Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 284

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PÁI-L IMSI.AND
dominates the fractionate. Clinopyroxene has dominated the fractionate in
previous cases but ranks as number two here. Titanomagnetite is an
important part of the fractionate, while both olivine and apatite are of less
importance, especially in the tristanites.
Jan 57, one of the low-Mg basalts, is here used to enter the intermediate
category. From this “mother liquid” the basaltic tristanites are derived
(Table 47) by fractionating the five above mentioned minerals. The clino-
pyroxene is an evolved titan-salite and the feldspar a slightly Or enriched
Ab rich plagioclase (An33). l’heir compositions are frequently encountered
in the analyses of the respective mineral groups of the low-Mg rocks of no or
slight Si enrichment. The subtracted titanomagnetite and apatite are good
representatives of these minerals in the rock groups as well. l'he olivine
fractionated is present in the “mother liquid” itself (Jan 57). The equilib-
rium approximations of these minerals to the magmatic conditions which
are to be imitated are thus fairly good, in spite of the fact that the rocks
under consideration are generally poor in phenocrysts. The plagioclase
subtracted amounts to 44 to 48 per cent of the fractionate and is thus the
dominating factor in the compositional changes. Clinopyroxene amounts to
17 to 27 per cent and the titanomagnetite to 17 to 18 per cent. Olivine and
apatite are subtracted in an amount of 6 to 9 per cent each and thus
considerably iníluence the result. The total fit obtained is fairly good. The
misfit is primarily caused by Ti and the alkalies. In the calculated “daugh-
ter”, Ti is too high in all cases and Na is too high in two cases out of three
while K is too low in the third case. Mg is slightly too high in two cases. P
shows the same erratic behaviour as in the low-Mg basalt case, as in fact
would be expected. Mn is systematically too high as in previous cases. This
fractionation leaves 66 to 80 per cent of the starting material as a liquid of a
basaltic tristanite composition.
In fractionating from the basaltic tristanites further into the tristanites
(Table 48) the same five minerals are needed but their compositional
evolution must also take place. The olivine thus becomes more fayalitic
(F051.5) and the feldspar becomes more enriched in the orthoclase and albite
components (An2o.s)- The titanomagnetite becomes richer in both Fe and
Mn, the elements usually enriched in the titanomagnetites of evolved rocks.
It also becomes slightly enriched in Ti but is much poorer both in A1 and
Mg, which are generally high in the iron-titanium oxides of this rock suite,
especially in the basalts. The clinopyroxene and the apatite are the same as
in the former case. The changes in equilibrium conditions are thus partly
traced and may be considered fairly matched. As in the previous case the
feldspar dominates the fractionate, making up 50 to 58 per cent of it. The
clinopyroxene amounts to 26 to 30 per cent of the fractionate and the
titanomagnetite to 11 to 15 per cent. The apatite and olivine subtracted
have decreased relative to the previous case. Apatite here amounts to 3 to 4
per cent and the olivine to 1 to 2 per cent. This fractionate gives a fairly good