Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 290

286
PÁLLIMSLAND
The apatite is from an intermediate rock but quite similar to the previous
one in composition. The equilibrium conditions may thus be concluded to
be fairly well matched. The orthoclase highly dominates the fractionate,
amounting to 82 to 84 per cent of it. The titanomagnetite comes next,
amounting to almost 10 per cent and is followed by the pyroxene of 4 to 7
per cent. Only minor amounts of the olivine and apatite (~1 per cent of
each) participate in the fractionation. The total fit is quite good. The
elements showing noticeable misfit are the alkalies and Mn, and A1 to some
degree. In the calculated “daughter” Na is systematically too high, while K
is too low in two cases out of the three. A1 is slightly high in two cases. Mn is
far too high in all the calculated “daughters”. In this case, the excess of
MnO of the calculated “daughter” is ~50 per cent of the MnO content of
the analysed “daughter”. This fractionation leaves a relatively small
amount of trachytic liquid or 21 to 39 per cent of the original tristanite.
Jan 82, a Sör-Jan tristanite, is here used as a “mother liquid” to the Sör-
Jan trachytes. This fractionation, likewise, needs five subtracted minerals.
Of these the clinopyroxene, titanomagnetite and apatite are the same (Table
50). The olivine of the Nord-Jan fractionate has disappeared. The feldspar
here is much less potassic, being a plagioclase of An28.s composition. To
compensate for these changes, a biotite enters the fractionate. All these
minerals are present in both the tristanites and trachytes and the equilib-
rium approximations are thus quite acceptable. The feldspar dominates the
fractionate and makes up from 67 to 71 per cent of it. The pyroxene amounts
to 9 to 14 per cent and the titanomagnetite to 7 to 11 per cent. Biotite closely
follows the titanomagnetite and amounts to 8 to 9 per cent of the fractionate
with one exception, where it is much lower. The apatite is subtracted in
minor amounts only, or 2 to 3 per cent. Regarding mineralogy, this
fractionate is thus quite different from that of the Nord-Jan case, but the
total fit obtained is equally good. The hitherto frequently found alkali misfit
is present. In the calculated “daughters” Na is too low in four cases out of
six and K too high in five cases. P is too low in five cases and Ti in at least
three cases. As in the previous case Mn is too high in all cases and mostly
much too high. Compared to the Nord-Jan case this fractionation leaves a
great amount of liquid. The trachytic “liquid” left here ranges from 42 to 65
per cent of the original “tristanite liquid”.
As in the case of the fractionation from basalts to the low-silica intermedi-
ate rocks, five different minerals are needed to obtain a good total fit in the
fractionation calculations of the trachytes. Even these five minerals are not
enough to prohibit a systematic misfit in certain elements. This fact casts
serious doubts on the possibilities of the crystal fractionation process being
responsible for the evolution of the rock group in a pure state. At least some
other process seems to be in operation as well, causing this systematic error.