Jökull - 01.12.1976, Page 30

DYPI
NÁMASKARÐ
HOLA B-6-
B-3 «
KRÍSUVÍK
HOLA 2
(Susceptibility) (Susceptibility)
Fig. 6. Susceptibilities of samples of drill chips
collected at 10 m intervals during drilling in
two deep thermal drill holes. Running average
shown as a thin curve. To convert to volume
units, in solid rock, multiply by approx. 2.5.
Mynd 6. Segulmœlingar á sýnum af svarfi úr
tveim djúpholum.
formations. This can be understood with refer-
ence to Fig. 5 which plots the room-tempera-
ture susceptibility of six fresh basalt samples as
a function of the maximum temperature to
which they have been heated. The samples are
of three types:
The top two samples are known to have a
Curie point of about 500° C with a minor low-
temperature component; the middle two have
a Curie point of about 560° C only, and the
bottom two samples have a dominating low-
temperature component. It is seen tliat tlie sus-
ceptibility of the samples with low Curie-point
components may increase significantly on heat-
ing to 200° C in air (possibly also on heating
to lower temperatures over longer periods) due
to formation of magnetite. The susceptibility
of all samples decreases on heating beyond
500° C, because of conversion of magnetite to
other minerals.
In rock samples containing a high-Curie-point
magnetic mineral (essentially pure magnetite)
only, it appears that the susceptibility is pri-
marily an indicator of the magnetite content
of the rock. Fridleifsson and Kristjánsson (1973)
have shown by saturation magnetization mea-
surements, that 1% magnetite by volume in this
case causes a susceptibility of about 2.2-10"3
c.g.s. volume units.
In order to test the variations of magnetite
content in subsurface basalt formations in Ice-
land, susceptibility measurements were made on
1300 samples (10—100 gm) of drill chips from
eiglit deep drill holes. The result are plotted
in c.g.s. weight units in Fig. 6 as function of
depth, for two of these. Others are shown in
Kristjánsson and Watkins (in press, 1976).
Only in the Námaskard drill hole has the
magnetite been destroyed by thermal activity;
in the others susceptibilities are commonly in
the range (0.5—2.0) • 10-3 weight units. This is
comparable with the means 1 to 5 times 10-3
volume units, given by Piper (1973) respectively
for Cenozoic basalt lavas and dykes in Icelandic
outcrops (excluding pillow basalts).
The susceptibilities in the drill holes are
quite variable and not well correlated with
lithology (Tómasson et al. 1974) but zones of
fairly non-magnetic material occur, e. g. at
1300—1500 m depth at Nesjavellir and 250—350
m depth at Reykjanes.
A large magnetic low occurs at Námaskard,
about 4000 y by 2—3 kilometers according to
National Energy Authority reports (1971), but
the area of magnetite destruction at 500 m
depth appears to be very narrow, since samples
from a drill hole 100 m distant (B 3 of Fig. 6)
are moderately magnetic.
CONCLUSIONS
Lateral variations in the local magnetic field
may present the largest obstacle to reliable
interpretation of drill hole magnetic surveys in
terms of the total magnetization of the rocks
in the immediate vicinity of the probe. Varia-
tions in this field are of order ± 1000 y in 100
m at 5 m above ground, and of order ± 1000 y
28 JÖKULL 26. ÁR