UNUSUAL INCLUSIONS IN DIAMONDS FROM THE ECLOGITE OF THE UDACHNAYA KIMBERLITE PIPE (YAKUTIA): NEW DATA ON THE NATURE OF MANTLE METASOMATISM

Among the different kinds of xenoliths in kimberlite pipes, the diamondiferous varieties are particular interesting. The studied eclogite xenolith is composed of omphacitic clinopyroxene and pyrope-almandine garnet grains. According to the composition of major minerals, this xenolith falls in between A and B eclogite groups [Taylor and Neal, 1989]. Diamond and sulfide are accessory minerals in this eclogite. We have emphasized the comparative study of inclusions in diamond and the primary minerals of the xenolith matrix. All extracted diamonds are represented by octahedral crystals, сolorless and brown, which contain the majority of inclusions. Coexisting garnet and omphacite inclusions in the same diamond hosts are entirely fresh. In the rim zone of omphacite inclusions, there is a sponge structure. The spongy margins are consisted of diopside-rich clinopyroxene and interstitial potassium feldspar. The formation of such structures around the omphacite inclusions can be explained by the reaction of K-rich liquid captured simultaneously with the pyroxene. In contrast, the garnet inclusions are free from any secondary alteration. Inclusions generally differ from matrix minerals by chemistry: clinopyroxene contains higher amount of Na₂O (6,0 - 6,27 wt.% in inclusions and 5,83 wt.% in matrix), Al₂O₃ (10,6 – 11,3 and 8,19 wt.%, respectively), K₂O (0,26 - 0,28 wt.% and 0,11 wt.%, respectively); garnet inclusions in diamonds have higher Ca-content (5,79 - 5, 83 wt.%) and low MgO content (12,1 – 12,6 wt.%) than garnets in matrix (3,71 wt.% and 17,1 wt.%, respectively). Diamonds contain fluid inclusions – liquid or the gas phases that was present during formation. The relative content of hydrocarbons and their derivatives by mass-spectrometric analysis ranges from 63 to 77%. The carbon dioxide content is 3.2-11.7%, and water content is 7.0-12.9%. Nitrogen is fixed in all diamonds both in molecular form and in the form of N-bearing derivatives of hydrocarbons (6.0-9.0%), as well as sulfur-containing compounds (2.8 - 6.0%). The difference in the composition of inclusions in diamonds and in the primary minerals of the xenolith suggests the formation of diamonds in a fluid or melt, which was not in equilibrium with the host rock.

The work is supported by RFBR (16-05-00841; 18-05-00761), RSF (14-17-00602P, 17-17-01154)