Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 168

164
PÁLL IMSLAND
slightly oxidized titanomaghemites. The rest contains grains of the high-
temperature oxidation exsolution trellis lamellae or grains pseudo-
morphically oxidized beyond the C3 oxidation stage of maximum trellis
oxidation exsolution (Haggerty, 1976a). The intermediate rocks show a
similar oxidation pattern. The general diíferences are: 1) unoxidized
samples are relatively few, 2) titanomaghemites (low-temperature oxida-
tion) are less common than in the basalts, but the process of maghemitiza-
tion is apparently carried further, 3) high-temperature oxidized samples
show a more intense oxidation, and 4) silicates (mostly biotite) are
frequently oxidized as well as the opaque oxides. The groundmass oxides of
the ankaramites are frequently oxidized. This oxidation is of a rapid type,
high-temperature, pseudomorphic oxidation, which leads to formation of
titanohematites and pseudobrookite minerals, but bypasses the stage to
stage evolution typical in most cases of high-temperature oxidation
(Haggerty, op.cit.). The amount of the oxidation varies from sample to
sample. In the most oxidized samples, olivine phenocrysts may be oxidized
as well.
a. The composition of the iron-titanium oxides
Eleven elements were determined in most of the iron-titanium oxides.
These are the same as in the case of the spinels described previously.
Representative analyses of the iron-titanium oxide minerals are given in
Tables 23 to 27. The compositions within the iron-titanium oxide triangle
are shown in Figs. 96 and 97, where the analyses are plotted in terms of total
mol. per cent compositions, RC>2—RO—R2O3. The high contents of minor-
and trace elements displace the points considerably if omitted. In case of
ilmenites e.g. the points fa.ll well on the Ti02 side of the ilmenite-hematite
solid solution line, when plotted as Ti02—FeO—Fe203 only.
The oxidation exsolution and its subsequent pseudomorphic oxidation,
the high temperature oxidation processes, result in major changes in the
Fe203/Fe0 ratios and the Ti distribution of the resulting phases. The low-
temperature oxidation of titanomagnetites, resulting in titanomaghemites,
apparently primarily affects the Fe203/Fe0 ratio. The Ti distribution is
only slightly affected by this process or not at all. The two minor elements of
the titanomagnetites occurring in greatest amounts, Mg and Al, show
noticeably lower concentrations in the titanomaghemites than in the unoxid-
ized titanomagnetites, while the other minor elements do not show a
significant difference. No other systemadc or general changes in the minor
and trace element contents of these minerals have been observed to accom-
pany the oxidation processes. The Mn content of the oxidized phases in
some of the rocks may be much higher than the maximum found in the
unoxidized ones. This enrichment is most probably not related to the
oxidation processes. This is indicated by the fact that the oxidized grains
containing the high Mn all occur in rocks of intermediate compositions,