Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 196
192
PÁLL IMSLAND
to the titanomagnetite structure at higher temperatures.
In general, little is known about the effects of temperature and pressure
on the behaviour of minor elements in iron-titanium oxides. The covaria-
tions shown and discussed here therefore merely indicate the possibility,
that these factors may aífect the composition. The mineral compositions,
though, probably reflect the more complex interplay of crystallization order
(and magnitude) and the consequent liquid compositional changes, which
are necessarily always of prime importance.
The contemporaneous pairs of ilmenite and titanomagnetite, crystallized
just before the groundmass solidification of the basic lavas, have crystallized
at 1040—1090°C, probably rather close to the earth’s surface. Uncommon
crystallization of earlier pairs of ilmenite and titanomagnetite has taken
place at deeper levels at around 1100—1150°C. The common titanomagne-
tite phenocrysts of the basalts, crystallized without contemporaneous ilmen-
ite crystallization, most probably took place under similar conditions. They
seem to have started to crystallize at the time the late spinel crystallization
stopped or slightly before. The late spinel is believed to have crystallized
around 1150°C. Thus the titanomagnetite crystallization seems to have been
going on from ~1150°C to at least 1040°C, occasionally accompanied by
ilmenite crystallization, especially at the lower temperatures.
F. FELDSPARS
I. Introduction
Feldspars are abundant as phenocryst phases in the Jan Mayen rocks and
occur as groundmass grains in all rock types of the rock suite, as well as
being common constituents of the xenoliths.
The feldspars of the basic lavas and the basic xenoliths are plagioclases,
while those of the intermediate lavas and xenoliths are both plagioclases and
alkali feldspars.
The plagioclase of the Jan Mayen rocks is a very heterogeneous group of
minerals as regards textural features, and individual rock samples clearly
have quite different plagioclase crystallization histories. In order to obtain a
general picture of the plagioclase evolution and its effect on the evolution of
the rock suite as a whole, some generalizations and simplifications of these
textural features must be made.
Plagioclase is a very common phenocryst phase in two of the basic rock
groups which together comprise almost 50 per cent of the studied samples
and the vast majority of the basic rocks of the island. These rock groups are
the pl-ol porphyritic basalts and the pl-ol-cpx porphyritic basalts. Of these
the pl-ol porphyritic type shows much simpler plagioclase features. The
plagioclase phenocrysts of these rocks are dominantly microphenocrysts or