Abstract
Microinclusions of high-density fluids (HDFs) were studied in coated diamonds from the Udachnaya kimberlite pipe (Siberian craton, Russia). The presence of C-centers in the coats testifies to their formation shortly before kimberlite eruption, whereas the cores have much longer mantle residence in chemically different mantle substrates, i.e., peridotite-type (P-type) and eclogite-type (E-type). The carbon isotope composition indicates an isotopically homogeneous carbon source for coats and a heterogeneous source for cores. Microinclusions in the coats belong to two groups: high-Mg carbonatitic and low-Mg carbonatitic to silicic. A relationship was found between high-Mg carbonatitic HDFs and peridotitic host rocks and between low-Mg carbonatitic to silicic and eclogites. The composition of high-Mg carbonatitic HDFs with a “planed” trace-element pattern can evolve to low-Mg carbonatitic to silicic during percolation through different mantle rocks. The compositional variations of microinclusions in the coats reflect this evolution.
Highlights
Due to their specific physical and chemical properties, diamonds can carry primary deep mantle material which remains protected from subsequent alterations during ascent to the surface
The fibrous rims of coated diamonds from the Udachnaya kimberlite pipe represent the last generation of the mineral and are closely related in time to their host kimberlite eruption
The variations in the carbon isotope composition suggest that the coats formed from an isotopically homogeneous source, whereas the cores formed from a heterogeneous source
Summary
Due to their specific physical and chemical properties, diamonds can carry primary deep mantle material which remains protected from subsequent alterations during ascent to the surface. Mineral inclusions in monocrystalline diamonds indicate that diamond formation mainly occurs in peridotitic (P-type) and eclogitic (E-type) host rocks at P = 4–7 GPa and T = 950–1400 ◦ C [1,2,3]. Mantle minerals are very rare in fibrous (including cuboid, coated, and cloudy) diamonds, but they often contain sub-micrometer inclusions (microinclusions) of fluids/melts from which the diamonds grew [4,5,6]. Coated diamonds consist of two contrasting growth parts: a monocrystalline core and a fibrous coat which is often enriched with numerous microinclusions. Considerable variations in the content and aggregation state of nitrogen and carbon isotopic composition between cores and coats of these crystals indicate their different thermal histories and carbon sources [18,22,23,24,25,26].
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