Role of capture and contamination of clastic material of lithopsheric
mantle in kimberlite emplacement
Institute of Geochemistry SB RAS, Irkutsk
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
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 FeOtotal 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
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.
Kostrovitskiy S. I.
Geochemical features of kimberlite minerals. Novosibirsk: Nauka.
48. № 3. pp.
49. № 4. pp.
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.
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.
1. Photo of porphyry kimberlite breccia from Internatsional’na pipe.
Macrocrysts of serpentinized olifine.