Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 231

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mineral chemistry and relationships
Mayen rocks the apatites of the more evolved rocks, trachytes, tristanites
and syenites, are clearly richer in Si than those of the basic rocks. 1 he
average of all analysed grains of the evolved rocks is 0.46 wt. per cent SÍO2
(r=0.14-0.97), while the average of the basic rock apatites is only 0.28 per
cent (r=0.17-0.53). The silica contents of apatites are thus apparently
related to the silica content of the liquid.
SrO (Fig. 115) was detected in all but one apatite of the extrusive rocks,
but only detected in two apatites of the coarse-grained xenoliths. The
concentrations are low, ranging up to 0.32 wt. per cent SrO. On the average
the SrO content of the apatites of the basic lavas is only slightly higher (0.17
wt. per cent) than that of the apatites of the intermediate lavas (0.13 wt. per
cent). The apatites of the Skaergaard intrusion (Nash, 1976) are low in SrO
(0.04—0.13 wt. per cent), while apatites of the Leucite Hills volcanic rocks
(Carmichael, 1967) are higher (0.14—0.46 per cent) and those of the
Shonkin Sag laccolith intrusion (Nash, 1972) are still higher (0.90— 1.60 per
cent). The “strontium fluor-apatite” given by Deer et al. (1962) containing
11.60 wt. per cent SrO, a major component concentration, comes from a
syenite dike in Montana. Apparently apatites of intrusive rocks may thus be
both poor and rich in Sr, depending on the composition of the rocks, relative
to those of extrusive rocks. This indicates that the Sr concentrations of the
bquid (or its crystallization history) is a most effective parameter in
controlling the Sr content of the crystallizing apatite. 1 he Sr rich Shonkin
Sag apatites e.g. crystallize from a liquid crystallizing no plagioclase (Nash
& Wilkinson, 1970 & 1971). This, most probably, is due to enrichment of Sr
in the liquid as the crystallization goes on. A late apatite is then the most
likely Sr host if alkali feldspar tends to reject Sr as it frequently does. I he
coarse-grained Jan Mayen rocks, on the other hand, crystallize plagioclase
at some stage. Jan 329 crystallizes apatite first and plagioclase later. The
apatite is practically Sr free while the plagioclase contains from 0.24 to 0.38
Wt. per cent SrO. This indicates that the distribution coefficient of Sr
between apatite and liquid is highly dependent on the Sr content of the
bquid; compositionally dependent, as numerous trace element distribution
coefficients have been shown to be.
FeO (Fig. 115). Iron was detected in all the analysed apatites. In the
syenitic xenolith, the iron-titanium oxides, with which the apatites are
closely associated, as previously described, are highly oxidized. The apatites
have suffered some alteration as well, which expresses itself in a brownish
colour caused by minute iron oxide needles in the apatite. 1 he mechanism
behind this alteration is unknown. That the iron is transported into the
apatite from the iron-titanium oxides seems to be a more likely process than
cxsolution of iron from the apatite itself (which is generally poor in iron), as
the intensity of the colour (and hence the amount of the iron rich needles) is
frequently strongest along cracks in the apatite crystals. The needles
themselves are far too small for analyses but analyses on the darkest areas of