Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 217

MINKRAL CHEMISTRY AND RELATIONSHIPS 213
found the transition between plagioclase-lherzolite and spinel-lherzolite to
take place at ~9 kb in the temperature range 1000 to nearly 1300°C.
In the experiments referred to above which were carried out under
varying pressure and temperature conditions, the crystallization of basic
magmas resulting in plagioclase together with olivine or olivine and clino-
pyroxene (±iron-titanium oxides) is therefore governed by some general P-
T restrictions. This is shown by the parallel to subparallel arrangement of
the relatively nearly juxtaposed lines encircling the fields of simultaneous
crystallization of these minerals. The experiments are made on composi-
tionally different basic magmas, ranging from LIL poor tholeiites to alkaline
basalts of high to low Mg contents. This generalized field of plagioclase-
olivine ±clinopyroxene and iron-titanium oxide crystallization of basic
magmas in the P-T projection is terminated by lines connecting the approxi-
mate points: 1200°C at 1 atm, 1250°C at 10 kb, and 1120°C at 17 kb and
then a generalized solidus which with decreasing pressure decreases from
1120°C at 17 kb towards 1000°C at 1 atm. Plagioclase and clinopyroxene
without accompanying olivine may crystallize at somewhat higher pressures
and temperatures.
The Jan Mayen basalts contain plagioclase phenocrysts together with
olivine or olivine and clinopyroxene (±iron-titanium oxides in both cases),
but never with clinopyroxene alone. Table 33 collects from the literature the
compositions and the first appearance plagioclase temperature of the alkal-
ine rocks, which most strongly resemble the Jan Mayen rocks (exccpt for the
trachytes). Here the temperature varies from 1130— 1170°C at 1 atm. lhese
experiments seem to be relevant for comparison with the Jan Mayen rocks,
which thus most probably crystallized the first plagioclases at depths at
somewhat higher temperatures or at -—1160— 1200°C. lhese depths probably
do not exceed 10 kb pressure or nearly 40 km. This crystallization results in
the formation of the cores of the plagioclase phenocrysts.
It has been concluded that the plagioclase microphenocrysts crystallized
during ascent of the magma. Iron-titanium oxide grains, slightly biggcr than
the groundmass grains and which were previously concluded to have
crystallized just before the groundmass therefore belong to the same category.
The oxide thermometer temperature of this crystallization is 1040— 1090°C.
The phenocryst cores are resorbed while microphenocrysts are not and
both may occur in the same sample. If the plagioclase cores crystallized at
temperatures above 1160°C and at considerable depth and the micropheno-
crysts at lower temperatures than 1090°C and during ascent, then the
resorption of the cores took place in the interval of these temperatures
(1160— 1090°C). According to the theory of Vance (op.cit.), in explaining
the resorption, a drop in pressure is necessary to introduce the resorption
conditions. This pressure drop would thus coincide with the temperature
drop from core to microphenocryst crystallization. 1 he marginal zone
crystallization of the resorbed phenocrysts would take place after the