Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 232

228
PÁLL IMSLAND
the crystals give apatite compositions with up to 7.83 wt. per cent FeO or
8.70 wt. per cent Fe2Os, which is probably the oxidation state of the iron in
this case. This iron enrichment is not accompanied by changes in the other
determined minor elements, but is compensated for by lowered CaO and
P205 contents.
Except for these altered grains the apatite contains FeO ranging from 0.22
to 1.11 wt. per cent, with an average of 0.47 per cent. On the average the
apatites of the intermediate rocks contain quite similar amounts of FeO
(0.48 per cent) to those of the basic rocks (0.45 per cent), while the
diíference of averages between fine-grained and coarse-grained rock apatites
is more prominent, or 0.55 and 0.36 wt. per cent respectively. Compared to
apatites of other rocks there is no contrasting difference to be seen. The
Leucite Hills (Carmichael, 1967) and Shonkin Sag (Nash, 1972) apatites are
slightly poorer in Fe than the Jan Mayen ones containing 0.12 to 0.31 (av=
0.23) and 0.16 to 0.34 (av=0.25) wt. per cent FeO respectively. The
Skaergaard apatites (Nash, 1976), on the other hand, are similar to the Jan
Mayen ones containing 0.21 to 0.71 (av=0.49) per cent FeO. Iron appears
to be generally present as a trace constituent in igneous apatites, mostly in
concentrations below 1 wt. per cent measured as FeO. Whether it occurs
primarily in the divalent or trivalent stage is uncertain. Analyses of apatites
given by Deer et al. (1962), which are older than the microprobe, report
both valency stages. F'rom the available data no clear pattern of a systematic
behaviour of Fe has emerged regarding diíferent rock types. Apatites of
lavas, though, appear to be slightly higher in Fe than apatites of plutonic
rocks.
MnO (Fig. 115) was detected in all analysed grains. The range is from
0.04 to 0.52 wt. per cent MnO and the average is 0.18 per cent. Extrusive
rock apatites are poorer in MnO, on the average (0.13 per cent), than those
of the coarse-grained xenoliths (0.24 per cent). Basic rock apatites are
poorer (0.07—0.17, av=0.11 per cent) than those of the intermediate rocks
(0.03—0.52, av=0.22 per cent). The most striking aspect of the Mn contents
of the apatites is enrichment in this element in the apatites of the most
evolved rocks (the syenitic xenolith, Jan 324; the trachyte, Jan 168 and the
tristanite, Jan 175). In all these rocks the apatites are closely associated with
the iron-titanium oxides, which similarly show a Mn enrichment as is
usually the case with late crystallizing iron-titanium oxides and especially
oxides of highly evolved rocks. The Mn rich apatites reported by Deer et al.
(1962) all come from pegmatites, a residual highly evolved magmatic
material. Cruft (1966) also found such Mn rich apatites to be of pegmatite
origin only. The MnO content of the apatites of the Skaergaard rocks (Nash,
1976) and the Shonkin Sag rocks (Nash, 1972) is low compared to the
analyses reported here, ranging from 0.07 to 0.09 and 0.05 to 0.08 wt. per
cent respectively. Apatite megacrysts in basalts from Southern Australia,
reported by Irving (1974), contain MnO in similar amounts (0.10—0.18 wt.