Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 206
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PÁLL IMSLAND
constant An content the plagioclases of the basic coarse-grained xenoliths
tend to contain less Si than those of the lavas. In the case offeldspars of the
intermediate coarse-grained xenoliths these relationships are reversed.
Fe. The iron content of feldspars is shown in Fig. 112 plotted against An
content of the ternary feldspar as FeOtot, as the microprobe does not
discriminate between the valence stages of Fe. Feldspar analyses made
before the microprobe revolution mostly report Fe as Fe203. According to
Deer et al. (1963) the Fe of both alkali feldspars and plagioclases is
primarily in the trivalent stage and considered to replace Al. The unimpor-
tant amounts of Fe2+ are considered to replace Ca or occur as impurity
inclusions. By stoichiometric correlations, Bryan (1974), on the other hand,
found 50—83 per cent of Fe in plagioclases of submarine basalts studied to
be Fe2+ and together with Mg to substitute for Al. The charge difference
resulting from this kind of substitution is balanced by further substitution of
A1 by Si and the resulting plagioclase molecule may be written as Ca(FeMg)
Si308 instead of CaAl2Si2Og. The valence stage distribution and the
substitution pattern of Fe in feldspars can probably not be simply described
by some general principle. The amount and kind of other trace elements,
£02, the structure of the feldspar and the liquid are some of the parameters
which may complicate matters. No attempt will be made here to solve this
problem for the Jan Mayen feldspars and Fe will simply be described as
FeOtot.
Iron was detected in practically all the analyses of the Jan Mayen
feldspars. Fe decreases as the An content of the feldspars decreases, but the
scatter of points is considerable. In the basic lavas the FeO content of the
plagioclases is mostly between 0.4 and 0.75 wt. per cent. In the plagioclases
of the intermediate lavas FeO is slightly lower, mostly between 0.2 and 0.5
per cent. In the alkali feldspars it varies between 0.1 and 0.5 per cent. In the
feldspars of the coarse-grained xenoliths Fe is generally lower than in the
lava feldspars. This is especially clear in the case of the basic xenoliths.
Longhi et al. (1976), on the basis of sparse data, found the distribution
coefficient for Fe and for Mg between plagioclase and basic liquid to be
lower for plutonic rocks than for extrusive ones. Their finding seems to be
confirmed by the distribution pattern of Fe in thejan Mayen rocks. Where
alkali feldspars and plagioclases occur together in the intermediate rocks,
the plagioclases are usually slightly richer in Fe than the alkali íeldspars. In
plagioclases Fe generally increases as crystallization continues, i.e. ground-
mass grains or phenocryst margins contain higher Fe than phenocryst cores.
This holds for alkali feldspars as well (see Table 32). Bryan (op.cit.) found
the Ca(FeMg)Si3Os substitution in plagioclases to be favoured as the
plagioclase becomes more sodic. The Fe enrichment in the late crystallized
plagioclases relative to the early ones found here is also accompanied by Mg
enrichment. This simultaneous Fe- and Mg enrichment of late plagioclases
is in a good agreement with Bryan’s findings.