Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 282

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PÁLL imsland
slightly too high. In f'our cases the Mn content of the “daughter” is too high.
This is rather serious, as the concentrations are so low and the fractionating
minerals contain Mn. This type of fractionation gives 37 to 60 per cent of the
starting material, a basalt of moderate Mg content (5.90 per cent), as a
“liquid” of low Mg content (<3 to~4 per cent).
Compared to the results of the early fractionation of the rock suite, the
results obtained for the basalts give a generally somewhat poorer match,
both in the case of total fit (2R2) and the misfit characteristics of individual
elements. Nevertheless, the compositional range of the subtracted minerals
is increased, as well as the number of mineral phases, but the compositional
range of the rocks is probably less. The equilibrium conditions between the
subtracted minerals and the “liquid” to be fractionated in the calculations in
the case of these basalts are poorly matched in most cases. The total fit
obtained is, though, quite good in numerous cases.
In a group of rocks as porphyritic as these basalts, one could take such
evidence as a sound indication of the operation of the crystal fractionation
process. But the heterogeneous nature of the phenocrysts and the pattern
that appears in the misfit of the calculations indicates that some other
process has been operating as well. Crystal fractionation within the basalts
of both high- and low Mg content is thus a very likely process, but most
likely it was accompanied, preceded or succeeded by another process. The
relative influence of this process on the final result is unknown, as is its
nature. Furthermore, the precision of these crystal fractionation calculations
is uncertain when one bears in mind the difficulties in imitating the
equilibrium conditions, the heterogeneous nature of the phenocrysts, and
the best-result seeking nature of the calculations. Changing mineral
relationships, e.g. in the fractionate, would result in a worse total fit, but this
could, nevertheless, increase the precision of the calculations, if the other
process worked in the appropriate direction.
IV. Crystal fractionation into and within the
low-silica intermediate rocks
The low-silica intermediate rocks are the basaltic tristanites and the
tristanites. The most primitive of these are similar to the low-Mg basalts in
Mg content but Si02 is increased to 51.75 wt. per cent or more (compared to
>50 per cent in the basalts).
This is a small group of rocks. Compared to the bulk of the basalts and the
trachytes, these rocks are poor in phenocrysts. This is especially the case
regarding the basaltic tristanites.
The crystal fractionation calculations carried out here are made in two
steps: from a low-Mg basalt to the basaltic tristanites and from a basaltic
tristanite to the tristanites. In both cases five minerals are subtracted;
feldspar, clinopyroxene, titanomagnetite, olivine, and apatite. The feldspar