Greinar (Vísindafélag Íslendinga) - 01.01.1977, Page 98

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ment against such an assumption that a dense high-pressure
nanocrystalline state takes over here and remains so in the
Upper Mantle. The viscosity at the Moho is high and the
temperature not elevated.
The real question is, whether nanocrystalline state could
have originated here for some other reason than high litho-
static pressure. A positive answer would lead to a simple solu-
tion of various fundamental problems in geoscience. An Upper
Mantle in a dense solid nanocrystalline state, and of nearly
the composition of “primary” basalt could for instance lead
to an easy solution of the problem of the origin of basaltic
magma, i.e. be the latent magma state. One would only need
such a heat source — frictional heat — which sufficed to dis-
rupt the “twinning bonds”, and this would be much less than
the heat of melting. Thus would be created a porridge of loose
nanocrystals and such low viscosity which allowed the dense
crystals to expand and change into light forms in mineralogical
equilibrium with a basaltic melt.
But can we give an acceptable explanation of how light equili-
brium crystals can turn into dense nanocrystals at the Moho?
Provisorily we refer to our conclusion from the frequency of
shallow earthquakes, that in the oceanic layer there must be
yield to stresses by plasticity. This really goes far to show
that in the uppermost mantle one must expect to find the re-
sults of all the known effects of shear in crystals, and among
these is the formation of dense polymorphs. Other authors
have already stressed the potentialities of shear in the mantle:
“Such transformations (as are caused by shear) may be signi-
ficant in the mantle“ ((52), p.109). We suggest that a diminua-
tion of the (strongest and densest) crystals down to nano-
crystalline size is not an unrealistic expectation.
One possible result of the mentioned shear processes in the
mantle and in the oceanic layer would be the expulsion of argon
towards the surface. This would be a simple explanation of
the fact that excess argon in volcanic materials is always
strangely small.
Let us now mention a further point. On the basis of shear