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

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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.

Mineral inclusions of iron ores of the Bakchar deposit (Western Siberia)

Asochakova E.M.

Tomsk State University, Tomsk, Russia

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The composition of the oolite iron ores from the Polynyanka site of the Bakchar deposit (Western Siberia) has been studied by the energy dispersion analysis combined with the scanning electron microscopy in the Analytical Centre of the natural systems geochemistry of Tomsk State University (analyst Ph.D. O.V. Bukharova).

The Bakchar deposit is confined to the Upper Cretaceous and Paleogene deposits overlapped by a rather thick Neogene-Quaternary rock body (160-200 m). Iron ores are related with several horizons: Narymskian, Kolpashevskian, Tymskian and Bakcharskian. The depth of productive strata varies from 2 to 40 m. The iron ore horizons are traced throughout the entire area of the deposit, as well as outside its borders and are separated by the barren and weakly ferruginous rocks that overlap each other often with washouts.

When studying the ore horizon of the Polynyanka site, we have distinguished the iron ore types that differ in their structural-textural characteristics, mineral compositions of the cementing mass, as well as in the specificity of their position within a section. Three essential ore types are distinguished in the section structure. Related to the first type are gothite-hydrogothitic (oolitic) ores representing cemented or loose sediments of brownish colour. The second ore type is glauconite-chloritic; these ores are distinguished for their dense, weakly cemented structure and the greenish-gray colour. The third transitional ore type bears the signs of both gothite-hydrogothitic and glauconite-chloritic formations.

Oolites as chief concentrators of iron hydroxides are the principal components of the Bakchar's iron ores. Oolites have always attracted the attention of scientists, because these are unusual spherical, ellipsoidal or similar mineral aggregates of conic-lamellar (shelly) structure 2 mm across. Formations similar to oolites, but of size over 2 mm, are called pisolites. Besides oolites and pisolites, ooids are recovered from the sedimentary iron ores. Those latter are mineral aggregates of a spherical or ellipsoidal form, sized from a fraction of mm to 2 mm, without signs of inner structurization, i.e. containing no nuclei.

Oolites of the Bakchar ores diversify in a form and colour. They are mainly spheroidal, oblate and angular by form; by colour they are black sniny, brown (fallow) shiny and dull. Sometimes, oolites are of irregular form, angular, flattened with a uneven surface, dull at times. In gothite-hydrogothitic ores, oolites are often oxidized and coloured brownish-rusty; in glauconite-chloritic ores, oolites are shiny, black, of regular shape. In contrast to greenish glauconite-chloritic ores and transitional varieties, in gothite-hydrogothitic ores there are often oolite fragments and clasts.

The REM-data indicate that oolites have often complicated, concentrically zonal texture with two pronounced zones: central and peripheral. The central part (100-200 µm in diameter) may be represented by waste minerals: quartz, magnetite, more rarely common potash feldspar; otherwise, they may have the nonuniform fabric resembling an ooid. The ooid often contains inclusions of the same waste minerals less than 50 µm in size. The peripheral oolite zone (20-100 µm thick) represents the successive concentric alternation of layers. The alternation of oolite concentres is so thin that their chemical composition is presented by mineral mixtures, the most part of which is composed of gothite and hydrogothite. The zonality of concentres is of the inversion character; the uniform alternation of the Σ Fe₂O₃ content in concentres is marked. Along with gothite and hydrogothite, if judged from the spectrum elements set, there are kaolinite, hydromicas, chlorites and phosphates in these ultrafine concentres.

The studies of mineral inclusions (~1μm) in oolite ores have demonstrated the presence of sulfids, free silver, zircon, ilmenite, rutil and rare-earth minerals. By the location and isolation character, all mineral inclusions can be classed as syngenetic (sulfids, free silver and rare-earth minerals) and epigenetic (zircon, ilmenite, rutil). The epigenetic minerals are commonly found in the terrigenous part of oolite ores together with quartz fragments. The syngenetic minerals mainly encountered in oolite aggregates and more rarely in the dominant bulk of glauconite-chloritic ores. Three associations are distinguished among these minerals: sulfide, phosphate and sulfide-phosphate.

The sulfide association comprises microinclusions in the contact of fragmented minerals and concentres of gothite-hydrogothite and leptochlorite composition. Pyrite is the essential sulfide mineral in oolite. Two generations of pyrite are distinguished: framboidal and euhedral. The pyrite framboids are composed of crystals > 1 μm in size. The euhedral pyrite is composed of aggregates in the form of several octahedral crystals 5-10 μm in size. Most likely, the growth of euhedral crystals is accounted for by the framboid enlarging. Trace elements As, Au and Pt are characteristic for pyrite.

Among other sulfides, sphalerite, covellite and antimonite have been revealed in the oolite aggregates. Covellite forms impregnations in oolites and the main ore mass. Sphalerite has been found in the nuclear part of oolites as colloform aggregates 20-50 μm in size. Antimonite has also been found within the oolite nuclei as microinclusions.

The phosphates are mainly represented by the association of REE minerals. In composition, they are solely Ce (Ce₂O₃ up to 27.63%), other rare earths identified La (La₂O₃ to 12.79%) and Nd (Nd₂O₃ up to 9.82%). In the rare-earth phosphate oolite occur very often in concentration and in the central parts, the number of micro-inclusions, the size of ~ 1 mm in one oolite may not exceed 10. Among the impurities observed Ca, Fe, Si, Al.

The sulfide-phosphate association presented by REE phosphates, sulfides, silver and free silver. The free silver is in the form of point or hair-like inclusions in the concentration oolites, localized along the concentric layers. In the central parts of the oolites silver forms a thin non-uniform impregnation, the number of inclusions in an oolite is rarely more than five points, less than 1 micron. In the ore horizon of the underlying mudstones are found the free silver dendrites (dimensions 250 × 300 mm), coated with silver sulfide (acanthite).

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