Unusual Inclusions And Genesis Of Microdiamonds From The Zelenyi Yar Neogene Placer (The Ukrainian Shield): A Tem Study

Abstract

16 individual diamond microcrystals from the Neogene placer of Zeleny Yar on the Ukrainian Shield have been studied using TEM methods. Placer sediments are fine-grained sands of coastal marine origin redeposited in a river valley. The placer heavy minerals include ilmenite, rutile, zircon, kyanite, sillimanite, staurolite, tourmaline, mantle and impact type diamonds. Undisputed minerals genetically related with mantle diamond were not found in the placer. The studied mantle diamonds have different morphologies: octahedron, octahedroid, octahedron-rhombic dodecahedron transition crystal, cube, cuboid, crystals of {100} + {111} + {110} combinational form, fragments of octahedra. The size of the crystals does not exceed 0.5 mm. They differ in color - from colorless to brown and gray, and growth mechanisms - tangential for octahedra and transitional crystals and fibrous for cubic crystals. Some optically clear diamond crystals contain distinct dark inclusions already visible under a binocular microscope. 21 TEM foils 15 × 10 × 0.150 µm in size each, prepared from typical crystals, have been studied. Two groups of diamonds are distinguished based on inclusions: 2 diamonds with sets of inclusions typical for diamonds from ophiolites and the remaining 13 crystals with sets of multiphase inclusions and structural defects typical for diamonds from kimberlites, lamproites and lamprophyres. Crystals of the first group are cube-octahedron and a fragment of an octahedron. Their inclusion are: Mn-olivine, MnNi alloy with minor Si content, MnNiO₃ oxide, Mn₅C₂ carbide, rhodonite (Mn(SiO₃) and amorphous silica. MnNiO₃ oxide is associated with silica and chlorine. Most of the second group crystals are characterized by curved dislocations, carbon platelets were found in several crystals and nitrogen platelets in one crystal. Inclusions in these diamonds are: dolomite, ilmenite, magnetite, apatite, mica, chlorite and orthopyroxene. One crystal contains a Fe-rich forsterite inclusion. They belong to the typical eclogite assemblages of mantle diamond from kimberlites. Crystals of this group experienced significant annealing. Possible original sources for crystals of the first group are proposed serpentenites, for second group - kimberlites or lamproites or lamprophyres.