Udachnaya-East kimberlite pipe

Surgutanova E. A.

V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia

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Investigation of the samples of deep-seated rocks such as xenoliths of deformed peridotites found in kimberlites has a great significance for mantle petrology.

Most of the previously performed investigations concerned with secondary altered and serpentinized xenoliths. But a reliable analysis of wall-rock can't be obtained in this way. In my study collection of unaltered samples xenoliths of deformed peridotites from the Eastern Udachnaya kimberlite pipe were used. This pipe is a diamond deposit in the Daldyn-Alakit kimberlite field of the Yakutian kimberlite province. Since samples selected for the study are uniquely fresh, and therefore the objective data about the composition of these rocks can be provided.

Deformed peridotites are consists of pyroxenes, garnet and olivines porphyroclasts set up in the matrix of fine-grained olivine neoblasts of the second generation. It is believed, that textures deformed peridotites are occurred through a process of increasing deformation, dynamic recrystallisation and grain size reduction from a coarse-grained protolith. The temperatures and pressures of the last equilibrium of mineral associations in xenoliths estimated using a geobarometer [2] are 1260–1400°C and 56–70 kbar. Judging from the high PT -parameters of equilibrium of deformed peridotites, they are located in the lithosphere–asthenosphere transition zone [1].

Samples of wall-rock were analyzed for major and minor elements. To detect the concentrations of trace element in minerals, method of mass spectrometry with inductively coupled plasma with laser ablation (LA ICP-MS) were used. Concentrations of the major elements were determined by microprobe analysis of minerals. Geochemical analyses of Cpx shows decreasing relative abundance of each REE from La to Lu (Fig. 1, a). The pattern of the spectra of Срх corresponds to lherzolites [4]. From geochemical analysis of Gar can be concluded subdivision of Gar into two groups. First have a sinusoidal pattern of spectrum (Fig. 1, b), which corresponds to harzburgites and inclusions in diamonds. This Gar are not in eqilibrium with Cрх. The dominant REE pattern shown by garnets from second group is an HREE-enriched plateau pattern (Fig. 1, c). This Gar are in eqilibrium with Cрх, as it corresponds to lherzolites.

The compositions wall-rock samples were calculated using a modal-mineral chemical composition of rocks and geochemical composition of garnets and clinopyroxenes, which concentrate the largest number of incompatible elements.

Then, measured geochemical compositions of deformed peridotites were compared to calculated ones. Spectra of concentrations of rare and rare-earth elements normalized on the composition of chondrite for each sample were plotted to compare results produced by both methods (Fig. 1, d). According to the chart discrepancies are observed in LREE concentrations, minerals are depleted in incompatible elements, but rock in bulk is saturated. Therefore, incompatible elements can be concentrated in submicron interstitial phases such as apatite, perovskite, phlogopite, carbonates. Presence of these phases indicates a cryptic mantle metasomatism process which doesn't appear in the modal mineralogy of the studied samples.

References:

1.Boyd et al. (1997) Composition of the Siberian cratonic mantle: evidence from Udachnaya peridotite xenoliths, Contrib Mineral Petrol, 228-246 pp

2.Brey G.P and Kohler T. (1990) Geothermobarometry in Four-phase Lherzolites II. New Thermobarometers, and Practical Assessment of Existing Thermobarometers, Journal of petrology, Vol. 31, Part 6, 1353-1378 pp

3.Agashev A.M., Pokhilenko N.P. et al. (2010) Geochemical Evolution of Rocks at the Base of the Lithospheric Mantle: Evidence from Study of Xenoliths of Deformed Peridotites from Kimberlite of the Udachnaya Pipe, Doklady Earth Sciences, Vol. 432, Part 2, 510-513 pp

4.Sobolev N.V. (1974) The deep seated inclusions in kimberlites and the problem of the upper mantle composition, Nauk Publish, Novosibirsk

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