Rit (Vísindafélag Íslendinga) - 01.06.1984, Side 189

mineralchemistry and relationships
185
26) and three porphyritic basalts (Jan 48, 69 and 146). Table 30 lists the
compositions and the temperature and ÍD2 values obtained for these and
other mineral pairs from the Jan Mayen rocks and Fig. 102 shows the ÍÖ2
versus T° plot. According to these determinations, these oxide pairs crystal-
lized at 1040 to 1090°C and at 10-10'4 to 10-9'9 atm fD2, close to the FMQ
buífer. These values are within the range of other published determinations
from basic extrusive rocks according to the collective data of Haggerty
(1976b). Compared to the data obtained by Anderson (op.cit.) on the
alkaline basalts of Tristan da Cunha both the temperature and the ÍD2 are
on average only slightly lower in the Jan Mayen case. The grains analysed
by Anderson are described as phenocrysts and could thus be expected to
have formed at somewhat higher temperatures and fD2 than the relatively
later grains in the Jan Mayen case. The only groundmass pair analysed by
Anderson gives considerably lower values, or 890°C and 10-13'8 atm. The
temperature and fD2 values obtained for these oxide grains in the Jan
Mayen basic rocks are thus not unreasonable and give ~1060°C as the
temperature of the magma slightly before crystallization of the groundmass
itself and a fD2 value of ~10-101 atm.
Only three samples give information on the temperature and ÍD2 condi-
tions of the magmas at an earlier stage of oxide crystallization. One of these
samples is a porphyritic basalt (Jan 27), which contains discrete grains of
ilmenite and titanomagnetite occurring as inclusions in titan-salite pheno-
crysts. A pair of these, included in the same phenocryst, was analysed and
gives 1150°C and 10-7 6 atm f02. This is the same temperature at which the
late spinels were believed to have crystallized. The sample further contains
megacrysts of sandwich type ilmenite and titanomagnetite. Analyses from
the inner parts of the megacrysts give 1080°C and 10-9'5 atm f02, which is
quite similar to the values obtained for the relatively late grains of the
groundmass. The origin of the megacryst is uncertain. At the extreme edge
of the megacryst a later overgrowth of these minerals has occurred. This late
overgrowth represents lower temperature than the other late minerals, or
975°C and 10-12 atm f02. The late overgrowth occurred in the basalt liquid
and could give the temperature and f02 of the liquid at the final groundmass
stage.
Jan 148 is a gabbro xenolith brought up by basalt magma and is
composed of the same minerals as characteristically occur as phenocrysts
(plagioclase, titan-salite and minor olivine and oxides) in the basalts. The
oxides are composite type grains of ilmenite and titanomagnetite, which give
1100°C and 10-8 4 atm fD2. The sample most probably represents cumulat-
ing phases from the crystallizing basalt magma. Jan 329 is a hydrous
mineral xenolith of uncertain relationship to the Jan Mayen magmas. It
contains coarse grains of sandwich type ilmenite and titanomagnetite, which
give 1210°C and 10-7 6 atm fD2.
If the titanomagnetite phenocrysts of the basalts are crystallized under