PECULIARITIES OF METAMORPHIC DIAMONDS
DOI:
https://doi.org/10.30970/min.72.04Keywords:
diamond, morphology, carbon isotopic composition, nitrogen impurities, solid inclusions, ultrametamorphic rocksAbstract
For the first time a new type of diamond-bearing non-kimberlite rocks with microdiamonds was discovered in the Kokchetav massif of Kazakhstan – diamond microcrystals were found in rocks of ancient metamorphic complex and placers of different ages. Subsequently, similar microcrystals were found in the metamorphic rocks of the Dabie Shan massif (Central China), the Fjortoft Islands (Northern Norway), the Erzgebirge Massif (Germany) and the Bohemian Massif (Czech Republic), in metamorphic massifs of the Rhodopes (Greece and Bulgaria), the Western Alps (Italy) and the Eastern Alps (Slovenia), and in many other localities. The mineralogy of diamonds of the Kokchetav and the Erzgebirge massifs is studied in the most detail. We summarized original and literature data on the mineralogy of metamorphic diamonds. Information on the geology of their diamond-bearing rocks and their indicator minerals is briefly covered – distribution, age, geological position of ultrametamorphic rocks, indicators of their diamond- bearing capacity and minerals of diamond paragenesises. Peculiarities of metamorphic diamonds are characterized by often big content in rocks, their micron size and diversity of crystal shape, structural imperfection and normal growth mechanism of many crystals, dominant light isotopic composition of their carbon, high content of nitrogen impurities and their low type of aggregation, spectral types Ib–IaA, enrichment of impurities, the content of specific solid inclusions and the unusualness of mineral associations. There are many hypotheses about the formation of metamorphic diamond crystals, including their metastable growth. The hypothesis of their subduction origin in the mantle at rather high PT-parameters is considered to be the most probable. Kokchetav microdiamonds from metamorphic rocks of Kazakhstan and Samotkan microdiamonds from Neogene sands of the Middle-Dniprо area (Ukrainian Shield) are compared. According to the comparison indicators, Samotkan microdiamonds and Kokchetav microdiamonds differ markedly in crystal size and morphology, content of nitrogen impurities and degree of its aggregation, the ratio of spectral types, the ratio of helium isotopes 3He /4He and a set of mineral inclusions. Samotkan diamonds are larger, no skeletal forms have been identified among them, the range of their carbon isotope composition is wider, among them, there is quite a lot of nonnitrogen and low-nitrogen crystals, different spectral types of crystals represent them, and some of them contain mineral inclusions of peridotite association.
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