2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Melt inclusions in mantle xenolith from kimberlite — first direct evidence for existence of the deepest mantle-derived carbonatite melts.

Golovin A.V., Sharygin I.S., Korsakov A.V.

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

e-mail: avg@igm.nsc.ru

In this paper we present the results of the study of melt inclusions in minerals from the one of the deepest mantle xenoliths — sheared garnet-bearing lherzolites from unique ultra-fresh kimberlite from Udachnaya East kimberlite pipe.  P-T conditions of the origin of these xenoliths estimated as high as 6.0-7.3 GPa and 1230-1370 ^оC. Secondary melt inclusions (up to 100 microns in size), consisting of bubbles and fine-grained aggregate of carbonates, sulphates and chlorides, as well as transparent daughter phases and ore minerals, were identified in sealed cracks in rock-forming olivine and clinopyroxene (Fig. 1).

Mineral assemblages and the composition of the minerals were investigated by means of Raman spectroscopy, Scanning Electron Microscopy and Microprobe  analyses. Northupite, shortite, nyerereite, Na-Mg-carbonates, Ba-Na-Sr-carbonates, dolomite, aragonite, burkeite, aphtitalite, halite, silvine, chlorine-magnezite, pyrrhotite, pentlandite, djerfisherite, K-Fe sulfid, apatite, perovskite, chromite, magnetite, ilmenite, phlogopite, tetraferriphlogopite, olivine, diopside, sodalite were identified among daurhter phases within the inclusions. Homogenisation tempereature is as high as 800 ^оC. This temperature estimates is the lowest temperature of the melt-mantle xenolith interaction. The melt inclusions represent the relicts of this melt. It is worth noted that aragonite is the CaCO₃ polymorphs in the melt inclusions. The Raman bands 153, 181, 208, 704, 1086, 1464 cm^-1 are characteristic bands of aragonite. Experimental results and observation on natural high-pressure mineral assemblages confirm that aragonite is reliable geobarometer. The aragonite-calcite transformation allow to define the minimum pressure formation of the inclusions as high as 2 GPa at 800C. Presence of dolomite within the inclusions may indicate higher pressure values about 3 GPa based on reaction Mg-Calcite = Aragonite + Dolomite (Fig. 2a). Detail Raman imaging of 10 melt inclusions reveals that carbonates occupy about 50 vol.%. Furthermore alkali carbonates are predominant carbonates, while aragonite occurs quite rare. Thus, the composition of the melt, which interact with xenoliths, was alkali carbonatite melt. Findings of aragonite and dolomite in the melt inclusions indicate that this interaction occur at depth >90 km (mantle conditions) (Fig. 2b). Up to now the kimberlite considered as ultramafic mantle-derived rocks, and usually chemical composition of kimberlite melts reconstructed based on bulk chemistry of kimberlite rocks. SiO₂, MgO, CaO and H₂O and CO₂ are the major chemical components (up to 90% in total), which define the evolution trend of crystallization of kimberlite worldwide. Concentration of SiO₂ + MgO in kimberlites are ≥ 55 wt.%. Serpentine or olivine (in less altered varieties) and carbonates are main rock-forming minerals. Generally olivine/serpentine content exceeds 50 vol.%, thus presence of serpentine or olivine, which are concentrators of SiO₂ and MgO in kimberlites define their ultramafic nature.

Previous investigation of melt inclusions, which have alkali-carbonatite composition, in olivine from kimberlites (Kamenetsky et al., 2004; Golovin et al., 2007) and zonation pattern of olivine microphenocryst from kimberlites worldwide make questionable existing paradigm on ultramafic nature of kimberlite magmas. The study of internal morphology and variation of compositions of microphenocrysts of olivine from unaltered kimberlites reveals that predominant part of olivine cores are xenocrysts and they represent restites of mantle xenolith (Kamenetsky et al., 2008; Brett et al., 2009). Brett et al. (2009) suggested that up to 95 % of olivine microphenocrysts in kimberlites are indeed relics of mantle rocks and only about 5% of olivine grew from kimberlite melt. But this hypothesis is only under development and it requires reliable statistics confirmation.

Results of the melt inclusions study of secondary melt inclusions in minerals from mantle xenoliths display a lot of similarity with previous results on melt inclusions in olivine from kimberlite from Udachnaya East kimberlite pipe (Kamenetsky et al., 2004; Golovin et al., 2007). Furthermore, chlorides, alkali-carbonates and sulphides were found in ground mass of kimberlites. Concentration of alkalies and Cl in some samples may exceed 8 and 6 wt.%, respectively (Kamenetsky et al., 2007). All this information indicates that there are genetic relations between melts, which interact and sheared lerzolithes, and kimberlite magmatism. We believes that the presence of melt inclusions in the sheared lerzolithes is the first direct evidence that at least on the early stage kimberlite melts had alkali-carbonatite compositions. Recently, there has been increasing evidence that carbonatite melts/fluids play important role in diamond origin (Zedgenizov et al., 2007; Araujo et al., 2009). If the protokimberlite melts indeed were alkali-carbonatite melts this rise question that at least some diamond may have close relation with kimberlite magmatism (e.g. Araujo et al., 2009). Findings of the melt inclusions in mineral form one of the deepest mantle-derived xenoliths confirm the possibility of generation alkali-carbonatite melts at lithosphere-asthenosphere boundary.

This study was financially supported by RFBR grant № 10-05-00575-a.

References:

Golovin, A.V., Sharygin,V.V., Pokhilenko, N.P. Melt inclusions in olivine phenocrysts in unaltered kimberlites from the Udachnaya-East pipe, Yakutia: some aspects of kimberlite magma evolution during late crystallization stages // Petrology. 2007. v. 15. P. 168-183.

Kamenetsky M.B., Sobolev A.V., Kamenetsky V.S., Maas R., Danyushevsky L.V., Thomas R., Pokhilenko N.P., Sobolev N.V. Kimberlite melts rich in alkali chlorides and carbonates: A potent metasomatic agent in the mantle // Geology. 2004. v. 32. No. 10.  P. 845–848.

Kamenetsky V.S., Kamenetsky M.B., Sobolev A.V., Golovin A.V., Demouchy S., Faure K., Sharygin, V.V., Kuzmin, D.V. Olivine in the Udachnaya-East kimberlite (Yakutia, Russia): types, compositions and origins // J. Petrology. 2008. v. 49. P. 823–839.

Brett R.C., Russell J.K., Moss S. Origin of olivine in kimberlite: Phenocryst or impostor? // Lithos. 2009. v. 112S. P. 201-212.

Kamenetsky V.S., Kamenetsky M.B., Sharygin V.V., Golovin A.V. Carbonate-chloride enrichment in fresh kimberlites of the Udachnaya-East pipe, Siberia: a clue to physical properties of kimberlite magmas? // Geophys. Res. Lett.. 2007. v. 34. P. 9316-9321.

Zedgenizov D.A., Ragozin A.L., Shatsky V.S. Chloride-carbonate fluid in diamonds from the eclogite xenolith //

Doklady Earth Science. 2007. V. 415. I. 6. P. 961-964.

Araujo D.P., Griffin W.L., O'Reilly S.Y., Grant K.J., Ireland T., Holden P., van Achterbergh E. Microinclusions in monocrystalline octahedral diamonds and coated diamonds from Diavik, Slave Craton: Clues to diamond genesis // Lithos. 2009. V. 112S. P. 724-735.