Greinar (Vísindafélag Íslendinga) - 01.01.1977, Síða 84

82
ally into consideration, but some hypothetical mixture of
peridotite and basalt seemed recently attractive and with the aid
of laboratory results, it could lead to ingenious explanations
of the various basalt varieties, if the place of derivation had
a prescribed depth and temperature. But physically highly
questionable, not to say impossible processes, had to be in-
troduced to extricate the so formed basalt magma from the
mother rock. Partial melting of the mother rock was assumed,
based on a laboratory analogue. This process means an ionie
exchange throughout a certain volume of the mother rock,
and the formation of new minerals, interspersed with and in
mineralogical equilibrium with basaltic magma.
This state of mineralogical equilibrium with the melt is
only in existence as long as there is intimate contact between
both phases. If gravitative settling of the heavy minerals takes
place, and the basaltic magma is so separated, and left just
above the original depth for a prolonged time, the same mine-
rals, from which it escaped, must now be formed in it, for they
are the equilibrium minerals. Thus, either differentiation goes
on in the magma, if it separates and rests at depth, or it must
come immediately after the partial melting to the surface and
quite certainly it will then maintain the original chemical
composition! Filter pressing, i.e. pressing of very viscous
magma through a weak mesh of crystals, is physically mean-
ingless but if possible, it would obviously lead to the same
result as gravitative separation. Partial melting in a very basic
mantle is no solution of the great problem of the origin of
basalt magma. If formed by melting of eclogite, the magma
would be crowded with lumps of the mother rock, and very
essentially have the bulk composition of the heavy mantle, if
it was derived from it by partial melting, and without extrica-
tion from small crystals or a mesh of such. Hence, the upper
mantle must be nearly of basaltic composition.
But there are other very weighty reasons which also lead
to this conclusion. The chemical inference from seismology,
isostasy, and laboratory densities concerning the upper mantle,
is simply wrong.