Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 180

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PÁLL IMSLAND
Gunha, which further show a general increase in Ti from core to edge of
each titanomagnetite grain. It is, on the other hand, in contrast to the
statement of Carmichael et al. (1974), that throughout an evolutionary
magma sequence, the ulvospinel-magnetite series shows some depletion in
the Usp component in response to falling temperature, and which would be
expected from the relation of the FMQ- and other buífer curves to the
isoconcentration lines of Usp in magnetite in the fö2 versus T° diagram of
Buddington & Lindsley (1964).
When the concentrations of elements in minerals range up to several per
cent, as in the case for Al203, MgO and MnO in the Jan Mayen oxides,
these elements in fact no longer fall within the minor- or trace element
category. Nevertheless, in most iron-titanium oxides these elements are
described as such. The minor- and trace element components determined in
the Jan Mayen oxides are: Si02, A1203, Cr203, V203, MnO, MgO, CaO,
NiO and ZnO. Of these A1203, Cr203, MnO and MgO were determined in
all cases. The rest was determined in about 75 per cent of the analyses, with
the exception of ZnO, which was determined in 30 per cent of the analyses
only. The concentrations of these elements in the oxides are greatly variable.
Some occur only in trace-concentrations, while others show a range of
several per cent.
AI2O3 in the iron-titanium oxides ranges up to nearly 15 wt. per cent. In
Fig. 98, A1203 and MgO are plotted against Ti02 and distinction is made
between grains from basic and intermediate rocks. In the primary
titanomagnetites A1203 ranges from 1 to nearly 15 per cent. The same range
is found in the high-temperature oxidized magnetites, while A1203 only
ranges up to 7.5 per cent in the low-temperature oxidized titanomaghemites.
In contrast to the titanomagnetites, A1203 in the primary ilmenites is very
low, or only ranging up to nearly 1 per cent. The high-temperature
oxidation products of the ilmenites, the pseudobrookites, similarily contain
less than 1 per cent A1203. The secondary ilmenites and titanohematites, on
the other hand, which are high-temperature oxidation products as well, but
of original titanomagnetites, contain up to nearly 8 per cent A1203. Thus it
seems that during the high-temperature oxidation process A1 remains
unaffected in its original position in the crystal. Whether the low A1 contents
of the titanomaghemites, relative to the titanomagnetites, reflect original
differences or are caused by the low-temperature oxidation process is not
clear. Fig 98 reveals that the titanomagnetites of the basic rocks contain
much more A1 than those of the intermediate rocks. In the case of the
ilmenites this relationship, if it exists, is obscured by the low concentrations.
When grouped into groundmass grains, phenocrysts and inclusions, the
titanomagnetite phenocrysts show clearly higher A1 contents than do the
groundmass grains. Furthermore the inclusions tend to be still higher in A1
than the phenocrysts (see Table 29). This shows that A1 enters the early
formed titanomagnetites in greater amounts than the late formed ones. In