New aspects of kimberlite-related metasomatism

围绕金伯利岩的岩石学问题是地幔岩石学的前沿问题。我们还不清楚金伯利岩的成因，以及它们在侵位前在地幔和浅地壳中的熔体成分。我将探讨交代作用在以下两个方面的作用：1)使地幔适应金伯利岩熔体的诞生，2)控制地壳中金伯利岩的矿物学和结构。金伯利岩熔体与周围地幔不平衡，金伯利岩与地幔之间神秘的缺失环节被认为是地幔交代作用形成的。我们将研究与碱性-超镁铁质地块碳酸盐岩接触的巨晶和辉长岩，作为交代地幔与金伯利岩熔体之间地质关系的代用品，在200公里深处无法直接观测。对交代剂的独立约束将来自作为钻石包裹体捕获的流体，这些流体被捕获在纤维状和八面体生长的宝石级钻石中，并由交代地幔矿物的地球化学推断。第二类研究涉及硅质地壳岩石包体与寄主金伯利岩之间的交代反应。金伯利岩中的地壳污染可达金伯利岩体积的一半以上，严重影响钻石品位和该矿的经济效益。地壳污染导致硅质包体的熔融杂化和交代交代，包体周围有微型夕卡岩反应带。我将研究这一过程中元素的交代转移以及随后的金伯利岩重结晶和织构发展。拟议的研究将以详细的岩石学、地球化学和矿物学研究以及地球化学模型和岩石学实验为基础。该项目在许多方面都是创新的。它的基础是以前不为人知的巨晶和钻石产状、钻石裂隙中地幔流体的新圈闭地点、金伯利岩母岩的新地块类似物以及关于火山碎屑金伯利岩成因的新观点。该研究对基础岩石学和实用地质学具有重要意义。地幔交代作用的研究将有助于解决地幔富碳酸盐原生熔体的成因、组成及其交代地幔源区的组成等突出问题。该提案的实际意义涉及钻石矿石稀释的量化和对影响矿石开采的混合金伯利岩矿物的认识。如果我们找到金伯利岩结构、矿石贫化和围岩结构之间的因果联系，我们就可以简单地根据地壳地质来预测金刚石矿床的许多主要特征。

Petrological problems surrounding kimberlites are on the forefront of mantle petrology. We do not yet understand the genesis of kimberlites, as well as their melt compositions in the mantle and in the shallow crust before their emplacement. I will explore the role of metasomatism in 1) pre-conditioning the mantle to the birth of kimberlite melts, and in 2) controlling the mineralogy and textures of kimberlites in the crust. Kimberlite melts are not equilibrated with the ambient mantle, and the enigmatic missing link between kimberlites and the mantle is believed to have formed through mantle metasomatism. We will study megacrysts and fenitized olivinites in contact with carbonatites of alkaline-ultramafic massifs as proxies for the geologic relationships between the metasomatized mantle and kimberlite melts, unavailable for the direct observation at depths >200 km. Independent constraints on metasomatic agents will be obtained from fluids trapped as diamond inclusions in fibrous and octahedrally-grown, gem-quality diamonds and deduced from geochemistry of metasomatic mantle minerals. The second line of research deals with metasomatic reactions between xenoliths of silicic crustal rocks and host kimberlites. Crustal contamination in kimberlites can amount to more than half of the kimberlite volume, significantly impacting the diamond grade and the economics of the mine. Crustal contamination leads to melt hybridization and metasomatic replacement of silicic xenoliths, with mini-skarn reaction zones around the xenoliths. I will investigate the metasomatic transfer of elements in this process and subsequent kimberlite recrystallization and texture development. The proposed research will be based on detailed petrographic, geochemical and mineralogical studies as well as geochemical modelling and petrological experiments. The project is innovative in many ways. It is based on previously uncharacterized occurrences of megacrysts and diamonds, novel trap sites for mantle fluids in diamond fractures, novel massif analogues of kimberlite parent rocks and novel ideas about the genesis of pyroclastic kimberlites. The proposed research will be significant for the fundamental petrology and practical geology. The study of mantle metasomatism will contribute to solving the outstanding problem of the origin of carbonate-rich primary melts in the mantle, their composition, and the composition of their metasomatized mantle source. The practical significance of the proposal relates to the quantification of the diamond ore dilution and recognition of hybrid kimberlite minerals impacting the ore extraction. If we find the causal link between the kimberlite textures, the ore dilution and the country rock architecture, we will be able to predict many major characteristics of diamond deposits simply on the basis of crustal geology.