Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 222
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PÁLL IMSLAND
basalt lava. The feldspars are rather homogeneous in composition and
cluster rather closely into two compositional groups (Fig. 108). The average
composition of these feldspar groups has been used to calculate the cryst-
allization temperature according to the two-feldspar thermometer method
revivified by Stormer (1975). The temperatures obtained, assuming 1—3 kb
pressure as the crystallization depth, are between 796 and 827°C. The other
xenolith (Jan 324) is fully crystalline and shows a slightly more hetero-
geneous composition of the feldspars, especially the plagioclase, was
brought to the surface by a trachyte lava. Using those feldspars, which are
supposed to have crystallized latest and most probably simultaneously, in
the same way gives over 1100°C for the same pressure range. The tempera-
tures of Jan 142 are much lower and the sample is partially remelted to a
glass of feldspar-like composition. Incorporation ofjan 324 in the trachyte
liquid, on the other hand, does not lead to its immediate remelting. Its
crystallization temperature might thus be suspected to be similar or slightly
higher than those of the trachytes. This could mean that the crystallization
temperatures of the feldspars of the trachytes do not much exceed 1000°C,
which is identical to the temperatures shown in Table 34 for 1 atm
experiments on trachytes crystallizing an alkali feldspar first and with a
similar composition to those occurring on Jan Mayen.
One tristanite sample (Jan 175) contains two feldspars in the ground-
mass. Similar temperature calculations have been carried out for these. At 1
atm, crystallization would have taken place at 856°C according to the two-
feldspar thermometer.
It is therefore concluded qualitatively, that the more evolved trachytes of
Jan Mayen crystallize an alkali feldspar as the first mineral and do so at
around or slightly above 1000°C. The less evolved trachytes and the
tristanites, on the other hand, crystallize plagioclase first and do so at much
higher temperatures, probably over 1100°C. The groundmass crystallization
of these rocks seems to have taken place around 850°C.
G. APATITES
I. Introduction
Apatite occurs as minute slender, long needles in the groundmass of the
Jan Mayen lavas, but these are too small for analysis. In the basic lavas
their occurrence is most conspicuous among the mineral grains encircling
the vesicles, indicating a late crystallization.
As phenocrysts apatite occurs in a few samples only and in small
amounts. These are small, subhedral to anhedral equant to round grains
and occur mostly in close association with titanomagnetite phenocrysts. The
apatite phenocrysts mostly occur in the more evolved rocks of the rock suite