110
Probably the most important change in physico-chemical conditions in the
ascending kimberlite column is related to self-oxidation of volatile “bubble”. It is
commonly accepted that kimberlite magmas are being initiated by the process of
fluid diapirism. This fluid is chiefly composed of hydrogen methane that was
accumulated in the mantle under the platform plates. During the ascent of the
magmatic column, if the pressure of magmatic system drops below 20 kbar, the
oxygen fugacity will increase, that resulting in the oxidation of the fluid to CO,
CO
2
and H
2
O (Figure10; Portnov, 1982).
However, the rapid movement of the magma at the pressure lower than 20
kbar may preserve diamonds. Near the surface, water is absorbed by the
kimberlite magma at the serpentinization. Due to hydrogen and methane
oxidation, the initial volume of the kimberlite rocks may vastly decrease.
8. PETROCHEMISTRY OF KIMBERLITES AND FACTORS
OF DIAMONDIFEROUS
It is necessary to note the importance of establishing the dependence of
grade diamond-bearing from the chemical composition of kimberlites and
paragenic minerals of the megacrysts and nodules. For example, some specific
kimberlite minerals with favorable geochemistry (high-Cr, low-Ca pyropes and
picroilmenite ) can be used as direct indicators of the presence diamond. To the
same lines concerns the idea of correlating the chemical composition of the
kimberlite to the diamond content.
So V.N. Milashev (1965) suggested, that the polymorphs transformation
of diamond into graphite within an ultramafic melt is a form of catalytic process.
The catalyst is related to metatitanium acid ions, which are constantly present in
ultramafic magma and later result in the formation of titanium acid salts
(titanates): ilmenite (FeTiO
3
) and perovskite ( CaTiO
3
). The catalyst follows
this chemical reaction:
diamond
C + [ Ti
4+
O
3
]
2-
= CO + [Ti
3+
O
2
]
-
+ e .
graphite
The action of the catalyst tears carbon atoms from diamond and return them
back to the diamond. If the pressure in the melt remains high the diamond is
stable, the anions of the metatitanic acid may support the growth of large crystals.
But when the pressure decreases, diamond is unstable and the carbon atoms will
tear off under the influence of metyatitanic acid [ Ti
4+
O
3
]
2-
and will form
graphite, which is stable under the lower P-T conditions.
The economic value of diamond deposits are determined not only by weights
content, but also by the quality of diamond crystals, digital expression of which
serves the price in money ($) for 1 (one) carat. The weights content of diamonds