Alkalic and carbonate components of kimberlites, - their origin

Kostrvitsky S.I.

Institute of Geochemistry, SB Russian Academy of Sciences, Irkutsk, Russia

Following the definition by the Canadian petrologist R. Mitchell (Mitchell, 1986) kimberlites include the variety of potassium ultramafic rocks enriched by volatiles (mainly СО₂). In the standard classification the foreign researchers divide kimberlites into two groups: 1) with low K₂O content, mainly of carbonate-olivine (serpentine) composition and 2) with high K₂O content, with the matrix including phlogopite and diopside. The basis to distinguish two groups of kimberlites (Smith, 1983) was their sharp difference in trace element and isotope compositions which predetermined different mantle sources (for the 1^st and 2^nd groups – weakly depleted and enriched sources, correspondingly).

Kimberlites of the Yakutian province are not included in this systematics. Despite wide variations of both chemical, and mineralogical composition including K₂O content. In terms of isotope composition in Sr-Nd system almost all kimberlites of the province can be regarded as kimberlites of the 1^st group.

Within the province it is conventional to distinguish the south diamond-bearing and north poor-diamond kimberlite fields. Specific features in the composition of kimberlites from the north fields as opposed to kimberlites from the south fields are found for such oxides as TiO₂, FeOtot, MnO, Na₂O, K₂O and P₂O₅. The increased ferruginosity, titanium content and alkalinity are distinctive features in the composition of kimberlite rocks from the majority of fields of the Yakutian province of both Mesozoic, and Paleozoic age.

Kimberlites, characterized by relatively high К₂O content occur in the south diamond-bearing fields as well. High-K petrochemical type involves diamond-rich kimberlites of the Nakyn field and poor-diamond micaceous kimberlites of Zagadochnaya and Bukovinskaya pipes. We found different correlations between the alkalinity and other characteristics of the composition for the south and north kimberlites. High-Mg kimberlites of the south field as compared with high-Fe varieties turned out to be more carbonatized and contain more K₂O. On the contrary, the kimberlites from the north fields demonstrate a direct dependence between the alkalinity and TiO₂ and FeOtot contents in kimberlites. We believe that the major K₂O source for diamond-bearing kimberlites (pipes of the Nakyn field, Dachnaya pipe of the Malobotuobinsk field) included the assimilated material of hosting sedimentary rocks. Moreover, the contamination of kimberlites of these pipes was accompanied by the carbonatization. The higher carbonate component in the kimberlite melt most likely contributed to disintegration of host rocks, their involvement into the kimberlite rock and the subsequent assimilation.

The abnormal high REE concentrations and isotope studies indicate that the alkalinity source for micaceous high-Mg kimberlites from pipes of the Daldyn field (Zagadochnaya and other pipes) was initially mantle.

Based on studies of the so-called non-altered kimberlite from the Udachnaya-east pipe number of scientists (Kamenetsky et al, 2004, 2008, 2009) arrived at the conclusion, that Na₂O significance for the origin of kimberlites was earlier underestimated. A "non-altered " kimberlite shows increased Na₂O content (from 0.5 up to 2-3 wt. %) owing to occurrence of such secondary minerals, as zemnokorite, halite, shortite. The superimposed surficial sedimentary-marine source of chlorine-containing mineralization is confirmed by halite occurrence from modern brines, observed in the mine of Udachnaya pipe.

Studies of isotope Sr and Nd systematics for kimberlites of north fields of the Anabar Region indicate that the lithosphere metasomatized mantle is responsible for alkaline composition of kimberlites. Studies of isotope Sr composition of phlogopite megacrysts from kimberlites of both north and south fields suggest the asthenosphere source, being the major mantle source for kimberlite too.

In order to understand the petrogenesis of kimberlites we have to solve the problem what amount of volatiles resulted from endogenous processes and what amount is due to superimposed processes. The carbonate component of kimberlites mainly includes calcite which together with serpentinite composes the kimberlite cryptocrystalline or microlite matrix, makes up pseuodomorphs in olivine, forms clusters, veins, separate sites of hydrothermal mineralization. The concentration of oxides (CaO+CO₂) in kimberlites varies from the first up to 80 % and more.

The major factor of kimberlite composition variability is the carbonatization. An inverse correlation of CaO, CO₂ with other rock-forming oxides is the most typical dependence for kimberlite rocks of the Yakutian province. The reverse correlation with carbonate components is the strongest for SiO₂ and MgO. There is also an inverse correlation of TiO₂, Al₂O₃, MnO, K₂O, P₂O₅, FeOtot with CO₂ though it is significantly weaker. The cluster analysis of petrochemical oxides made both for the whole set of data, and for analyses of separate pipe clusters divides the rock-forming oxides into three groups: 1) SiO₂ and MgO, 2) TiO₂, Al₂O₃, MnO, K₂O, P₂O₅, FeOtot, 3) CaO, CO₂. These groups correspond to the basic mineral phases of kimberlites 1) olivine-serpentine, 2) perovskite-picroilmenite, phlogopite, apatite; 3) calcite which are responsible for main trends of the kimberlite melt differentiation.

The composition of the surrounding host rocks significantly influences the total amount of carbonates in kimberlites. This fact is a strong argument in favor of sedimentary-marine source in the formation of carbonate component of kimberlites.

Kimberlites from veins, dykes and sills (bodies, which are united by the intrusion demonstrate abundance of carbonates of the mantle origin.

Isotope-geochemical studies indicate the occurrence of three calcite types: 1) kimberlite proper; 2) sedimentary and 3) hydrothermal, differing in Sr, Ва and REE concentrations. In terms of Sr concentration various calcite types are one order different.

A deep mantle origin of the certain part of carbonates, their role as major minerals-carriers and minerals-concentrators of incompatible elements, and role in formation and uplifting the kimberlite fluid from a mantle depth indicate that they are an obligatory part of kimberlite rocks. It also suggests that kimberlite physical- chemical system cannot be considered separate from its carbonate component. At the same time, we should not identify the carbonate component with carbonatites, which are different both in concentrations of incompatible elements and element ratio, different distribution of trace element composition.

References:

Kamenetsky M.B., Sobolev A.V., Kamenetsky V.S., et al. Kimberlite melts rich in alkali chlorides: A potent metasomatic agent in the mantle. Geology. 2004. V. 32. № 10. P. 845-848.

Kamenetsky V.S., Maas R., Kamenetsky M.B., Paton C., Phillips D., Golovin A. V., Gornova M. A. Chlorine from the mantle: Magmatic halides in the Udachnaya-East kimberlite, Siberia. Earth Planet. Sci. Lett. 2009. V. 285. P. 96-104.

Mitchell R.H. Kimberlites: mineralogy,geochemistry and petrology. New York, Plenum Press. 1986. 442 p.

Smith C.B. Pb, Sr and Nd isotopic evidence for sources of southern African cretaceous kimberlites. Nature. 1983. V. 304. N 5921. P. 51-54.

This study was financially supported by RFBR integration projects N 23.1 and 72.