Jökull - 01.01.2013, Side 6

L. Kristjánsson
isfactory within error margins of a few percent. Búa-
son also noted that the mean inclination of the rema-
nence was +81◦ with a standard deviation of only 4◦.
The mean inclination in 827 Quaternary lavas >1 m.y.
old in Iceland (L. Kristjánsson, unpublished data) is
71◦ with a standard deviation of 10◦; current secular-
variation changes in the field direction are of the or-
der of 4◦ per century. We have later ascertained that
very little viscous or other secondary components are
present in the remanence (see below). Neither is there
any pronounced anisotropy of the magnetic suscepti-
bility χ in the core material (within 1%, in 9 speci-
mens measured) which might have explained the uni-
form remanence inclination. Assuming that the re-
manence is of primary origin, it is therefore possi-
ble that all the lavas in the 41–143 m depth inter-
val were emplaced as part of caldera-filling material
within a period which was short relative to geomag-
netic secular variation time scales, say less than a mil-
lennium. It may be mentioned that very uniform re-
manence directions have been found in sequences of
comparable volume, for instance in lava units at dif-
ferent levels within the large Quaternary tablemoun-
tain Hlöðufell in Southern Iceland (L. Kristjánsson,
unpublished data).
Búason (1971) also measured the susceptibility of
powdered samples from 41–200 m depth. He found
that material in drill cuttings from below 143 m depth
was similar to the core samples in this respect, and he
obtained an overall solid-rock average χ of 0.07 SI
volume units from his total of 34 samples.
Rock-magnetic studies on samples from the
drilling
Friðleifsson and Kristjánsson (1972) obtained strong-
field thermomagnetic curves in air on five samples
from the Stardalur core and two from cuttings be-
low 143 m. Curie points were in the range 560–
620◦C, with considerable drop in room-temperature
saturation magnetization being caused by the heating.
This points to the presence of fairly pure but cation-
deficient magnetite (Fe3O4), becoming oxidized to-
wards maghemite (γ-Fe2O3) and then converting to
hematite (α-Fe2O3) which is much less magnetic than
magnetite or maghemite. Similar behavior on heat-
ing is fairly often observed in samples from Icelandic
lavas, especially if they have suffered some in-situ ox-
idation or zeolite facies alteration (e.g. Kristjánsson,
1972 and unpublished data; Goguitchaichvili et al.,
1999).
It is known that the effective size of magnetite
grains greatly influences their magnetic behavior (see
Dunlop, 2002; Tauxe, 2010). Thus, in very small
grains (<1 µm) all the molecular dipole moments tend
to be aligned even in the absence of any external
magnetic field. If these small grains (called single-
domain grains) are non-spherical in shape, it can be
quite difficult to change their remanent magnetization
by external magnetic fields, i.e., they have high mag-
netic coercivity. Larger grains are composed of re-
gions having different magnetization directions. Such
multi-domain grains have low coercivity because the
domain boundaries will migrate under the influence
of moderate magnetic fields. One possible explana-
tion for the high intensity of natural remanence in
the core was considered to be the presence of single-
domain magnetite. This was tested by alternating field
(AF) demagnetization of the natural remanence in five
samples from the Stardalur core (Fig. 4 of Friðleifs-
son and Kristjánsson, 1972). Four demagnetization
curves had shapes which are fairly typical for Ice-
landic Tertiary basalt lavas, with median destructive
fields (MDFs, where half of the original remanence
has been removed) around 20 mT. Single-domain
magnetite (especially in elongated grains) may be ex-
pected to have higher values of this parameter which
is closely related to their coercivity. One of the five
core pieces which was unusually fine-grained and
highly oxidized, indeed had an MDF exceeding 60
mT. All five samples exhibited very stable directions
of remanence, varying by only 1–2◦ on AF demag-
netization to 25 mT peak field. This indicated that
almost no viscous remanence is present, a result con-
firmed by a six-month storage experiment. Other soft
(i.e. of very low coercivity) secondary components
such as might be caused by drilling equipment, were
also not in evidence.
The presence in a rock of a major magnetization
component due to single-domain grains can also be
to some extent inferred from the ratio Mr/Mi between
6 JÖKULL No. 63, 2013