106
Reprinted with the permission of the Publishing House “Nedra”.
Figure 7. Regional schema of the distribution facies of kimberlites and
comagmatic to them alkaline-ultrabasic rocks and carbonatites in Africa.
Reprinted with the permission of the Publishing House “Nedra”.
The fields of kimberlites subfacies: 1 – diamond’s ( a – Phanerozoic
age,
b – Proterozoic age); 2 – diamond’s and pyrope’s (a – Phanerozoic age,
b - Proterozoic age ); 3 – pyrope’s (a – Phanerozoic age, b – Proterozoic
age );
4 - fields of pikrites and pikrite porphyrites ( a – Phanerozoic age,
b – Proterozoic age); 5 – fields of alkaline-ultrabasic
rocks and carbonatites
( a – Phanerozoic age, b – Proterozoic age ).
The borders of distribution: 6 – kimberlite diamond’s subfacies,
7 – kimberlite diamond-pyrope’s subfacies, 8 – kimberlite pyrope’s
subfacies,
9 – ultrabasic and alkaline-ultrabasic porphyres rocks of pikrites subfacies.
Kimberlites provinces: I – Transvaal (PR
3
), II – Kalahari (MZ),
III – Congolese (MZ), IV – Tanzania (MZ), V – Liberia (PR
3
, PZ
2
, MZ),
VI – Gabon (PR
3
).
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7. PHISICO -CHEMICAL CONDITIONS OF CRYSTALLIZATION DIAMONDS
According to the newest data, the crystallization of diamonds from
ultramafic magmas could occur at the reaction of the volatile components of
hydrogen, carbonic oxide and hydrocarbon in hard carbon cubic habit (diamond)
at the high temperatures 900-1300
o
C and pressure 4-7 GPa on the depth of 170-
240 km in the non-depleted mantle (Portnov, 1982; Kaminsky, 1984; Navon,
1991; Hunt et.al.,1992; Marakushev et al.1995).
The analysis of the isotopic equilibrium of different gases such as CO,
CH
4
and CO
2
show that at the rising of oxidation increases the concentration of
heavy carbon isotopes in the fluids CO, CH
4
and CO
2
. As suggested
A.A.Marakushev (1995) the carbon isotopes will become heavier during the
process of diamond crystallization through the following gas reactions:
CO + H
2
= C
(diamond)
+ H
2
O; CH
4
+ 2CO = 3C + 2H
2
O;
CH
4
+ CO
2
= 2C + 2H
2
O.
The discrete changes in the isotopic compositions adequate
13
C several
per mil (6-10
o
/
oo
) as has been shown by the study of diamond-in-diamond
structure from the Udachnaya pipe (Galimov et al., 1990). The natural evolution
of carbon isotopes of diamond in eclogitic systems resulted in positive values of
13
C = 2
o
/
oo
, but in peridotites it is limited to
13
C= -1
o
/
oo
.
Such difference supports
more higher oxidation environment for fluids at the crystallization diamonds
from eclogite in comparison with the diamonds from peridotite.
It appears that the main process in the evolution of kimberlite magma is a
layer magmatic differentiation. At different stages of layering the melt takes a
chemical liquation and the magma is separated into two immiscible melts.
Accumulation of heavy crystals in the bottom parts of the melts and squeezing
will separate a portion of magmatic liquid from the earlier crystallized layer. Deep
in the upper mantle, the magmatic differentiation into two squeezing of melts had
passed with the different silica contents, that resulted in lower peridotite and in
upper eclogite layers.
It is assumpt that within the upper mantle there is a bimodal segregation
of the rock-forming elements into two magmas: a relatively reduced by ultramafic
components with higher hydrogen content; and a relatively enriched by oxidized
components with lower water content. This difference in deep-seated magma
chambers provides contrasting environments for the formation a different types of
diamond (Marakushev et al., 1995).
The differentiation of the rock-forming elements in the initial ultramafic
melts is accompanied by contrasting distribution of the fluid components, that
results in formation two different types of diamond crystals – eclogitic and
peridotitic (Figure 8,9).