Jökull - 01.12.1980, Page 41

lation also observed by Ade-Hall et al. 1968).
Maghemitisation is not evidently controlled
by deuteric oxidation state (Fig. 5), but does
exhibit a close correlation with the degree of
hydrothermal alteration as estimated
qualitatively from thin sections.
THERMOMAGNETIC
CHARACTERISTICS
Thermomagnetic spectra (saturation mag-
netisation, Js, versus temperature) were
measured for the 78 dykes by heating steadily
in air to 700°C and using a strong permanent
magnet to saturate samples. Sixty-two dykes
exhibit a single Curie point and 16 dykes
exhibit two Curie points with the highest
populations occúrring at 350—450°C and
500—550°C (Fig. 5). The Curie points are
defined to í 10°C as shown in Fig. 5 and could
be reproduced for individual dyke samples to
Fig. 5. The definition of single and double
Curie points from thermomagnetic curves,
and a histogram of the temperature dis-
tribution of Curie points for the 78 dykes of
this study (right hand side). The vertical axis
refers to the number of samples (N) and the
horizontal axis to the Curie point (Ct) in
degrees centigrade. Double Curie points are
alphabetically paired together.
Mynd 5. Vinstra megin: skilgreining á Curie-hita-
stigi (CJ bergsýnis. Hægra megin: súlurit Curie—
hitastiga úr göngunum 78; bókstafir eiga við sýni með
tveim Cfgildum.
Fig. 6. Distribution of Curie points (Ct) with
respect to oxidation state (M) for the 78 dykes.
Closed circles are single Curie points and open
symbols are double Curie points.
Mynd 6. Tengsl Curie-hitastigs og oxunarstigs í
göngunum.
a few degrees by remeasuring. Of the 16 dykes
with double Curie points, 11 pertain to dykes
with no visible alteration, 3 to slightly altered
dykes and only 2 to highly altered dykes; in
addition all of these 5 hydrothermally altered
dykes with double Curie points exhibit virtu-
ally no deuteric oxidation and 80% of the
dykes with double Curie points are unoxid-
ised. It is also noteworthy (Fig. 6) that the
temperature field of single Curie points is very
large in unoxidised dykes but becomes res-
tricted to a narrower and higher range of
temperatures as the oxidation class increases
to 3.
The saturation magnetisation axis of Fig. 5
is arbitrary but some comparison of the phases
in the dykes with double Curie points can be
made by comparing the ratio A/B or the
JÖKULL 30. ÁR 39