Paleomagnetic laboratory,
University of Iceland
Historical (to 1968; see L. Kristjansson: Terra
Nova 5, 6-12, 1993).
The potential of the paleomagnetic method for geological
research in Iceland was first demonstrated in the early 1950’s by the work of Jan
Hospers, a Dutch Ph.D. student in Cambridge, U.K. His research was followed up
by prof. Trausti Einarsson of the University of Iceland, working with Thorbjörn
Sigurgeirsson who was at the time Director of the National Research Council.
Sigurgeirsson who became professor at the University in 1958, designed
equipment for making accurate remanence measurements on hand samples. With this
equipment and an alternating field (AF) demagnetizer, pioneering research on
the paleomagnetism of the lava pile in Iceland was done in the late 1950’s.
Among its notable early results were the demonstration of the suitability of
the AF method to remove viscous magnetization from basalts, and the discovery
of intermediate directions at boundaries between polarity zones in the lava
pile. In 1964, Sigurgeirsson installed an astatic magnetometer and improved
AF-demagnetizer. These were used e.g. for some measurements on core samples
from a large U.K.-Icelandic expedition in 1964-65.
In 1957, Sigurgeirsson set up
a geomagnetic observatory at Leirvogur near Reykjavik which has been in
continuous operation since then.
A typical
sampling locality for paleomagnetic studies: Mt. Tradarhyrna above Bolungavik
village, NW-Iceland, with approx. 50 lava flows of 15 Ma age (Geophys. J. Int.
155, 2003)
Developments
1968-80.
Leo Kristjansson was employed
in paleomagnetic research and magnetic surveys at the University of Iceland in
1968-69 and from 1971. An extensive paleomagnetic research effort on Icelandic
rocks (totalling some 2400 lava flows in several regional surveys) was begun in
1972 by N.D. Watkins of the University of Rhode Island in collaboration with I.
McDougall of the Australian National University, L. Kristjansson, and other
Icelandic scientists. Watkins also collaborated with G.P.L. Walker of Imperial
College on completing other projects in East Iceland. Most of the measurements
were made at the U.R.I. through 1978, but after the untimely death of Dr.
Watkins from cancer in 1977, the paleomagnetic measurements from the major
surveys were processed and written up by L. Kristjansson with the other
collaborators. A Foster spinner magnetometer, presented to the University by
Dr. Watkins in 1975, was in service for a few years.
Using a
proton magnetometer and other instruments designed by himself, Sigurgeirsson
carried out an aeromagnetic survey over most of Iceland at 3 km spacing in
1968-80. Kristjansson surveyed parts of the insular shelf by ship in 1972-75.
Magnetic anomalies observed reach 2 mT or more in amplitude. Various rock types
in Iceland were sampled and measured in order to aid in the interpretation of
the surveys; the average remanence intensity in basalt lavas is around 4 A/m
but values up to an order of magnitude greater occur sporadically.
Developments
1980-2005.
In order to maximize the
scientific return on the available resources, research at the University of
Iceland paleomagnetic laboratory has concentrated on simple measurements of
remanence directions in a large number of basalt lava flows. Mostly, these
efforts have had direct connections to stratigraphic mapping of various parts
of the 0- 15 Ma lava pile. Only a few minor studies have been made on e.g.
sediments, intrusives, or altered rocks.
Thermal
demagnetizations, microscope work, paleointensity measurements, and
rock-magnetism projects including anisotropy have not been carried out here to
any significant extent. Such studies tend to be time-consuming and require
expensive equipment; the results are often ambiguous and of limited relevance
to specific geological or geomagnetic problems.
Left: A stratigraphic section of 2.9 km thickness in Eyjafjördur, N-
Iceland. It is estimated to cover the time interval 9-5 Ma ago. 320 lavas were
measured. (Int. J. Earth Sci. 93, 2004)
Right: L.
Kristjansson collecting samples in Talknafjördur fjord, NW-Iceland 2005.
In 1978 the
University acquired an “Institut Dr. Förster” static four-probe fluxgate
magnetometer which has been in constant use since then. In this instrument the
x,y, and z components of the dipole moment of a 1” rock specimen are each
measured in eight positions, and the results averaged. It is very convenient in
use, and comparisons with results from other magnetometers abroad have given
excellent agreement. A “Molspin” AF tumbler demagnetizer was obtained in 1989,
replacing older equipment constructed at the Science Institute.
Due to the
high stability of the primary directions in the lavas (after removal of viscous
remanence, usually complete by 10 mT treatment), collecting four samples from
each lava flow and demagnetizing at 3-5 steps has been found to be quite
adequate to obtain excellent within-site agreement (average directions with a
95% confidence radius of 5° or less in recent years). Low-field susceptibility
data and thermomagnetic curves (obtained with a “Bartington” MS 2W furnace
meter which the University acquired in the early 1990’s) and ARM observations
(using an attachment provided with the Molspin demagnetizer) have aided in
interpreting some of the remanence measurements.
Emphasis has
been on obtaining results from lava flows with minimal alteration (< 100°C
secondary heating) and small tectonic tilts (< 6-8°). Most summers since
1979, of the order of 100 or more lava flows have been sampled by L.
Kristjansson. Mostly, lavas of > 2 Ma
age have been targeted, as in younger rocks stratigraphic complications occur
due to glaciations and rapid erosion. However, some studies have involved young
lava flows, e.g. on the “Skalamaelifell geomagnetic excursion” in SW-Iceland
estimated to have occurred some 42 ka ago. Part of a study on Quaternary lavas
around Skaftafell, SE-Iceland carried out by J. Helgason, also used the
laboratory’s facilities.
As a
by-product of these stratigraphic surveys, a large body of high-quality
paleomagnetic direction and intensity data from Iceland is available, in fact
mostly published in detail. These data are in many ways unique, as there are no
other regions in the world where a comparable number of relatively fresh lava
flows covering a complete 15-Ma interval are easily accessible. They have been
used for statistical studies of some overall properties of paleomagnetic field
in Iceland. It should be kept in mind that such studies have limitations due
firstly to the time elapsing between successive lava flows in the pile (which
is highly variable but averaging 5-10 ka), and secondly to the rather small
number of radiometric age determinations so far carried out on Icelandic rocks.
Nevertheless the analysis of these data has led to interesting results. Among
these is the observation of a significant reduction in the scatter of
paleomagnetic poles taking place since 15 Ma ago, and an estimate of the
variation of the mean geomagnetic dipole moment as the virtual pole moves away
from the geographic pole. See the following diagrams.
Left: Angular standard deviation of grouped virtual geomagnetic poles in
Icelandic lava flows. 4230 flows with a stable reliably determined remanence
direction are included in the top graph. Various rejection criteria are applied
in the lower ones (Phys. Chem. Earth 27, 2002).
Right: Variation of the Earth’s magnetic dipole moment with the latitude
(north or south) of the virtual geomagnetic pole. The diagram was derived from
remanence intensities in 3514 Icelandic lava flows. Triangles: arithmetic
averages; circles: geometric averages. Typical standard errors are shown.
(Phys. Earth Planet. Interiors 1999).
In addition
to the instruments already mentioned, the paleomagnetic laboratory has three
portable fluxgate magnetometers, two gasoline-powered 1” diamond core drills,
and various other minor equipment. The only permanent staff member is L.
Kristjansson; assistance with computer work etc. is provided by G. Jonsson
(part-time, since 1987), and students are employed occasionally in field work
and routine measurements. The laboratory has not been involved in any graduate
student theses. Through the years, rock samples and paleomagnetic data have been
supplied on request to several parties abroad; advice and logistic assistance
has also been provided to numerous expeditions.
In
continuation of the magnetic surveys mentioned above, aeromagnetic measurements
over parts of the shelf were carried out by the laboratory in 1985-86 and
1990-92. A detailed survey of the Reykjavik area, made in 1993, has also been
published. No surveys have been made subsequently, but the data from the older
ones have been digitized and interpreted. The laboratory owns two proton
magnetometers which are used for teaching purposes and as backup for
instruments at the University’s observatory.
Recent and current projects; opportunities.
Projects recently completed
include one on 76 lavas in the Fljotsdalur valley of East Iceland (paper
published in Jökull 55, 2005) and one on 17 igneous units from the Antarctic
Peninsula (paper published in Antarctic Science 17, 2005). A review of some
aspects of paleomagnetism and magnetic surveys in Iceland is appeared J.
Geodynamics vol. 43 in early 2007, and a paper on a project in Skagafjördur,
North Iceland (250 flows sampled in 2002-04) was published in Jökull 56, 2006. The
main current project concerns the Patreksfjördur-Arnarfjördur area of
NW-Iceland where 330 lavas of 12-13 Ma age were sampled for paleomagnetic
measurements in 2004-07. The laboratory has participated in investigations
initiated by the University of Aarhus in Denmark, comparing magnetic minerals
in Icelandic rocks with those analysed by Mars landers (paper published in
Phys. Earth Planet. Inter. 154, 2006).
In the last decade or so, only a handful of foreign
expeditions have visited Iceland to collect samples for paleomagnetic or rock
magnetic studies. However, many opportunities exist for interesting and
significant projects in both fields. This applies e.g. to stratigraphic mapping
and correlation, analysis of long-term properties of the secular variation,
radiometric dating of geomagnetic polarity transitions and major excursions,
within-unit variations of magnetic properties, magnetic anomaly modelling, and
effects of hydrothermal alteration.
(Revision
Dec. 2007, L.Kr.)
Bibliography on
paleomagnetism and magnetic surveys in Iceland