Jökull - 01.01.2013, Side 10

L. Kristjánsson
majority of Vahle et al.’s (2007) nine samples, the
median destructive fields (MDFs) of their remanence
were in the range 17–23 mT.
Gradual systematic changes in remanence direc-
tions during the AF demagnetization were noted in
some cases. The cumulative amount of these changes
was of the order of 3◦ of arc or less after 60 mT treat-
ment (when 1–9% of the NRM intensity was left).
Such changes might be due to a small amount of
their magnetization being a secondary high-coercivity
CRM in the magnetite that is derived from olivine.
Alternatively, this could reflect a gradually changing
direction of the ambient field during original cooling
of the lavas through the temperature interval in which
their main remanence component became fixed.
On storage of the seven samples in the Earth’s
field for over two years, the intensity of the VRM ac-
quired amounted to only about 0.1% or less of their
original NRM intensity. Subsequently, these seven
samples were demagnetized at 90 mT and then given
an anhysteretic remanence (ARM) by subjecting them
simultaneously to a steady 50 µT magnetic field and a
parallel sinusoidally alternating field of amplitude 90
mT for five seconds. The steady field was supplied
by an attachment furnished with the Molspin demag-
netizer. The characteristics of ARM have generally
(Soffel, 1991) been found to be analogous to those
of TRM. In each of the seven samples the MDF of the
ARM was a little lower than the MDF of its NRM: av-
erages were 18 and 20 mT. With stronger alternating
fields (not attainable in our equipment) these averages
would have been even more similar, which supports
the idea that the NRM in the Stardalur core is TRM.
The intensity of the ARM in the seven samples was
only a fraction (7–22%) of their NRM intensity, but
comparison studies on other Icelandic basalts would
be required for an interpretation of that observation.
NEW GROUND SURVEY AT
STARDALUR IN 2011
A ground magnetic survey of the main peak of the
Stardalur anomaly was carried out on five magneti-
cally quiet days in 2011. The number of points mea-
sured was 432, using Barringer GM-122 and Geo-
metrics G-856 proton free-precession magnetometers.
The probe height was 2 m above ground. Positions
were obtained from a hand-held Garmin Etrex GPS
instrument to about 4 m relative accuracy. The survey
was intended to delineate approximately the shape of
the main peak of the anomaly within the 58 µT con-
tour, and to improve estimates of the highest fields
attained. Results are shown in Figure 3, after some
smoothing (F. Pálsson, pers. comm., 2011) to atten-
uate small-scale features clearly caused by the land-
scape and rocks in the top few meters of the ground.
Maximum field values observed were in the range
81.2–81.4 µT. We have not managed to locate the drill
hole 1 from 1969–1970.
The anomaly peak strikes 20–25◦ east of north.
This is not very different from the strike of the fissure
swarm transecting the Stardalur volcanic center which
is 30–35◦ east. The similarity in trends indicates
that the source rocks of the Stardalur anomaly peak
were generated in eruptions on a short fissure. The
broad aeromagnetic lineations in southwestern Ice-
land which are probably due to Brunhes, Matuyama
and Gauss age formations (Jónsson et al., 1991) reach
greater deviations from north. This could be due to
lateral offsets between neighboring fissure swarms, as
is evident in the Reykjanes peninsula (Sigurgeirsson,
1970a; Kristjánsson et al., 1989).
The anomaly peak was simulated with a simple
model of its source (Figure 4). With the following
constraints (which are not very realistic), a tolerable
fit to the smoothed contour map of Figure 3 was ob-
tained:
– The source is assumed to be composed of three flat-
topped vertical columns in partial contact. Each has
a square cross section, and a total magnetization in-
tensity Mt of 64 Am−1 in a vertical direction. The
columns reach great depth.
– That part of the observed field in Figure 3 which is
due to sources other than the above columns, is as-
sumed to have a constant strength F of 57.6 µT, an
inclination of +75◦ and a declination of 0◦.
– Increasing altitude of the ground towards the east
by up to 30 m east of the road to the farm was al-
lowed for. Otherwise the altitude of the probe above
sea level was assumed constant at 190 m, and minor
10 JÖKULL No. 63, 2013