Classification peculiarities of mantle eclogites from Udachnaya-East pipe (Yakutia)

Pokhilenko L.N., Agashev A.M.

Sobolev Institute of Geology and Mineralogy, SB RAS, Novosibirsk, Russia

The 54 eclogites from Udachnaya kimberlite pipe were investigated. The 22 of them were studied using SEM. The wide variety of compositions of the rock-forming minerals (garnet and pyroxene) was established in scale of the same sample. According to conventional classification (Taylor, L.A., and Neal, C.R., 1989) the eclogites can be divided into three groups (АВС), mainly by MgO-content in rock-forming garnet and pyroxene (fig.1,2).

It is a view that the grading by garnet and pyroxene coincide. But it is happened not any time. The detailed study shows that garnet or/and pyroxene can demonstrate more than one composition within the limits of the section. It makes difficult the determination of group for given rock. Primary pyroxene (normally omphacite) is represented by relicts of greenish grains, so its identification is not hard. Secondary pyroxene (usually diopside) can be present either as small grains in the cracks of primary pyroxene or simplectites with feldspar in the rims around primary pyroxene. (fig.3a,b).

So, we will have the discrepancy between “garnet” and “pyroxene” group grading if the primary pyroxene is completely replace by secondary one. It is not simple sometimes to establish the primary garnet. Fig.4 shows two relicts of garnet grains located on either side of the elongate kyanite grain:

They have close size, morphology, wide rim around. Here we have analyzed the big grain of phlogopite with diopside (group A) inclusion in it. It is secondary pyroxene (in equilibrium with ga2) because the ga1 inclusion was found in kyanite grain definitely proving the equilibrium between two last minerals.

The whole rock composition was defined for 17 studied samples (fig. 5). The data for rocks containing garnet or/and pyroxene of group С were included in this group. There is a positive correlation between MgO and Fe2O3+CaO (arrow on the diagram). А and В groups can be symbolically separate by the arrow showing the negative correlation between MgO and Fe2O3+CaO for these groups. Diamondiferous eclogites are marked by diamonds.

So, the classification of eclogites supposes first of all the identification of primary rock-forming garnet and pyroxene from the late similarly named minerals which were forming during the post-crystallization period of the rock origin under the influence of methasomatic fluids/melts.

Taylor, L.A., and Neal, C.R., 1989. Eclogites with oceanic crustal and mantle signatures from the Bellsbank kimberlite, South Africa, part I: Mineralogy, Petrography, and whole rock chemistry. J. Geol. 97, 551-567.