Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 185

MINERALCHEMISTRYANDRKI.ATIONSHIPS 181
Cr contents are titanomagnetites, which have in common that they occur in
ankaramites and mafic basalts, and are unaltered by oxidation. They are
mostly groundmass grains, but phenocrysts and inclusions in silicate pheno-
crysts are also found. The Cr203 concentrations of these grains range up to
2.48 wt. per cent. The lowest Cr203 content of the spinels is found in the late
spinels, where it may be as low as 13.5 wt. per cent. There is thus a clear
compositional gap between the spinels and the titanomagnetites as regards
Cr content as well as the Usp compnent previously described. Except for
these Cr rich titanomagnetites there is no observable pattern in the distribu-
tion of'Cr in the iron-titanium oxides. In 30 per cent of the analyses Cr was
not detected (i.e. Cr203<0.005 wt. per cent).
Si02 (Eig. 100) occurs in very low concentrations in the iron-titanium
oxides, mostly below 0.25 wt. per cent. There is no definite pattern in its
distribution. Those grains which contain more than 0.25 per cent are mostly
from intermediate rocks in the case of titanomagnetites, but from basic rocks
in case of ilmenites, but there are too few samples to indicate any tendency.
In only 5 per cent of the analyses where Si was determined was it not
detected. Some authors (see Vincent & Phillips, op.cit.) believe Si in the
oxides to be contained in accidental minor silicate inclusions rather than
being more or less evenly distributed throughout the crystal structure. The
low and even concentrations and rare absence of Si obtained here (and in
other microprobe analyses) would rather suggest an even distribution
throughout the crystal structure than such minute silicate inclusions. An
even distribution of Si in the crystal structure does not exclude silicate
tnclusions and the rare higher Si contents might well be caused by such
inclusions.
CaO (Fig. 101) in the oxides ranges up to 0.54 wt. per cent, but is mostly
below 0.15 wt. per cent. A definite pattern of distribution is not observed,
except that many ilmenites contain rather high amounts of Ca relative to
titanomagnetites. These ilmenites occur in basic rocks only. In 42 per cent of
the analyses, where CaO was determined, was it not detected (i.e. <0.005
tvt. per cent). During crystallization of the oxides, Ca is certainly available
tn the liquid, but does not seem to be readily accepted by the iron-titanium
oxide crystal structure, especially not the titanomagnetite one. Vincent &
Phillips (op.cit.) report Ca in the Skaergaard oxides. The concentrations are
generally slightly higher than in the Jan Mayen oxides and clearly decrease
m the oxides of the latest liquids to crystallize.
NiO (Fig. 101) in the oxides ranges up to only 0.21 wt. per cent and does
not show any systematic distribution. In 50 per cent of the analyses, where
Ni was determined, was it not detected. Crystallization of olivine removes
Ni effectively from the liquid. Olivine has crystallized from the Jan Mayen
liquid over löng periods and could thus probably have kept the liquid nearly
constantly free of Ni. This may be the reason for the low Ni contents of the
oxides rather than a rejection by the oxide crystal structure. The Ni content