About origin of kimberlites and megacryst minerals

Kostrovitsky S.I.^*, Solovjeva L.V.^**, Alymova N.V.^*, Yakovlev D.V.^*, Suvorova L.Ph.^*

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

^** Institute of the Earth’s crust SB RAS, Irkutsk, Russia

serkost@igc.irk.ru 
 

It is well known that kimberlites are hybrid heterogeneous rock of the subvolcanic origin. They began to be formed at the depth of the upper mantle and completed on the surface of the Earth. The questions which are still open include: what components of rocks have the magmatic origin, what are of metasomatic one and what of them have the xenogenic origin due to the destruction of rocks of the lithosphere mantle; what is a role of metasomatic processes in forming rocks; composition and value of fluid component; what is the subvolcanic process of kimberlite intrusion; what are the reasons of their evolution.

The kimberlites are marked by the so-called minerals-satellites of diamonds which include garnet (Gar), picroilmenite (Ilm), chromospinelides(CrSp), olivine (Ol) demonstrating  barophyle conditions of crystallization. The above minerals are found in kimberlites in different quantities, showing different composition and different crystallization trends. The leading factor in variability of mineral composition is petrochemical.

We distinguished regional and local heterogeneities of kimberlites, found within the Yakutian province both inside separate fields and bunches of pipes, separate pipes. If regional difference is stipulated presumably by initially different mantle sources of kimberlite  substance  local heterogeneity is related to secondary factors of chemical component redistribution. The regional difference between kimberlites in FeOtot, TiO2, K2O contents was the basis to distinguish 5 petrochemical types (Kostrovitsky et al., 2004). Three types of kimberlites, found in the southern part of  the Yakutian province 1) high-Mg, low-K, 2) high-Mg, high-K and 3) Mg-Fe are diamond-bearing forming deposits of diamonds. Two Fe-Ti petrochemical types are widespread in the northern part of the province and are poor in diamonds. 

It is remarkable, that each of petrochemical types demonstrates a certain composition of both rock-forming minerals, and accessory minerals-satellites. For example, the magnesia- ferriferous type of kimberlites contains as a rule olivine showing wide variations in the composition (from 7 up to 14 % fayalite minal); moreover the heavy fraction contains mainly picroilmenite. High-Mg kimberlites contain olivine in which the fayalite minal makes up, as a rule, no more than 8-9 %. The heavy fraction contains manly garnet and chromospinelides instead of picroilmenite (for example, in pipes Aikhal and  International). In Fe-Ti 4^th and 5^th types the content of fayalite minal in olivines reaches as high as 18-20 %.

The megacryst mineral association (the size of grains > 1 cm ) is of particular  value for understanding the kimberlite nature.  Low-chromium high-titanium paragenesis is the most abundant among megacrysts. It’s crystallization is most likely related to the  kimberlite  formation. Studies of isotope systematics  of megacryst association minerals (garnet, clinopyroxene and phlogopite), as well of the garnet from  coarse-grained deformed lherzolites shows that it is similar to the systematics, characteristic of kimberlites. Almost all isotope Sr-Nd compositions lie on the plot in the area of values common to HIMU source. It should be noted, that for Phl megacrysts (10 samples) we studied only Sr isotope composition, while for the majority of garnet grains (9 samples) only Nd isotope composition was studied.

We determined the age of crystallization for Phl, Grt and Cpx megacrysts from Udachnaya-east pipe using  Rb-Sr isochrones. Isochrone  age of Phl and Grt are almost the same and made up 401,5±3,3 and 400,8±5,5 Ma, correspondingly. The corresponding initial 87Sr/86Sr values determined from isochrones amount to 0,70304 and 0,70359 (εSr =-20,7 and-12,8). As the age of formation of  Udachnaya-east pipe makes up 367 Ma  (Kinni et al., 1997), we can arrive at the conclusion that the major part of the megacryst association crystallized during the prekimberlite period from the asthenosphere liquid that was accompanied by the metasomatic transformation of the lithosphere mantle.

We believe that the formation of kimberlites is related to the crystallization of both megacrysts and the certain part of macrocryst minerals, including, accessory minerals (chromospinelides and garnet).

The lack of correlation between isotope-geochemical and petrochemical characteristics suggests the existence of independent mantle sources for petrogenic and trace incompatible elements of kimberlites (Kostrovitsky, et al., 2007). It is supposed, that a strong flow  of fluids from the asthenosphere source when uplifting in the setting of the heterogeneous lithosphere provoked the formation of local kimberlite centers which gave rise to petrochemical types of kimberlites. Thus, geochemical specialization of kimberlites results mainly from a common (for all pipes of the field, and most likely for several fields of similar age) mantle fluid, which was predominant in rare element balance of the hybrid melt in the center. The clastic material of the mantle origin was significant for major element composition of kimberlites. The lithosphere source is in particular significant for the origin of high-Mg kimberlites, which are often diamond-bearing. For example, the International pipe contains varieties of porphyry kimberlite, which includes macro-megacrysts as fragments of megacrystalline dunite-harzburgites occupying 60-70% of the kimberlitic rock. Disintegrated material was not a simple mechanical impurity. We believe that assimilation of clastic lithosphere material by the kimberlite fluid is of particular importance. The composition of the hybrid melt was responsible for specific features of the composition of barophyle association of macrocryst minerals, crystallized in the kimberlite melt.

The deep faults were significant for initiation of kimberlite volcanism that is confirmed by a linear tectonic control of spatial distribution of kimberlite pipes.

Kimberlites should be regarded as rocks which generated as a result of deep faults in the lithosphere mantle, activization of the asthenosphere layer, the subsequent uplifting- breakthrough of the asthenosphere melt-fluid to the surface of the Earth, accompanied by decomposition and the subsequent assimilation of rocks of the lithosphere mantle. 
 

References:

Kinni P.D., Griffin B. J., Kheamen L.M., Brakhfogel F.F., Spetsius  Z.V. Determination of U-Pb age of perovskites from Yakutian kimberlites by ion-ion mass-spectrometer (SHRIMP) method. Geology and geophysics. 1997. v. 38. № 1. P. 91-99.

Kostrovitsky S.I., Morikyo T., Serov I.V., Rotman A. Ya. On kimberlite origin // Doklady RAS. 2004. V. 399. № 2. P. 236-240.

Kostrovitsky S.I., Morikyo T., Serov I.V., Yakovlev D.A., Amirzhanov  A.A. Isotope systematics of kimberlites of the Siberian platform // Geology and geophysics. 2007. V. 48. № 3. P. 350-371.

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