Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 210

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PÁLL IMSLAND
That this is actually the case will be shown later.
BaO was detected in practically all alkali feldspar analyses and in 64 per
cent ofthe plagioclase analyses. The concentrations are generally low in the
plagioclases but increase with decreasing An content (Fig. 112), a pattern
found by Heier (op.cit.) for plagioclases of a collection of rocks. Plagioclases
of over Ango composition usually contain less than 0.15 wt. per cent BaO,
while plagioclases of An60_35 compositions contain up to 0.25 wt. per cent
and plagioclases below An35 composition (which only occur in the
intermediate rocks) as well as the alkali feldspars contain up to 1 per cent
and occasionally more. The highest BaO content detected is 3.83 per cent in
the core of an alkali feldspar phenocryst in one of the trachytes. Where alkali
feldspars and plagioclases occur together in a sample, the alkali feldspars
are generally richer in Ba than the plagioclases. In the case of alkali
feldspars, the early formed crystals are generally richer in Ba than the late
formed ones. In the case of plagioclases, on the other hand, these relations
are reversed (Table 32). The plagioclases of the ankaramites, which are
groundmass grains and scarce microphenocrysts, typically contain less than
0.005 wt. per cent BaO. Drake & Weill (op.cit.) found in their experiments,
that the wt. ratio partition coefficient of Ba between plagioclase and liquid
was less than 1 at temperatures above approximately 1060°C and strongly
dependent upon the temperature. At high temperatures plagioclase takes
less Ba than at lower temperatures. Above 1060°C plagioclase is thus
depleted in Ba relative to the liquid and plagioclase fractionation will enrich
the liquid in Ba. The authors found this to be in accord with observations of
actual igneous differentiation trends. Furthermore, they found stoichiometric
considerations to indicate that in plagioclase Ba substitutes for Ca only.
As noted previously Ti, Cr, Mn and P were analysed in a few grains. In all
cases the analysed minerals were plagioclases of basic lavas. The analyses
are only 16 in number. Cr and Mn were detected in two cases, P in five cases
and Ti in fifteen. The Cr203 and MnO values obtained were in all cases 0.03
wt. per cent. The range of P205 was from 0.02—0.08 wt. per cent and the
Ti02 from 0.05—0.22 wt. per cent. Of the 16 analyses made, 14 are of
plagioclase contained by one ankaramite sample (Jan 30). All but one of
these analyses revealed the presence of Ti. The Ti distribution in the grains
of this sample shows a tendency of Ti to be concentrated in late formed
crystals relative to early formed ones (see Table 32). Cr, Mn and P thus only
occur sporadically in the plagioclases, while Ti is generally present and
shows systematic behaviour in its distribution, which is comparable to that
of Fe, Mg, Sr and Ba. Evans & Moore (1968) report the presence of Ti in the
plagioclases of the tholeiitic Makaopuhi lava lake, where the Ti02 content is
in the order of0.05 wt. per cent. Rahman & MacKenzie (1969) found Ti, by
wet chemical analyses of separated alkali feldspars from the trachytes of
Ischia, to be in amounts comparable to those reported here from plagio-
clases. They claim the separates to be 99.5 per cent pure. Ti occasionally