Kyanite eclogite xenolith from Udachnaya pipe: whether there was coesite in the rock

Alifirova T.A., Pokhilenko L.N.

V.S. Sobolev Institute of Geology and Mineralogy sb ras, Novosibirsk, Russia

taa@igm.nsc.ru

Considering all the deep-seated xenoliths from kimberlite pipes worldwide, kyanite eclogites and grospydites take a special place of mantle rock types. A number of works were devoted to the investigation of these rocks found in kimberlites of Daldyn-Alakit region (e.g. [1, 2] and references therein). The presence of pressure-indicative minerals like coesite and diamond in the xenoliths is often described [2].

Mantle xenolith of garnet–clinopyroxene–kyanite rock (kyanite eclogite) from Udachnaya kimberlite (sample LUV134/10) has been studied by us in detail. The eclogite shows fine- to medium-grained mosaic texture: kyanite and garnet-1 grains of 0.5 to 1.5 mm in size are poikiloblastically included to the clinopyroxene matrix. The xenolith also contains symplectitic intergrowths of vermicular garnet-2 and polycrystalline quartz, and kyanite and pyroxene grains as well (see Fig.a). The large symplectites (up to 8 mm) look like porphyroblasts with subhedral pyroxene outlines. Despite the exceptional freshness of the sample, the precursor clinopyroxene (pyroxene-1) from the eclogite is nearly all replaced by tiny pyroxene-2 + plagioclase symplectites (Fig. b), preserving unreacted relics at the central parts of the pyroxene porphyroblasts and inclusions in kyanite and garnet.

Rock-forming minerals usually contain globular inclusions of each other, and rounded and/or subhedral SiO₂ inclusions observed are more often included into kyanite grains. The SiO₂ inclusions partially or near-covered seem to be monocrystalline quartz with no evident palisade textures or polycrystalline shell. Larger ones are frequently surrounded by medium to strong radial crack patterns (Fig. c), whereas smaller subhedral inclusions when unexposed demonstrate a weak birefringent halo. These small inclusions may be an indicator of their high-pressure origin [3].

Raman spectroscopic study reveal a common quartz band of 464 cm^-1 for partially or near-exposed SiO₂ inclusions in kyanite, and slight Raman frequency shift of quartz band to 466 cm^-1 in uncovered SiO₂ inclusions is observed. Polycrystalline quartz from porphyroblastic symplectites shows a clear 464 cm^-1 mode without any frequency shift.

Relatively large kyanite and garnet-1 grains contain numerous needles and platelets of rutile (Fig. d, e). Rutile lamellae are considered to be of exsolution origin according to their preferred orientation, near-uniform composition and distribution within host mineral. Clinopyroxene also contains rutile rods frequently resorbed and mantled by ilmenite (Fig. b). Rutile lamellae within kyanite and garnet-1 hosts are in some cases intergrown with transparent phases like pyroxene and SiO₂.

Kyanite with FeO about 0.3 wt % and TiO₂ up to 0.06 wt % is generally characterized by low exsolution proportions, no more than 0.3 vol.%. Combining the volume proportions of the host kyanite with exsolved rutile lamellae of known chemical composition and computed densities, the reconstructed kyanite major-element compositions were obtained. Precursor kyanite had the TiO₂ content as high as that described by Pearson & Shaw [4] in Al₂SiO₅ polymorphs and equal to about 0.28-0.34 wt %.

Major-element compositions of garnet-1 and garnet-2 cover a range of grossular, almandine and pyrope contents (Grs_54.8–58.3 Alm_22.4–25.1 Prp_14.5–17.4). According to the grossular-rich garnet composition (> 50 mol. %) with significant part of pyropic component, rock type considered may be named ‘grospydite’. Garnets have Na₂O content up to 0.15 wt % at central parts of large garnet-1 grains. Garnet compositions are typically characterized by Si content up to 3.02-3.04 atoms pfu. It was suggested to consider garnets with Si>3.03 atoms pfu as majoritic ones. Garnets with that Si content are stable at pressures more than 6 GPa.

Clinopyroxene (pyroxene-1) has the omphacitic composition with approximate proportions of Jd 46 mol. %, Di 33 mol. %, Hd 8 mol. %. Significant admixtures of Ca-Tschermak and Ca-Escola molecules are observed, in average 7 mol. % of Ca-Ts and 4 mol. % of Ca-Es. Ca-Escola component becomes stable at clinopyroxene at pressures more than 3 GPa.

Porphyroblastic symplectites made up of garnet-2 and quartz were possibly the result of complex decomposition of Ca-Escola molecule in initial pyroxene that was stable at pressures about 6 GPa and temperatures >1100 °C according to compositional reconstructions. The general reaction responsible for the symplectite formation is Ca-Es-Px + Di-Px → Grs-Grt + 3SiO₂ (Cs) taking place under decompression. The formation of porphyroblastic symplectites was decreased under the pressures about 3 GPa, and the next Px-2+Pl symplectites begun to form through the following reactions: Jd + Qtz → Ab and Ca-Ts + Qtz → An. Coesite stable at pressures >3 GPa was transformed into quartz polymorph now observed at the large symplectites and mineral inclusions.

A number of decompression-indicative textures are presented in the unusual kyanite eclogite LUV134/10 from Udachnaya pipe. Two types of symplectite after pyroxene point out to stepwise pressure decrease under mantle conditions long before the entrainment of kimberlitic melt. Ca-Es- and Ca-Ts-rich clinopyroxene became unstable during subsequent decompression from ~6 GPa to <3 GPa, whereas garnet preserved its majoritic features having exsolved only Ti-component in the form of rutile needles. Preservation of majoritic garnet may be explained if the reaction Mj → Grt + Px was buffered by SiO₂ and Al₂SiO₅. During the decompression (and cooling) of the rock Ti-bearing kyanite had exsolved rutile lamellae. With pressure decreasing to <3 GPa coesite was transformed into quartz partly retaining its high-pressure formation.

Fig. Textural features observed in the kyanite eclogite LUV134/10 are shown, where (a) – ‘porphyroblastic’ symplectite composed of garnet (Grt), quartz (Qtz) and kyanite (Ky) and surrounded by diopside (Di) pyroxene intergrown with plagioclase (Pl); (b) – resorbed rutile (Rt) rod with ilmenite (Ilm) in diopside+plagioclase symplectite; (c) – large partially covered quartz inclusion with strong radial crack pattern around; (d) and (e) – rutile exsolution lamellae within garnet and kyanite, respectively.

References:

1. Sobolev, N.V., Kuznetsova, I.K., and Zyuzin, N.I. (1968) The petrology of grospydite xenoliths from the Zagadochnaya kimberlite pipe in Yakutia // Journal of Petrology, V. 9, P. 253-280.

2. Spetsius, Z.V. (2004) Petrology of highly aluminous xenoliths from kimberlites of Yakutia // Lithos, V. 77, P. 525-538.

3. Korsakov, A.V., Perraki, M., Zhukov, V.P. et al. (2009) Is the quartz a potential indicator of ultrahigh-pressure metamorphism? Laser Raman spectroscopy of quartz inclusions in ultrahigh-pressure garnets // European Journal of Mineralogy, V. 21, P. 1313-1323.

4. Pearson, G.R., Shaw, D.M. (1960) Trace elements in kyanite, sillimanite and andalusite // American Mineralogist, V. 45, P. 808-817.