Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Role of capture and contamination of clastic material of lithopsheric mantle in kimberlite emplacement

Коstrovitsky S.I., Esenkulova S.А., Yakovlev D.А., Kalashnikova Т.V.

 Vinogradov Institute of Geochemistry SB RAS, Irkutsk

 serkost@igc.irk.ru

The common definitions of kimberlite rocks primarily imply chemical and mineral compositions (Mitchell, 1986).  However, to understand the nature of kimberlites is not feasible without evaluating the contribution and extent of hybridization process in formation of these rocks; though, the hybrid nature of kimberlites is accepted by some leading researchers. Moreover, in the publications (Davidson, 1964), dedicated to the issues of rock origin, the kimberlites are referred to as “slurry”, i.e. hydro mix or cement solution. It is noteworthy that in describing the mineral composition of hypabyssal kimberlites Р. Mitchell pointed out, that they consist of olivine macrocrysts (~25 vol. %), olivine phenocrysts (~25 vol. %) submerged into carbonate-silicate mesostasis (Mitchell, 2008). Normally the term «macrocryst» is applied by the authors as neutral, when the origin of mineral remains uncertain. It is true, that macrocrysts of olivine (Ol) in kimberlites could appear due to crushing of peridotites of lithosphere mantle, and as a result of crystallization in the primary deep magmatic chamber, or in time of kimberlite melt-fluid ascending from mantle depths.  The xenogenous nature of olivine is proved by an angular clastic shape of grains, their micro fracturing, in some cases presence of two-peak histograms  of distribution of the first-rate component in Old and, finally, available xenoliths of peridotite proper. It is believed (Kostrovitsky 1986), that Mg-rich Ol (91-94% Fo) originated due to disintegration of high-Mg lithosphere mantle. And Ol with a more ferriferous composition (86-90% Fo) crystallized from the primary kimberlite melt. It is to note that olivine, as the mineral of megacryst low-Cr association, began to crystallize in the asthenosphere layer.  

The ratio of high-Mg and relatively low-Mg olivine in kimberlites of different pipes and even within one pipe, but in different structure-textural varieties of kimberlites, varies widely from 50 by 50 % up to 100% of only high-Mg olivine (Kostrovitsky, 1986). In our opinion, just the ratio of olivine of varying composition might indicate the rate of enrichment or «contamination» of kimberlite melts with xenogenous material. The approximate character of contamination evaluation is associated with that this index does not consider the amount of melt-contaminated xenogenous material. The ratio of olivine can only qualitatively indicate the extent of contamination. It is also distinctly correlated both with a chemical composition of rocks and picroilmenite abundance in kimberlite. With increased concentration of FeO_total in kimberlites the relative amount of high-Mg olivine is markedly reduced; the content of picroilmenite in high-Mg kimberlites falls down to zero.

A wide variability of chemical composition of kimberlites and steady differences in the composition of kimberlites filling some pipes, pipe clusters and even fields of pipes became the foundation for developing the petrochemical classification of kimberlites (Kostrovitsky et al., 2007). The total of 5 petrochemical types of kimberlites were distinguished by the contents of FeO, TiO₂ and K₂O including two main types for diamond-bearing kimberlite fields: high-magnesium and magnesium-ferruginous. It is evident that kimberlites of different petrochemical types are differentiated by mineralogy. The heavy fraction of high-Mg kimberlites is dominated by garnets and spinellids, and low-Cr megacryst association of minerals is practically absent. Picroilmenite predominates in the fraction of magnesium-ferruginous kimberlites. The fact of affinity of isotope-geochemical characteristics of kimberlites of different petrochemical types served as the basis for the authors (Kostrovitsky et al., 2007) to conclude on the existence of different mantle sources in their emplacement, e.g. (i) asthenosphere source defining the isotope and microelement (only incompatible elements) composition, and (ii) lithosphere one responsible (the same as asthenosphere) for formation of petrochemical composition.

We believe that the high-Mg type of kimberlites basically results from disintegration, capture and partial contamination of rocks of lithosphere mantle. This conclusion is exemplified by the breccias of porphyry kimberlite from pipe Internatsionalnaya (Fig. 1). It is composed of 40-60% clastic macro- and megacrysts of olivine referred to dunite-harzburgite paragenesis. This is concluded in considering composition specifics of breccia and massive kimberlites making up the pipe and dyke bodies of Kuoisky field, and pipe Obnazhennaya, in particular. Compared to massive varieties the breccia kimberlites show higher contents of SiO2, MgO and lower CaO and CO₂. The onset of breccia formation should evidently be referred to the moment of penetrating the lithosphere mantle by kimberlite melt-fluid, and it is associated with the processes of disintegration and capture of its rocks.  

 Origin of low-Cr megacryst association of minerals is linked with asthenosphere melt (Boyd, Nixon, 1975), which contributed to formation of primary kimberlite melt. It is indicated by affinity of isotope Sr-Nd systematics, age characteristics for megacrysts and kimberlites, model calculation of composition of primary melt for garnet megacryst (Kostrovitsky et al., 2008; Solovjova et al., 2008; Nowell et al, 2004). If we accept crystallization of megacryst association of minerals as beginning of kimberlite emplacement, and crystallization of olivine bulk mass as the end, then the compositional evolution of kimberlite melt will proceed towards increase of its mg# coefficient. This abnormal trend of magmatic melt development may be explained by a continuous process of contamination with highly magnesium rocks of lithosphere mantle. The required amount of contaminated olivine for raising mg# of primary kimberlite (asthenosphere) melt, capable to crystallize olivine of bulkmass, reaches 70% of melt volume. The question arises how realistic it is to produce such a volume of contamination.

Thus, the occurrences of kimberlite volcanism are regarded as the channels for bursting of asthenosphere melt-fluid through lithosphere mantle towards the Earth surface accompanied by disintegration of partial contamination of mantle rocks. Origination of different petrochemical types of kimberlite is possibly due to a different ratio of fluid and melt component of asthenosphere source and so different volume of captured clastic material of lithosphere mantle. The highly-magnesium type of kimberlite formed in bursting of primarily fluid part of asthenosphere producing a more intense disintegration and subsequent capture of clastic lithosphere material. The magnesium-ferriferous type of kimberlite was derived with both fluidal and melt parts of asthenosphere substance involved. The latter explains the heightened content in kimberlites of FeO and TiO₂, and presence in them of megacryst olivine, garnet and picroilmenite.

The research is performed with support of Integration Grants N 27.1,  59  and 115.

References:

Kostrovitskiy S. I. Geochemical features of kimberlite minerals. Novosibirsk: Nauka. 1986. 263 p.

Kostrovitsky S. I., Morikio Т., Serov I.V., Yakovlev D.А., Amirzhanov А.А. Isotope-geochemical systematics of kimberlites of the Siberian platform. Geol. & Geophys. J., 2007. V. 48. № 3. pp. 350-371.

Solovjova L.V., Lavrentiev Yu.G., Egorov K.N., Kostrovitsky S.I., Koroljuk V.N., Suvorova L.F. Genetic link of deformed peridotites and megacrysts of garnet from kimberlites with astnehospheric melts.  Geol. & Geophys. J., 2008. V. 49. № 4. pp. 281-301.

Boyd   F.R., Nixon P.H. (1975) Origin of the ultramafic nodules from some kimberlites of Northern Lesotho and the Monastery Mine, South Africa. In:  Physics  and  Chemistry of the  Earth. New York: Pergamon  Press. V. 9, 431-454.

Davidson E.F. (1964) On diamondiferous diatrems. Econom. Geolog. V. 59. pp. 1368-1380.

Mitchell Roger H. (2008) Petrology of hypabyssal kimberlites: Relevance to primary magma compositions. Journal of Volcanology and Geothermal Research. V. 174. P. 1–8.

Nowell G.M., Pearson D.G., Bell D.R., Carlson R.W., Smith C.B. and Noble S.R. Hf isotope systematics of kimberlites and their megacrysts: new constraints on their source regions. J. of Petrology. 2004. V. 45. N. 5. P. 1583-1612.

 Fig. 1. Photo of porphyry kimberlite breccia from Internatsional’na pipe. Macrocrysts of serpentinized olifine.