氮含量变化在0.5 ~ 2697.4 at. ppm；δ13C值分布集中在-6.46 ~ -2.41‰。不同样品内部δ13C值和N组成具有不同的变化模式，部分颗粒可能经历了两个甚至更多有差异的源区生长过程，金刚石结晶过程中地幔流体/熔体碳同位素和氮的组成存在复杂的交换，源区存在来自壳源碳的循环。.5）贵州原生金刚石内新发现四方硫铁矿包裹体，属于亚稳定态矿物；湖南砂矿金刚石蓝晶石和柯石英等硅酸盐矿物包裹体精细氧同位素测试（澳大利亚国立大学），获得了金刚石原生蓝晶石和柯石英包裹体δ18O变化范围分别为6.63‰~7.50‰和7.32‰~7.98‰，有力证实了扬子克拉通岩石圈地幔存在古老板块的俯冲作用。