Jökull - 01.01.2009, Page 48

Leó Kristjánsson
would have an inclination at +77.2◦ in the present
area. The average of virtual poles from all the lavas is
at 88.3◦N, 130◦E, with a vector sum of R = 308.5, and
a 95% confidence angle of 3.1◦. It should be noted
that the occurrence of groups of low-latitude virtual
poles may cause some bias in these averages, for in-
stance the ten flows VR 9–10, VS 3–8, 29–30 with
similar poles near the Equator.
Low-field thermomagnetic curves were obtained
in air from five samples, using a Bartington MS2W
furnace and susceptibility meter. These curves have
quite variable shapes, as noted also by Kristjánsson et
al. (2003). Their final Curie points are in the range
500–600◦C but there is always evidence of some al-
teration and irreversibility on heating, cf. also the next
paragraph. It may therefore be difficult to find lava
samples in Northwest Iceland suitable for paleointen-
sity studies by conventional methods.
Kristjánsson (1967) contemplated the possible use
of magnetic properties as a geothermometer, i.e. an
indicator of maximum temperatures experienced by
lava flows on burial within the pile. If a significant
permanent change occurs in for instance the rema-
nence intensity, coercive force, or susceptibility of a
stably magnetized sample upon heating in the labora-
tory to a temperature T and cooling to room tempera-
ture, a simple interpretation would indicate that previ-
ous secondary heating of the sample while still in situ,
did not reach T. This simple model was tested by heat-
ing eight very stable lava specimens of both polarities
from near sea level in Arnarfjörður and Tálknafjörður
to 100◦C and 160◦C in field-free space. The room-
temperature susceptibilities increased in all cases, by
an average of 2.5% and 6% respectively, and inten-
sities had decreased altogether by an average of 1.5%
after the 160◦C heating. Similar results were obtained
by Kristjánsson (1967) on samples collected near Ísa-
fjarðardjúp and heated in air.
As a precaution against oxidation of the magnetic
minerals, the heating to 160◦C was in the present
case carried out in oil. It may be tentatively con-
cluded that the exposed lava pile in these two parts
of the Northwest peninsula has not experienced heat-
ing above 100◦C. Reasonable estimates of geothermal
gradients, as well as the observed near-absence of ze-
olite infillings in most of the present profiles, also
make it unlikely that temperatures at the present sea
level ever reached 100◦C (cf. Kristjánsson and Jóns-
son, 2007, p. 33).
CONCLUSIONS
This study is a first attempt to i) use variations
in the geomagnetic field, along with other infor-
mation, to strengthen correlations between strata on
the three southwesterly promontories of the North-
west peninsula of Iceland, between Arnarfjörður and
Breiðafjörður ii) set up a common scheme of polar-
ity zones for the area. The lava pile in the area has
suffered little alteration, compared to other areas of
similar or even younger age in Iceland. These lavas
may therefore present good opportunities for radio-
metric dating of polarity-zone boundaries and geo-
logical events, as well as for geochemical investiga-
tions. Measurements of magnetic remanence direc-
tions yielded very stable and consistent results, and
the possibility of using these and other magnetic prop-
erties as a qualitative geothermometer in the basalt
lava pile is suggested. Like other Icelandic basalts
however, the lavas in this area do not seem to be ideal
material for paleointensity studies using conventional
whole-rock methods.
Polarity zones in the Arnarfjörður-Breiðafjörður
area appear to be relatively thick (for instance over 40
lavas in profiles TE and AB), and there are other in-
dications that the lava production has sometimes been
proceeding quite rapidly. Thus, Table 2 includes some
50 cases of two or more successive lavas having rema-
nence directions that are similar within 10◦ of arc or
less. This may be compared to current secular varia-
tion rates of the order of 5◦ per century, and the num-
ber of such groups is well in excess of what may be
attributed to chance. In addition to flows VS 3–8 and
AC 1–6 mentioned above, three or more similar suc-
cessive directions occur for instance in TE 7–13, AJ
7–10, RS 8–11 and TW 6–9.
It appears that by the use of reversals and large ap-
parent geomagnetic excursions (Figure 4a,b) recorded
in the lava pile, correlation has been established be-
tween all the profiles FD and TR/TS on the south
side of Tálknafjörður, VR on the north coast of Pat-
48 JÖKULL No. 59