Jökull - 01.12.1980, Side 42

Fig. 7. The ratio A/B (see Fig. 5) for double
Curie point samples plotted as a function of
Maghemitisation index Mh.
Mynd. 7. Hlutfallið A/B (sjá Mynd 5) fyrir sýni
með tvö C, teiknað sem fall af ummyndunarstuðlinum
Mh.
relative proportions of saturation magnet-
isation contributed by each component (this is
necessarily only approximate because a frac-
tion of the initial saturation magnetisation of
the high Curie point component will be de-
stroyed when the temperature of the lower
Curie point is reached). When the ratio A/B is
plotted against Mh (Fig. 7) A is observed to
decrease with respect to B in increasingly alt-
ered samples. As already observed, Mh corre-
lates closely with the degree of low tempera-
ture alteration, and it thus appears that
regional hydrothermal alteration has the
effect of raising the low Curie point towards
the high Curie point until it is ultimately in-
distinguishable.
Ade-Hall et al. (1971) suggest that in a
moderately-deuterically oxidised basalt a few
unoxidised grains may yield a low Curie point
(0—300°C) whilst even only slightly-oxidised
grains will result in a high Curie temperature
(500—580°C) because the ilmenite lamellae
then occur within Ti-deficient grains. When
deuteric oxidation state (M) exceeds a value of
about 3.3 a single high Curie point only results
because there are then few if any unoxidised
grains left in the rock. Increase in regional
hydrothermal alteration also causes low Curie
temperatures to migrate towards the high
Curie temperatures until they are too close to
be resolved. According to these authors this
occurs at around zone E of the regional depth
burial zonal sequence.
The Reydarfjördur dykes lie within zones D
and E (Walker 1960, Gibson et al. 1966, Ade-Hall
et al. 1970) and predictably a great many of
the Curie points lie within the high tempera-
ture bracket. The degree of alteration also
accounts for the fact that many of the low
single or double Curie points that might be
expected from deuterically oxidised dykes
where M never exceeds 3 have been elevated
to the higher temperature bracket. There are a
few exceptions (Table 1) and these may dis-
tinguish late stage dykes that postdate most or
all of the regional hydrothermal alteration.
In conclusion, we find in the dykes of
Eastern Iceland a positive correlation between
titanomagnetite deuteric oxidation state and
magnetic polarity in addition to a number of
ancilliary correlations explicable in terms of
either deuteric or hydrothermal alteration.
The polarity-oxidation correlations is in the
same sense as that found in other Tertiary and
Mesozoic igneous rocks from intermediate to
high palaeolatitudes. Ade-Háll and Watkins
(1970), however, found no significant corre-
lation in Canary Islands volcanics and Reilly et
al. (1976) also reported no correlation in other
low latitude volcanics írom East Africa but
did not give the experimental basis of their
investigation. The explanation of these
various results is, at present, uncertain.
ACKNOWLEDGEMENTS
The samples were collected on an expedition run
under the auspices of Educational Expeditions Inter-
national (E.E.I.). We are very grateful to Dr. I. L.
Gibson, Dr. L. Henage and student members for
valuable discussions, and the National Research
Council of Iceland for permission to undertake this
work.
40 JÖKULL 30. ÁR