Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 167

mineral chemistry and relationships 163
liquid structure are unlikely to affect the distribution of Mg and Fe2+. The
possibility that the spinels of Jan 87, which are all anhedral grains, are in
fact xenocrysts and thus do not represent a trend of simple magma evolu-
tion, cannot be excluded. If that is the case, the spinels of the basalts show
only one trend, i.e. the late spinel trend of strongly decreasing Mg/(Mg+
lm2+)) slightly decreasing Cr/(Cr+Al) and slightly increasing Fe3+/(Fe3+ +
Cr+Al) accompanied by a final strong increase in Ti.
The late spinels are of extremely small amount in the rocks and occur as
small groundmass grains or a thin overgrowth on early spinel phenocrysts.
Olivines of compositions which could be in equilibrium with these spinels at
reasonable temperatures are found in most of the rocks, but the equilibrium
relationships are almost impossible to recognize in thin-sections. At 1150°C
the late spinels are in equilibrium with olivines of Fo62-80- Relative to the
early spinels they show decreased Mg/(Mg+Fe2+) and Cr/(Cr+Al) ratios
and a much higher Fe3+/(Fe3++Cr+Al) ratio and Ti amount, as seems to be
typical of late spinels. The only difference between the late spinels of the
ankaramites and the basalt is a slightly lower Cr/(Cr+Al) in the basalts.
Fhe crystallization of these Fe- and Ti enriched late spinels has occurred
before the crystallization of the iron-titanium oxides, which may mantle the
late spinel overgrowth of spinel phenocrysts. The extreme edge of the titan-
salite phenocrysts frequently shows a slight enrichment of Cr relative to the
bulk of the grains, which might indicate that during the crystallization of the
titan-salite the remaining liquid was enriched in Cr, which then lead to the
crystallization of the late spinels, when the titan-salite crystallization ceased.
Olivine is still crystallizing at this time, but plagioclase does not seem to
have started to crystallize.
III. Iron-titanium oxides
Titanomagnetites are the characteristic original opaque oxide minerals of
the basalts and the intermediate lavas as well as of the coarse-grained
xenoliths, except for wehrlite. Original ilmenite occurs in relatively few of
these rocks. In the lavas these minerals occur as phenocrysts, micropheno-
crysts and groundmass grains. The phenocrysts and microphenocrysts are
mostly euhedral or nearly so, more or less equant in form, while the
groundmass grains frequently exhibit skeletal or lath-like forms. Mostly the
minerals form discrete grains of titanomagnetite and ilmenite, but they may
occur as composite grains, sandwich laths and trellis lamellae. Of these
latter types, trellis lamellae and their later pseudomorphically oxidized
products are the most common, followed by the composite grains. In the
lavas the phenocrysts are nearly always titanomagnetites only, while the
groundmass grains are both titanomagnetite and ilmenite.
The oxides of about half of the basalt samples do not exhibit any
secondary oxidation. Most of the basalts that do, contain low-temperature