Jökull - 01.12.1984, Síða 69

Notes on paleomagnetic sampling in Iceland
LEÓ KRISTJÁNSSON
Science Institute
University of Iceland, Dunhaga 3, 107 Reykjavík
ABSTRACT
This paper presents various generalpaleomagnetic
results from Iceland, which may be of use in
stratisgraphic research, especially in the mapping
of polarity zones in the field. It includes estimates
of mean magnetization intensities in basalts and
angular frequency distributions of remanence dire-
ctions, as well as a discussion ofseveral problems
commonly encountered in stratigraphic correla-
tion usingpaleomagnetic methods. Advice on pro-
cedures and precautions for this type ofresearch is
given, based on experience in several large
surveys.
INTRODUCTION
During the last 12 years or so, the present
author has participated in quite extensive paleo-
magnetic studies on lavas from several Miocene
to early Pleistocene areas in Iceland. Results
from these studies have been published in several
papers, each dedicated to a specific area or topic.
In the course of this research, various results
have also been obtained, which were too general
for inclusion in the above papers, but which may
be of some value to other current and future
studies of related character in Iceland and else-
where. Specifically, one is aiming to improve the
method of mapping of lava magnetic polarities as
a reliable stratigraphic tool, both as done on hand
samples in the field, on cores from wireline drill-
ing operations, in the laboratory, and by
interpretation of aeromagnetic anomalies. This
paper relates some of the above mentioned gene-
ral results. It presupposes knowledge of certain
physical principles and measurement methods, as
outlined e.g. by Kristjansson (1983).
REMANENT AND INDUCED
MAGNETIZATIONS
Any sample of rock contains both a remanent
and an induced magnetization vector. The natu-
ral remanence (N.R.M.) is the vector sum of an
original thermal remanence (T.R.M.) and secon-
dary magnetizations such as viscous (V.R.M.)
and isothermal (I.R.M.) remanences.
Of these, the I.R.M. is manifested mostly as
lightning remanence or drilling remanence. The
former occurs mostly on ridge and crag outcrops;
it is rare in NW- and N-Iceland, occurs occa-
sionally in Esja and Borgarfjördur, but is com-
monly observed in the Snaefellsnes and Reyk-
janes peninsulas. It is easily recognized by its
effect on a compass close to the outcrop, by its
very high intensity and by a coercivity of only
100—250 Oe. (1 Oe = 10-4 T). Drilling reman-
ence also has a low coercivity and is very
inhomogeneous within samples. It is probably
caused by a combination of high fields and shear
stresses near some drill bits and/or tools used in
extracting cores.
V.R.M. is in most samples of small intensity
compared to the original T.R.M. before a.f. de-
magnetization treatment, and is eliminated by
100—150 Oe peak a.c. fields. In NW-Iceland, the
intensity of V.R.M. may average 0.4 A/m, but in
other areas investigated it seems to average 0.2
A/m. (1 A/m = 10-3 Gauss). However, it may be
envisaged that the V.R.M. resides in large or
unexsolved titanomagnetite grains in a sample,
whereas the T.R.M. lies in small, elongated and/
or exsolved grains. Accordingly, there is a gene-
ral inverse correlation within lavas, of V.R.M.
and induced magnetization on one hand and
original T.R.M. on the other, and in a significant
number of cases the V.R.M. dominates. Thus, in
about 6% of our lava samples from Snaefellsnes
and Myrar collected in 1980—82, directional
changes between N.R.M. and 200 Oe demagnet-
ization exceeded 90° (whereof about 1% may
include lightning effects). This is of course most
noticeable in lavas having reverse T.R.M., as the
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