Jökull - 01.12.1972, Síða 75

105.4 M DEPTH
It is evident from this table that strong
contributing factors to the broad magnetic ano-
maly are the fine grained near surface intru-
sives (dykes, sheets, plugs). The magnetization
values depend very much on the chemical com-
position of the rocks, with the iron rich varie-
ties (tholeiite and basaltic andesite) having
higher magnetization than the rocks with lower
iron contents (olivine tholeiites).
The only intrusive to be magnetically studied
in detail was a basaltic andesite plug at Múli.
Eight normally magnetized samples were found
to be rather poorly stable, with an average
median destructive a.c. field of 120 Oe and a
mean remanence direction of 320°, + 70° (a 95
= 9°) after 100 Oe treatment. Tlie Curie point
of one sample was 400 °C.
DllILL CORE
A 200 m deep drill liole was sunk at the
center of tlie above mentioned maximum in
1969—1970. Below fresh olivine tholeiite lava
flows, ancl tuffs, both of low magnetic inten
sity, the drill entered highlv magnetic tholeiite
flows at 41 m depth (Fridleifsson and Tómasson
1972; Fig. 3 of Steinthórsson and Sigvaldason
1971).
Natural remanent magnetization (NRM) in-
tensity and inclination values were measured
in 103 normally magnetized specimens from the
drill core between 41 ancl 143 m depths, using
an astatic magnetometer (Búason 1971). The
inclination values averaged 81°, with a stand-
ard deviation of 4° which is much lower than
would be expected from the long-term disper-
sion of geomagnetic field directions in Upper
Cenozoic times; this dispersion is about 22° in
NW-Iceland basalts (Kristjánsson 1970) and 10°
in the present and archaeo-magnetic field. The
Stardalur flows may therefore have been em-
placecl in a time interval rnuch shorter than
the major secular variation time scales (~ 2000
years), a situation similar to the Surtsey volcan-
ism. No useful indication of the age or the
tilt of these flows can be obtained from mag-
netic results.
The arithmetic average NRM intensity of the
103 specimens is 61 • 10~3 Gauss, and the average
susceptibility of 34 specimens is 5.5 • 10-3 Gauss/
N
MOMENT
(arbitrary units)
\
\
\
Hex = 400 OE
r
v
w
6Ó0°C Temp.
Fig. 3. Strong-field thermomagnetic curves ob-
tained from two Stardalur speciinens in air.
Procedure is as in Deutsch et al. (1971). Curves
from otlier pilot specimens and two specimens
from below 143 m were intermediate in shape
between these. Hex = external field; Tc =
Curie point.
Mynd 3. Segulhitaferlar í sterku sviði. Hcx fyrir
sýni úr tveim dýptum í Stardalsborholu 1. T0
= Curie-hitastig.
Oe, yielding a total magnetization of 64 • 10~3
Gauss. The correlation coefficient between these
variables in 23 specimens was 0.48.
JÖKULL 22. ÁR 73