Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Particularities of the reaction rims around garnets from the mantle xenoliths of different parageneses (by the example of basic and ultrabasic rocks from Udachnaya kimberlite pipe, Yakutia)

Pokhilenko L.N.

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

lu@igm.ru

Kelyphitic rims are a variety of secondary reaction rims around the pyrope grains in the garnet peridotites. They have a concentric-zonal structure and contain a number of minerals with hydroxyl group (amphibole, chlorite, serpentine, phlogopite) besides rhombic pyroxene and spinel; this is evidence that pyrope kelyphitisation has occurred in the melt with elevated amount of volatile components (Geological…, 1978). This paper considers the rims from the xenoliths of different mantle paragenesis. Olivine, sodalite, apatite, barite calcite (or aragonite) and sulfides have been found in the rims besides typical minerals (see Table).

Table.  Parageneses of mantle xenoliths from the Udachnaya kimberlite pipe. Mineral composition of the rims around the garnet. Amph – amphibole, ap – apatite, cat – calcite, cpx – clinopyroxene, fsp – feldspar, ol – olivine, opx – orthopyroxene, sp – spinelide, su – sulfide.

The rims thickness varies from the first microns to 400 microns. Zonal structure was not observed in the garnet rims of eclogite UV122/10, ilmenite ultrabasites UV162/09 and UV239/10. Gradual or abrupt enlargement of rims minerals in the direction from the garnet grain to the rim periphery is observed in the rest samples. The change of color and mineral composition from zone to zone mentioned by several researchers (Obata, 1980, Bobrov, 1997) is not observed. The rims commonly have phlogopite margins; phlogopite from the inner zones of the rim differs in morphology and composition from that of the margin. Probably K was repeatedly added into the system.

Pyroxenes from the rims around the garnets are enriched in alumina as compared to the rock forming pyroxenes resulting in enhanced contents of Ca-Chermakite minal (CaAl^VIAl^IVSiO₆) in the clinopyroxene and Mg-Chermakite minal (MgAl^VIAl^IVSiO₆) in the orthopyroxene (Fig. 1). Essential elevation of the iron content was observed for three clinopyroxenes and all orthopyroxenes from the rims. Both high alumina content and CaO entering into the orthopyroxene composition point to the high temperature of the formation of these pyroxenes.

Average compositions of cpx-opx of the garnet rims have been plotted on the scheme Ca in cpx – Ca in opx proposed by G.P. Brey for lherzolite paragenesis (Brey, 1991). Pyroxenes UV162/09, UV46/92, UV831/09 demonstrate non-equilibrium (disequilibrium); pyroxenes UV33/10, UV122/10 fall into the area of low pressures (1-1.5 GPa); pyroxene pair UV92/03 – in 4GPa; pyroxene of harzburgite parageneses UV207/10, UV834/09 -  in 5Gpa. All pyroxene pairs are in the high temperature part of the diagram.

Spinelides from the rims demonstrate lesser Cr content and slightly greater iron content as compared to the rock forming spinelides; a part of them have compositions similar to that of the spinels from spinel lherzolites and pyroxenites, the rest contain negligible chromium minal admixture and spinel from the eclogite garnet rim is a pleonaste with minor magnetite content. Spinelides from the garnet rim of the deformed lherzolite UV23/10 demonstrate large compositional variations: the grain composition has divided into chromite-pleonaste and magnetite. According to Sack and Ghiorso (1991) diagram the initial composition has been stable above 600^oC.

The olivine differing from the rock forming olivine by its higher iron content has been found among the minerals of garnet rims of two samples: UV831/09 Fo_Ol 90 and Fo_Ol-rim 87.8, UV162/09 Fo_Ol 88.2 and Fo_Ol-rim 84.7. Amphiboles among the minerals from the rims have not been found in every sample under study. They are mostly of pargasite type; tremolite has been found in UV239/10 and K-richterite – in UV831/09 (Fig.2). According to the experimental data (Konzett et al., 1997) K-amphiboles are stable at 10GPa and 1450^oC. This suggests a deep formation of garnet rims.

 Phlogopite occurs in the rims of all studied samples. Wide compositional variations in FeO, MgO, Al₂O₃ and TiO₂ have been found for phlogopites UV831/09, UV239/10, UV162/09, and to a lesser extent for the other samples. As a rule, the feldspars from the rims correspond in composition to orthoclase (UV23/10, UV92/03, UV46/92) sometimes with essential albite admixture. All of them fall into the high-temperature region of compositions miscibility on the diagram Ab-An-Or. Plagioclase (UV92/03) has been found in one case. Feldspars from the eclogite UV122/10 rim fall into the region of compositions immiscibility and are probably thin K-feldspar and plagioclase intergrowths. The total chemical composition of kelyphitic aggregate corresponds to garnet stoichiometry, but as compared to the garnet the drop of SiO₂, Al₂O₃ content and increase of MgO content are observed and K₂O, Na₂O impurities appear.

Conclusions. The rims around the garnets of mantle parageneses form as a result of heating with the active participation of deep fluids of wide range of compositions. The formation of rims is often a multi-stage process. The mineral composition of the rim is defined not only by garnet paragenesis (a given number of the main components in the system) but by the quality of the acting fluid. The time during which the rock experiences aggressive action before the removal to the surface is also an important factor. Oxygen fugacity also plays an important role. Garnet kelyphitisation – reaction at the garnet-olivine ga+ol→sp+2px boundary – varies in several cases. Thus alkaline water phases – phlogopite and amphibole (UV23/10) form instead of pyroxenes (+rare phlogopite) at high fluid concentrations; cpx, Ca-amphibole tremolite (instead of pargasite) and apatite (UV239/10) form with the addition of CaO. Olivine found in the garnet rims of UV831/09 and UV162/09 can be a result of the reaction 2px+sp→2ol+pl; but in our case sodalite forms along with olivine instead of plagioclase in the high temperature conditions under the action of aggressive fluid.

The work is supported by the RFBR grant 12-05-01043.

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