Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 183

mineralchemistryandrelationships 179
A low Mg is the case with analysed oxides from all tholeiitic rocks known to
the author. The Skaergaard oxides, nevertheless, show the same pattern as
do the Jan Mayen oxides, in that Mg prefers the oxides of basic rocks to
those of more evolved rocks.
MnO in the iron-titanium oxides shows a reverse pattern to that of both A1
and Mg. It is higher in the oxides of the intermediate rocks than those of the
basic ones, as shown by Fig. 99, and higher in the groundmass grains than
in the phenocrysts and inclusions (Table 29). It is thus favoured by the late
crystallizing oxides rather than the early ones. In the primary titanomagne-
tites it ranges up to 1.67 wt. per cent and up to 1.12 wt. per cent in the
primary ilmenites. The secondary oxidation products contain up to 6 wt. per
cent MnO. These oxidized grains of high Mn contents are scarce relative to
the unoxidized Mn poor ones and are, as previously mentioned, more
probably a reflection of original compositions than the results of the
oxidation process. This is indicated by the fact that the Mn rich oxidized
grains only occur in rocks of intermediate compositions, which are known
ffom other rock suites (Vincent & Phillips (op.cit.), Buddington & Lindsley
(1964), Lipman (1971)) to contain oxides of generally higher Mn contents
than those of basic rocks. Mn was detected in all analysed oxide grains.
According to Vincent & Phillips (op.cit.) Mn in the oxides of the Skaer-
gaard rocks is rather low, compared to the Jan Mayen ones, but shows the
same tendency to be higher in the more evolved rock oxides.
V203 in the primary titanomagnetites ranges from 0.14 to 1.14 wt. per
cent, while in the primary ilmenites the range is much narrower, or from
0.43—0.51 wt. per cent. Vincent & Phillips (op.cit.) found a high content of
V in the magnetites of the Skaergaard rocks, while the ilmenites are very
poor in V. This difference is much more contrasting than in the Jan Mayen
case. In the oxidized titanomagnetites, titanomaghemites, ilmenites and
titanohematites of the Jan Mayen rocks, the V2O3 range is up to 0.75 wt. per
cent, and up to 0.49 wt. per cent in the pseudobrookites. There is thus no
contrasting difference between the primary and the oxidized grains. There is
no clear pattern to be found in the V distribution between groundmass
grains, phenocrysts and inclusions (see Table 29). In the case of
titanomagnetites, the inclusions are on the average rather high in V2O3
(average=0.64 wt. per cent) compared to the phenocrysts (av=0.43),
which, on the other hand, are generally low in V2O3 compared to the
groundmass grains (av=0.60). Fig. 99, on the other hand, reveals a
tendency of V to be higher in the oxides of the basic rocks than the
mtermediate ones, which is in agreement with the Skaergaard oxides
(Vincent & Phillips, op.cit.), where V primarily enters the magnetites of the
early rocks. Where determined, V was detected in all analyses of the Jan
Mayen oxides but one.
Cr203 concentration in the iron-titanium oxides is very low, below 0.2 wt.
per cent, with a few exceptions (see Fig. 100). The grains with these higher