Rit (Vísindafélag Íslendinga) - 01.06.1984, Síða 264

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PÁLLIMSLAND
uncrystallized has the composition ofjan 26. This sample is practically free
of phenocrysts and has been erupted in a liquid state and then quenched
and crystallized. Its Mg value is 66. Primitive primary magmas of Mg
values from 66 to 82 thus exist in the Jan Mayen magma system and give
rise to the wehrlites, ankaramites, ankaramitic basalts and probably some of
the basalts as well upon crystallization, with and without modiíications as
will be discussed later. These magmas are formed at pressures exceeding 18
kb and above 1300°C. The ultramaíic parental material for these magmas
will not be considered here.
Of the more evolved rocks, aphyric basalts and basaltic tristanites occur
as practically phenocryst free rocks. 'I’hey are thus assumed to exist in a
totally liquid state and may thus represent primary magmas. The Mg value
of these rocks is from 35 to 43 which is quite different from that of the
primitive ones. If this evolved primary magma is generated in any amount,
the parental material to it must itself be evolved, relative to the ultramafic
one giving the primitive magmas. The aphyric basalts and basaltic trista-
nites comprise a very small part of the Jan Mayen lava sequence, indicating
infrequent eruptions of this magma type. If it represents a primary magma,
generated in considerable quantities, this magma either has difficulties in
reaching the surface, or it is generally modiíled before eruption.
The trachytes may also occur as nearly phenocryst free rocks and thus
exist as pure liquids. Their Mg values range in these cases from 21 to 25,
which shows them to be highly evolved liquids. A primary magma of such
an evolved composition produced in repeatedly eruptible quantities is
hardly of a direct mantle origin. An evolved crustal rock is more likely as a
parent to such a primary magma. Ifit is not a primary magma, on the other
hand, a number of evolutionary processes are possible, and the parent is a
magma or magmas of yet undefined composition.
C. CRYSTAL FRACTIONATION
I. Introduction
There is a long tradition in regarding the alkaline rock suites of oceanic
islands as having evolved through the process of crystal fractionation. The
process has been assumed to operate alone or to be the major process in an
evolution modified by other processes, in cases when compositional features
appear in the rock suites which cannot be accounted for by the crystal
fractionation alone.
The general broad covariation between phenocryst phases and their
crystallization temperature on one hand, and the composition of the rocks
and their liquid temperatures on the other, so frequently fulfilled in the
rocks of oceanic islands, is a primary indication of the validity of this process
in these cases. In addition in any rock suite evolution, fulfillment of such