Volcanism of the Siberian Platform — Sobolev 
Recently, in connection with the discussion 
of the composition of the subcrustal substratum, 
great attention is once more being paid to the 
eclogite problem. The suggestion has been made 
(Fermor, 1914; Lovering, 1958) that the sub- 
stratum below the Mohorovicic discontinuity is 
eclogite. Some authors are of the opinion that 
this eclogite layer extends to a depth of 900 km 
(V. V. Belousov, I960), i.e., to the base of the 
Galitzin layer. The only valid data available for 
discussion of this matter have been derived from 
study of the kimberlites — a fact that makes it 
desirable to treat this problem here. 
As is known, a great quantity of xenoliths of 
ultrabasic rocks, in some cases directly related to 
eclogites, are to be found in many kimberlite 
pipes. This fact suggests that such xenoliths are 
at least in part the remains of the partially- 
melted substratum, the more so as the compo- 
sition of the olivine in them resembles that of 
the first-generation olivine of the kimberlites. 
The question is, however, still open to discus- 
sion. There is some probability that these rocks 
were picked up by the kimberlites during their 
rise toward the surface, not only in the sub- 
stratum but also at much higher levels. The 
ultrabasic magma chambers are likely to have 
revived several times, and the formation of 
ultrabasite intrusions may have taken place dur- 
ing the first stages, further intrusions taking 
place later with the rapid movement of new 
portions of magma in new geologic conditions. 
Such intrusive massifs could have consisted of 
pyrope peridotite, such as that in Czechoslovakia. 
The absence of diamonds (at least in appre- 
ciable quantity) in the ultrabasite xenoliths 
speaks against the supposition that the xenoliths 
were brought directly from the deep magma 
chamber. Although the presence of diamonds in 
pyrope peridotites has been asserted by some 
workers, neither a xenolith with a diamond, nor 
its photograph, nor a detailed description of it, 
can be found anywhere. Many attempts to obtain 
diamonds by grinding and concentration of con- 
siderable quantities of ultrabasite xenoliths and 
eclogites have resulted in failure. This fact, how- 
ever, cannot altogether disprove the hypothesis 
of the subcrustal origin of these rocks, since the 
distribution of diamonds in the substratum may 
be nonuniform. Also, they may have, for the 
455 
most part, crystallized directly with the forma- 
tion of kimberlite magma. 
On the other hand, two findings of diamond- 
bearing eclogites, which are subject to no doubt 
and have been described in detail, are proof of 
the existence of subcrustal eclogites. Diamond 
formation in the zone of metamorphism is im- 
possible. The pressure there has never reached 
even that of coesite crystallization, which is 
lower than that of diamond formation. It is in- 
teresting to point out that the Jakutian diamond- 
bearing eclogite in its ratio of FeO to MgO is 
nearer to basic rocks than to ultrabasic rocks; 
it undoubtedly was not brought directly from 
the deep magma chamber, but was picked up 
from higher levels in the substratum. 
Comparing all the above-mentioned data, we 
come to the following conclusions regarding the 
constitution of the upper mantle and its rela- 
tionship to the earth’s crust: 
1. At comparatively small depths, probably 
less than 50-70 km, the subcrustal substratum is 
of peridotite composition, corresponding ap- 
proximately to the composition of meimechite 
or kimberlite, the latter being a magmatic rock. 
2. In the region of kimberlite distribution, 
higher than the peridotite layer but below the 
Mohorovicic discontinuity, the substratum is 
eclogite with chemical composition very near 
that of basalt. 
3. Pressures in the zone of metamorphism can 
vary greatly at one and the same depth, reaching 
at least 15 kilobars but never 20-25 kilobars. 
4. There is some reason to believe that higher 
pressures are characteristic of large parts of the 
earth’s crust, especially the border of the plat- 
forms. Pressure higher than simple hydrostatic 
pressure is also characteristic of the upper part 
of the subcrustal substratum in these same areas 
(probably down to a depth of about 150-200 
km). 
5. Higher pressure persisted in certain zones 
through considerable periods of geologic time, 
as is proved by the finding of ancient eclogitized 
schist in kimberlite of both Upper Paleozoic 
and Mesozoic age. 
Examining the above statements, we come to 
the conclusion that the hypothesis of a subcrustal 
eclogite layer (Fermor, 1914) has been con- 
firmed, but only in part. The author quite agrees 
