Table Of ContentOrigin of Thermoremanent Magnetization
ADVANCES IN EARTH AND PLANETARY SCIENCES
General Editor:
T. RIKITAKE (Tokyo Institute of Technology)
Editorial Board:
S. AKASOFU (University of Alaska)
S. AKIMOTO (University of Tokyo)
Y. HAGIW ARA (University of Tokyo)
H. KANAMORI (California Institute of Technology)
C. KISSLINGER (University of Colorado)
A. MASUDA (University of Kobe)
A. NISHIDA (University of Tokyo)
M. OZIMA (University of Tokyo)
R. SA TO (University of Tokyo)
S. UYEDA (University of Tokyo)
I. YOKOYAMA (Hokkaido University)
Special Issue of Journal of Geomagnetism and Geoelectricity
Origin of
Thermoremanent
Magnetization
Proceedings of AGU 1976 Fall Annual Meeting
December 1976, San Francisco
Edited by
David J. Dunlop
•
Center for Academic Publications Japan
Japan Scientific Societies Press
© CENTER FOR ACADEMIC PUBLICATIONS JAPAN, 1977
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JSSP No. 01362-1104
ISBN-13: 978-94-010-1288-1 e-ISBN-13: 978-94-010-1286-7
DOl: 10.1007/978-94-010-1286-7
Preface
Physical and chemical studies of the earth and planets along with their sur
roundings are now developing very rapidly. As these studies are of essentially
international character, many international conferences, symposia, seminars and
workshops are held every year. To publish proceedings of these meetings is of
course important for tracing development of various disciplines of earth and plane
tary sciences though publishing is fast getting to be an expensive business.
It is my pleasure to learn that the Center for Academic Publications Japan and
the Japan Scientific Societies Press have agreed to undertake the publication of a
series "Advances in Earth and Planetary Sciences" which should certainly become
an important medium for conveying achievements of various meetings to the aca
demic as well as non-academic scientific communities. It is planned to publish the
series mostly on the basis of proceedings that appear in the Journal of Geomagnetism
and Geoelectricity edited by the Society 'Of Terrestrial Magnetism and Electricity of
Japan, the Journal of Physics of the Earth by the Seismological SOCiety of Japan
and the Volcanological Society of Japan, and the Geochemical Journal by the
Geochemical Society of Japan, although occasional volumes of the series will include
independent proceedings.
Selection of meetings, of which the proceedings will be included in the series,
will be made by the Editorial Committee for which I have the honour to work as
the General Editor. I and the members of the Editorial Committee will certainly
welcome any suggestions that will promote the series. Whenever the convener of a
meeting related to earth and planetary sciences is in a position to have to look for
a medium for publishing the proceedings please contact us.
Tsuneji Rikitake
General Editor
Foreword
Thermoremanent magnetization (TRM) is of central importance to paleomag
netism, but its mechanism and even its diagnosis in nature have remained uncertain.
On December 9, 1976, a fUll-day session on 'The Origin of TRM' was held as part
of the fall annual meeting of the American Geophysical Union in San Francisco.
Of the six invited and sixteen contributed papers presented at the session (for abstracts,
see EOS, 57, 904-907, 1976), thirteen appear in final form in this special issue of the
Journal of Geomagnetism and Geoelectricity.
The papers fall into two broad groups, the first incorporating theoretical and
laboratory studies of TRM (traditional 'rock magnetism'), the second dealing with
TRM in natural materials (traditional 'paleomagnetism'). As the papers themselves
make clear, the artificial distinction between rock magnetism and paleomagnetism
is gradually disappearing: the problems that remain to be solved, the origin of TRM
among others, require a flexible point of view and attack on many fronts.
Six of the papers in this issue consider the physical origin of TRM in single
domain, pseudo-single-domain, and multidomain grains of magnetite and titano
magnetite. Three deal with other minerals: titanomaghemite, hematite, and iron.
The identification of TRM and its role compared to other remanence mechanisms
are considered for continental rocks in one paper, for submarine rocks and ophiolites
in two papers, and for extraterrestrial materials and their analogs in two papers.
I am indebted to Dr. Richard Blakely for his help in organizing the session, to
Dr. Minoru Ozima for invaluable editorial assistance, to Dr. John Verhoogen for
contributing an introduction to this issue and to those who so ably and speedily
reviewed or substantially commented on the papers: M.E. Bailey, S.K. Banerjee,
A. Brecher, R.F. Butler, C.M. Carmichael, D.W. Collinson, R. Day, M.E. Evans,
M. Fuller, W.A. Gose, R.B. Hargraves, H.P. Johnson, M. Lanoix, S. Levi, R.T.
Merrill, G.W. Pearce, P.H. Reynolds, E.J. Schwarz, P.N. Shive, and F.D. Stacey.
David J. Dunlop
Guest Editor
Introduction
The recognition that many rocks carry a remanent magnetization that reliably
reflects the direction, and in fewer rocks also the intensity, of the magnetic field
prevailing at the time and place of their formation, has lead in the past 25 years to
spectacular developments in several branches of geology and geophysics. The dis
covery of temporal variations of the earth's magnetic field on a time scale of 103_107
years, and in particular of frequent reversals of its polarity, has lead to new ideas
regarding the origin of the field and the behavior of the earth's core. Paleomagnetism
finally provided, after years of debate, the clinching arguments for continental drift,
and lead to the revolutionary concepts of sea-floor spreading and plate tectonics.
Yet, paradoxically, the very mechanism by which rocks acquire their tell-tale rem
anence has remained somewhat obscure, particularly so in the case of the thermal
remanent magnetization (TRM) acquired by igneous rocks as they cool in the earth's
weak field. The two features of TRM that have made it so useful in paleomagnetism
are its intensity and an extraordinary stability that enables it to survive over billions
of years. The intensity of TRM in rocks is usually greater by several orders of
magnitude than the remanence that can be imparted by exposure, at room tempera
ture, to the earth's field; and destruction of TRM by a-c demagnetization may require
an a-c field of several hundred gauss or more.
The theory of the acquisition of TRM has been satisfactorily developed, and
experimentally tested, in the two extreme cases of 1) magnetic grains small enough
to consist of single domains (SD), and 2) grains large enough to have a multi-domain
(MD) structure. (The threshold for single-domain behavior in magnetite appears to
be close to 0.05 pm for equidimensional grains.) It turns out that a field as weak as
the earth's field may induce in an assembly of non-interacting single-domain grains
a TRM comparable in magnitude to the saturation remanence induced at room tem
perature by fields of several thousand gauss, and of high stability. By contrast, weak
field TRM in multidomain grains is usually much weaker and less stable. A curious
feature is that grains of size between 0.05 and about 15 pm (for magnetite), and
which are clearly much too large to be SD, nevertheless exhibit an intensity of
remanence and a resistance to demagnetization that are typical of SD grains. Such
grains are said to show pseudo single-domain behavior (PSD). The TRM of PSD
grains also shows an inverse dependence on grain size that is not typical of MD
remanence, but the dependence of TRM on the magnitude of the inducing field is
not that predicted for SD behavior.
PSD behavior is now generally attributed to the presence of residual moments that
cannot be removed by demagnetization. Attempts to explain these residual moments
x Introduction
by invoking internal strains (dislocations), inhomogeneity, or Barkhausen discreteness
in domain-wall position, have generally failed to account for one or more charac
teristics of PSD, e. g., the inverse dependence of remanence on grain size. In the past
3 or 4 years, the search has focussed on the internal structure of domain walls ('walls
within walls') and their termination at the surface of grains. The hunt, lead mainly
by Stacey, Banerjee, and Dunlop, is still on, and the subject is lively enough to
warrant publication, as a special issue of papers presented at a session on the origin
of TRM held in December 1976 in San Francisco.
As the reader will judge, excellent progress is being made, but, as usual, much
remains to be done before we can claim to fully understand the TRM of rocks. It
is an unfortunate fact of nature that much of the TRM in rocks seems to be carried
by grains so small (less than 1 ,urn) that they can hardly be seen under the microscope;
neither their abundance, nor their size, shape or composition can be easily assessed.
Nor can we, in many instances, determine precisely when they formed, as they may
be products of chemical reactions (e. g., oxidation) occurring late in the cooling
history of the rock. It is amusing to reflect that if the pioneering paleomagnetists
of the early fifties had known, or even suspected, the full complexity (chemical,
mineralogical, textural, magnetic-structural) of the magnetic properties of rocks,
they probably would have thrown up their hands, declared rocks inherently unreliable,
and turned to lesser things. Ignorance, it would seem, can sometimes be a blessing.
J. Verhoogen
CONTENTS
Preface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v
Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. vii
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix
TRM and Its Variation with Grain Size: A Review. . . . . . . . . . . . .. R. DAY 1
Single Domain Oxide Particles as a Source of Thermoremanent Magnetization
· . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. M.E. EVANS 35
Domain Structure of Titanomagnetites and Its Variation with Temperature ..
· . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. H.C. SOFFEL 45
The Demagnetization Field of MuItidomain Grains. . . . . . . .. R. T. MERRILL 53
The Hunting of the 'Psark' . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. D.J. DUNLOP 61
On the Origin of Stable Remanence in Pseudo-Single Domain Grains ...... .
· . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. S.K. BANERJEE 87
The Preparation, Characterization and Magnetic Properties of Synthetic
Analogues of Some Carriers of the Palaeomagnetic Record ........... .
..................... .......... J.B. O'DONOVAN and W. O'REILLY 99
Reduction of Hematite to Magnetite under Natural and Laboratory Conditions
· . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. P.N. SHIVE and J.F. DIEHL 113
Characteristics of First Order Shock Induced Magnetic Transitions in Iron
and Discrimination from TRM. . . . . . . . . . . . . . . . . . . . .. P. WASILEWSKI 123
The Thermoremanence Hypothesis and the Origin of Magnetization in Iron
Meteorites ........................... A. BRECHER and L. ALBRIGHT 147
Thermal Overprinting of Natural Remanent Magnetization and KjAr Ages in
Metamorphic Rocks ................................ K.L. BUCHAN,
G.W. BERGER, M.O. MCWILLIAMS, D. YORK, and D.J. DUNLOP 169
Does TRM Occur in Oceanic Layer 2 Basalts? ............... J.M. HALL 179
The Effects of Alteration on the Natural Remanent Magnetization of Three
Ophiolite Complexes: Possible Implications for the Oceanic Crust ..... .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. S. LEVI and S.K. BANERJEE 189
Subject Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 209
Adv. Earth Planet. Sci., 1, 1-33, 1977
TRM and Its Variation with Grain Size*
Ron DAY
Department of Geological Sciences, University of California,
Santa Barbara, California, U.S.A.
(Received June 22, 1977)
Thermoremanent magnetization (TRM), the dominant mechanism in igneous
rocks, has been investigated for many years, yielding a large data base of experi
mental results and several theoretical models. However, there are still a large
number of discrepancies between the observations and the theories.
The theoretical models of TRM are reviewed, and then evaluated in the light
of recent experimental results from sized synthetic magnetites and titanomagnetites,
and igneous rocks.
1. Introduction
The fundamental assumption of paleomagnetism is that the direction and inten
sity of the natural remanent magnetization (NRM) of a rock are characteristic of
the ambient geomagnetic field present during its formation. The combined facts
that igneous rocks are magnetized by the geomagnetic field when they cool and that
the TRM which they thus acquire is usually very stable, even over geological time,
makes TRM a very important magnetic property of rocks. It has been known for
centuries that rocks carried a remanent magnetization, but it was only during the
last century (MELLONI, 1853) that it was established that the NRM contained a re
cord of the ancient geomagnetic field. The basic characteristics of TRM emerged
from the classical works of KOENIGSBERGER (1938), THELLIER (1938) and NAGATA
(1941, 1942) on natural materials. Recognizing the difficulties and uncertainties in
volved with magnetic measurements on natural materials, a second generation of
research began using synthetic powders (ROQUET, 1954; UYEDA, 1958; RIMBERT,
1959) and single crystals (SYONO, 1962). This was followed by experiments to deter
mine the effects of stress (LOWRIE, 1967; SHIVE, 1969b) and magnetocrystalline aniso
tropy (OZIMA et ai., 1964; PETROV and METALLOVA, 1968). A third generation of
work has been concerned with the effect of particle size on TRM (PARRY, 1965;
ROBINS, 1972; DAY, 1973; DUNLOP, 1973a; RAHMAN et ai., 1973; LEVI, 1974; LEVI
and MERRILL, 1976, 1977). The importance of this work is obvious; magnetic par
ticles in rocks are known to span a large size range covering single-domain (SD),
pseudo single-domain (PSD) and multi-domain (MD) behaviour, and it is well known
that magnetic properties are very sensitive to changes in grain size (PARRY, 1965;
* Paper presented at the special session on the 'Origin of TRM,' American Geophysical Union,
San Francisco, December 9, 1976.
