Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 236
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PÁU. ÍMSI.AND
oxides of Jan 69 (a basalt containing apatite phenocrysts) give a tempera-
ture of 1059°C. These titanomagnetite phenocrysts and the bulk of the
apatites would be crystallized under similar conditions, as indicated by the
close association of their appearance. The bulk of the apatites are thus most
probably crystallized around 1100°C in the case of the basic magmas.
Referring again to the conditions of oxide and feldspar crystallization, the
apatite crystallization in basic magmas would have taken place between 40
and 5 km depth. In the case of the more evolved rocks the apatite
crystallization seems to have taken place at similar temperatures but at less
pressure. The syenitic xenolith (Jan 324) contains two feldspars and the
compositions of these indicate that they were crystallizing around 1100°C at
the depth of 3 to 9 km. The apatites of this sample are most probably formed
under similar condidons. There is thus no contrasting difl'erence between
the temperature of formation of the apatites of basic and evolved magmas in
the Jan Mayen case. However, the pressure of this crystallization may
apparently, be quite diíferent. In the evolved liquids it seems to take place
only above 9 km depth, but in the basic rocks, apatite seems to be able to
crystallize down to 40 km depth. This depth difference may be of import-
ance, especially in connection with the supposed CI escape in a fluid phase,
as mentioned above.
H. MICAS
I. Introduction
Micas of two categories occur in the Jan Mayen rocks.
Micas of phlogopite composition occur as small allotriomorphic flakes in
the groundmass of the basic lavas and as more or less euhedral flakes,
extending from the walls into the vesicles and cavities of these lavas. The
xenolithic wehrlite (Jan 151) contains small amounts of phlogopite as well.
Here the phlogopite occurs as minute flakes or blades in very small cavities
at olivine-clinopyroxene grain contacts. The amount of the phlogopite in
each sample is in all cases small, probably not exceeding 1 vol. per cent.
These micas are unaltered.
Phenocrysts of biotite mica occur in the intermediate lavas. l'he pheno-
crysts are euhedral in shape, but with occasional embayments. The amount
of these phenocrysts is small in all cases, not exceeding a few vol. per cent of
the rock. In the hydrous mineral xenolith (Jan 329) compositionally similar
biotite is present and amounts to 12 vol. per cent of the rock. Here the biotite
occurs as panidiomorphic crystals of considerable size.
In some of the samples the biotite phenocrysts have suffered a secondary
alteration (see Figs. 41 & 42). Where most intense, this alteration has
resulted in a total breakdown of the original mica, which is now replaced by
a fme-grained aggregate ofopaque oxides and silicates. Where the alteration