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PACIFIC SCIENCE, Vol. XVII, October 1963 
an enormous scale leads us to the conclusion that 
regional melting of the basalt layer took place 
here, probably in its upper part. However, no- 
where in the platform did the magma chambers 
reach the sialic shell, and the small granitoid 
veinlets were formed wholly as a result of local 
crystallization differentiation. Taking into con- 
sideration the existing data on the structure of 
the earth’s crust in the platform, the depth of 
such magma chambers appears to have been 
about 25 km, and the geothermal gradient at the 
time of volcanism appears to have reached 40 C 
per kilometer. 
Some differences in composition of the traps 
might have resulted from differences in depth 
of the magma chambers in different parts of 
the platform and resultant differences in the 
differentiation phenomena. 
The presence of effusives of ultrabasic com- 
position ( meimechites, kimberlites) is definite 
proof of the existence of a corresponding 
magma. This magma could be formed only by 
the remelting of ultrabasic rocks, which in turn 
is proof of the existence of rocks of corres- 
ponding compositions below the Mohorovicic 
discontinuity. 
In the case of the formation of differentiated 
complexes, there is no doubt of the presence of 
big magma chambers and a relatively slow rise 
of magma either to the earth’s surface or to the 
corresponding intrusion chambers. A compli- 
cated evolution of the rocks takes place as a 
result of involvement of the higher levels of the 
earth’s crust in melting and, perhaps, as a result 
of assimilation and differentiation. 
In the case of the kimberlites the quantity 
of rising magma is very small. This can hardly 
be the result of the low penetrability of the 
earth’s crust. Rather, it is a proof of the forma- 
tion of very small magma chambers in which 
remelting was partial, and a magma which rose 
very rapidly up the deep fissures containing 
many suspended crystals that formed not only as 
a result of crystallization in the chamber but also 
that remained as a result of the partial melting. 
Not only theoretical calculations but also ex- 
perimental data now show that if diamonds 
were formed at a temperature of about 1200 C, 
the pressure must have been more than 40 kilo- 
bars. The notion that diamonds were brought 
by the magma from great depth, and not formed 
at the time of explosions near the earth’s surface, 
can be considered valid. Sometimes, on the basis 
of the above data, an attempt is made to de- 
termine the depth of the magma chamber from 
the implied hydrostatic pressure of the overlying 
rocks. This approach we cannot agree with, since 
the pressure in the earth’s crust, within the zone 
of metamorphism, can be as high as 15 kilobars, 
which is several times the pressure resulting 
from load at that depth, the difference between 
the pressure at the time of the mineral forma- 
tion and the calculated pressure due to the load 
being as much as 10 kilobars. Such zones of 
higher pressure may extend into the depths of 
the mantle, and it is to them that the regions 
of kimberlite development are likely to have 
belonged. The depth of formation of the magma 
chambers, in this case, may be less than calcu- 
lated — that is, not 150 km but 70 to 100 km. 
As a result, when magma rises to a higher level, 
pressure still remains high, though it falls by 
a quantity corresponding to the weight of the 
vertical column of magma. In places at a depth 
of 3-4 km (see above), a breaking of the earth’s 
crust occurred, accompanied by the formation 
of peculiar pipes and a sudden pressure decrease, 
constituting a kind of explosion. A great quan- 
tity of pyroclastic and xenogenic material rushed 
into the pipe, part of it being thrown up and 
then sucked back into the pipe again. The frag- 
ments filling the pipe may later be cemented 
by the rising magma. 
The pressure before the explosion is not only 
below that shown by the equilibrium curve of 
diamonds but also below that shown by the 
curve of pyrope, since the kelyphite rims around 
grains of the latter must have formed before 
the explosion. The fact that the diamonds are 
neither completely resorbed nor graphitized is 
due to the rapid rise of the magma and its com- 
paratively low temperature. The temperature of 
the magma must be lower than that shown by a 
curve corresponding to the region of metastable 
existence of diamonds (V. Sobolev, I960), 
which begins at 1200 C at normal surface pres- 
sure and rises to 2200 C at 30 kilobars. As 
is known, the diamonds show only traces of 
graphitization, which appear as graphitic ro- 
settes near some inclusions. 
