Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 156

152
PÁU. IMSI.AND
trivalent elements also clearly distinguishes these two trends as shown by
Fig. 90.
The late spinels of' the ankaramites f'orm a trend of slightly decreasing Cr/
(Cr+Al) values (~0.60—0.54) with strongly decreasing Mg/(Mg+F'e2+)
values (~0.50—0.15) and a strong increase in Fe3+/(Fe3++Cr+Al) values
(~0.10—0.45) at the low Mg/(Mg+Fe2+) values. The late spinel trend of
the basalts is similar to the late spinel trend of the ankaramites. It has
slightly lower Cr/(Cr+Al) values (~0.58—0.50) and has a slightly narrower
Mg/(Mg+Fe2+) range (~0.40—0.23), but is identical as regards the fmal
increase of Fe3+/(Fe3++Cr+Al). The late spinel trends are thus parallel and
differ slightly in Cr/(Cr+Al).
The directions of' these trends reflect the directions of evolution of the
liquids crystallizing the spinels. 'l'his is shown by mineral relationships and
covariations of' major elements in spinels and whole rock samples and is
further confirmed by both thermodynamic and experimental data, as will be
discussed.
The amount of major element oxides of the spinels is highly variable as is
obvious from the above description. The variations of these major elements
will not be described further, but the actual concentrations of these elements
can be seen in the represented analyses ( l'able 21) and from Figs. 90 and 91.
TÍO2 (Fig. 93) is a typical minor element in the early spinels with an
average of 1.3 wt. per cent and a narrow range, 0.62 to 2.28. On the average
TiO<2 is slightly higher (1.37) in the early spinels of the ankaramites than the
early spinels of the basalts (1.09), but the range is slightly less (0.86—1.79)
in the ankaramite early spinels compared to the early basalt spinels (0.62—
2.28). In the late spinels, on the other hand, TiO^ ranges from 1.21 to 18.96
wt. per cent. This Ti increase puts Ti among the major elements in the
spinel structure. The increase in Ti is accompanied by equivalent changes in
the spinel prism ratios: Mg/(Mg+F'e2+) decreases sharply, Cr/(Cr+Al)
decreases slightly and Fe3+/(Fe3++Cr+Al) increases sharply. This means
that there is a strong increase in Fe2+ and Fe3+ concentrations accompanied
by a decrease in Mg, Cr and A1 concentrations, or in other words, a
displacement from near the Fe3+ free spinel prism base (Fig. 89) towards the
corner of magnetite (or ulvospinel). The ulvospinel component of' the Ti
richest late spinel is nearly 50 per cent, which is considerably higher than
the ulvospinel component of'a great number of primary titanomagnetites in
the rock suite (see Fig. 88). The Ti enriched late spinels thus tend to bridge
the compositional gap between the early spinels and the titanomagnetites,
but these late spinels are extremely scarce minerals in this rock suite.
Arculus (1974) reports a compositionally continuous suite ofoxides, f'rom
aluminous chromites to titanomagnetites with abundant intermediate com-
positions, in alkali basalts from Grenada Island, Lesser Antilles, and Beeson
(1976) reports analyses of a similar oxide suite in transitional basalts from
East Molokai, Hawaii. These are two of the few published studies of