2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

The geochemical features of the peridotite garnets from  Udachnaya pipe (Yakutya)  

Pokhilenko L.N., Malkovets V.G., Agashev A.M.

V.S.Sobolev Institute of geology and mineralogy SB RAS, Novosibirsk, Russia

lu@igm.nsc.ru

Study of mantle xenoliths from the kimberlite pipes allows to get an important information about the origin and transformations of the lithosphere substance. The most deep-seated rocks of the upper mantle are the deformed peridotites, having the features of recrystallization and structures of the flow. Under the effect of asthenosphere meltings penetrating by the cracks and faults these rocks sometimes undergo significant transformations and enrich with different components.  (Agashev et al.., 2008). The most depleated rocks wich are not effected by the enrich processes are supposed to be the megacrystalline harzburgite-dunites (Pokhilenko et al., 1993).

The garnets from 5 deformed lherzolites (mineral composition: olivine, orthopyroxene, clinopyroxene, garnet), deformed harzburgite (mineral composition: olivine, orthopyroxene, garnet) and 12 megacrystalline dunites (mineral composition: olivine, garnet, sometimes chromite) from the kimberlite pipe Udachnaya (Yakutya) were analyzed by the technique of mass-spectrometry with linked plasma by laser ablation (LA-ICP-MS). The data are given in fig. 1-3.

The spidergrams of rare earth elements distribution of garnets from the deformed lherzolites are mostly characterized by the smooth increasing of LREE values with gradation in the field of HREE. (La_N/Yb_N = 0.01-0.12). The garnet of deformed lherzolite UV23/10 demonstrates sinusoidal form of distribution REE line with insignificant Pr-Nd peak, Gd minimum and gradual rising till Lu. (fig. 1). Only the lightest  REE values of garnet UV23/10 (La, Ce, Pr) are in the field of the deformed peridotites. The other values are in the dunite field where from Ho they coinside with these for dimondiferrous dunite LUV 833 (fig. 2).

The level of concentration and distribution type of REE in the garnet of deformed harzburgite UV207/09 have the features of both deformed lherzolites and dunites. The distribution curve after smooth rise till Sm, Eu, Gd goes down to Yb (fig. 1). The REE curve for the garnet from dunite UV121/10 demonstrates exactly the same distribution (fig. 2).

The distribution lines of REE in garnets of megacrystalline dunites have highly sinusoidal character (Nd_N/Dy_N sometimes gets 24). We can see it in fig. 2-3. The very big difference in values is seen in the width of dunite field and shows the different degree and time of their enrichment. The sample UV90/09 demonstrates almost flat surface without max and min (fig. 2) after the insignificant Ce enrichment. The garnet of dimondiferrous dunite LUV 833 contains the higher Ce concentrations, but the concentrations of the other REE from Sm till Er only slightly higher of those in chondrite. It is shown in fig. 2-3. On the contrary the samples UV75/09, UV836/09 have the rations max/min: Ce_N/Er_N = 65, Ce_N/Dy_N = 51, correspondently, wich is caused by the high fractionation of REE in their garnets.

The most of distribution curves for garnets of megacrystalline dunites have the peaks at the very beginning showing increased Ce, Pr, Nd concentrations proving early enrichment. The most enriched one is the garnet from UV7/09 (Pr_N > 10, Tm_N > 1), the garnet from UV836/09 left to be the most depleated (Ce_N > 1, Dy_N < 0.10) (fig. 3).

The sharp sinusoidality of REE distribution lines, early enrichment with the lightest REE prove that garnets of studied dunites were not in the equilibrium with the melt. The general the garnets like that coul be formed as a result of transformation of earlier chromites, which is confirmed by the increased Cr-concentration in them (for example, garnet UV167/09 contains 11.84 wt.% Cr₂O₃), either as a result of exsolution of primary Al, Cr-containing orthopyroxene, presenting in garzburgites.

References:

Agashev A.M., Pokhilenko N.P., Cherepanova Yu.V., Golovin A.V. Geochemical evolution of the base lithosphere mantle rocks on the results of study of the xenoliths of deformed peridotites from kimberlite Udachnaya pipe // Dokl. RAN. - 2010. - V. 432. - № 4. - P. 510-513

Pokhilenko N.P., Sobolev N.V., Boyd F.R., Pearson G.D., Shimizu N. The megacrystalline pyrope peridotites in the lithosphere of Siberian Platform: mineralogy, geochemical features and the problem of origin // Geol. and Geoph. - 1993. - V. 34. - № 1. - P. 71-84.