Jökull - 01.01.2013, Blaðsíða 9

The Stardalur magnetic anomaly, SW-Iceland
to the tip of the Kjalarnes promontory (Figure 1).
Gunnlaugsson et al. (2008) obtained Mössbauer spec-
tra on strongly magnetized intrusive rocks from the
Kjalarnes coast, suggesting that their remanence re-
sides in titanomaghemite.
Pálmason’s (1971) results on the upwarping of his
crustal Layer 3 at Stardalur were confirmed by Bjar-
nason et al. (1993). They found that the depth to the
lower crust (defined by a sharp change in the p-wave
velocity gradient at about 6.5 km/s) was 2.5 km at
Stardalur. A similar upwarping of the lower crustal
layers from its mean depth of 4.5 km has been noted
beneath both active and extinct volcanic complexes in
Iceland (Flóvenz and Gunnarsson, 1991; Brandsdóttir
and Menke, 2008).
RESEARCH ON THE STARDALUR
CORE IN 2008–2010
We have carried out measurements of magnetic sus-
ceptibility in those 95 of Þ. Búason’s (1971) 102 3-
cm core specimens which are still preserved at the
University of Iceland, using a Bartington MS2 audio-
frequency susceptibility bridge (lower graph of Figure
2). The average value of susceptibility χ in these (in-
cluding data for two of the lost specimens) is 0.067 SI
units, similar to that found by Búason (1971). A lower
average evident in Fig. 2 of Vahle et al. (2007) appears
to be due to a calibration error. Within- and between-
lava variations in susceptibility in the core are less
pronounced than those in remanence intensity (Fig-
ure 2), as is generally the case with lava flows (e.g.
Table 1a of Bleil et al., 1982). There is a slight posi-
tive correlation (R2 = 0.31) between the susceptibility
and the NRM intensity, assuming direct proportion-
ality. A positive correlation is in agreement with the
observation of Steinþórsson and Sigvaldason (1971)
who noted that the oxidation state of the titanomag-
netite was always high: large variations in oxidation
would tend to cause a negative correlation between
these parameters (Wilson et al., 1968). Vahle et al.
(2007) who fitted the susceptibility and intensity data
from 20 samples (including some at less than 41 m
depth) with a power law, had found a higher correla-
tion coefficient. Roughly, the susceptibility of a basalt
sample is proportional to its magnetite content, with 1
vol. % magnetite causing a χ of 0.027 SI units. If al-
lowance is made for the variable purity of magnetite
in Icelandic Tertiary basalts, it may be estimated that
they contain on average about 1% of magnetite by vol-
ume, and the Stardalur lavas 2.5%.
The question of the presence of single-domain
magnetite in the Stardalur core has already been con-
sidered above. A common procedure for evaluating
the overall domain state of a magnetic sample is to
take it through a hysteresis loop to its saturation mag-
netization. The ratio of saturation remanence to sat-
uration magnetization (Mrs/Ms) obtained from such
a loop may be plotted against the ratio between two
coercive forces (Hcr/Hc), i.e. the external fields re-
quired to null the remanence and the total magneti-
zation respectively. A relatively high value (0.5–1)
of the former ratio in a sample combined with a low
value (which usually means less than 1.5 or 2) of the
latter is considered to indicate the presence of single-
domain magnetite. Several published studies on Ice-
landic basalt lavas, connected with attempts at deduc-
ing paleo-field intensities (e.g. Fig. 3a of Brown et al.,
2006), have shown that their Mrs/Ms vs. Hcr/Hc val-
ues mostly fall in an intermediate category on these
so-called Day plots. The intermediate behavior may
be due to all the magnetic grains being composed of
a small number of magnetic domains each (termed
pseudo-single-domain grains), and/or to the sample
consisting of a mixture of single-domain and multi-
domain grains. Such plots therefore provide only a
rather qualitative criterion for the effective grain size,
and opinions on their detailed interpretation have var-
ied considerably (Dunlop, 2002).
Day-plot data reported by Kristjánsson et al.
(2010; S. A. McEnroe, pers. comm., 2010) on 12
samples from the original Stardalur core showed that
they had Mrs/Ms values well below 0.5, like Ice-
landic lavas in general. Further work on these as-
pects is in progress. Alternating field demagnetiza-
tion of NRM was carried out on small arbitrarily ori-
ented pieces from seven strongly magnetized samples
(depths: 47.6, 59.8, 84.3, 87.7, 96.6, 101.0 and 139.7
m), using a Molspin two-axis tumbler demagnetizer.
As had been found for four samples in 1970 and the
JÖKULL No. 63, 2013 9