Download Crustal Magnetism, Lamellar Magnetism and Rocks That Remember

Crustal Magnetism,
Lamellar Magnetism and
Rocks That Remember
Suzanne A. McEnroe1, Karl Fabian1, Peter Robinson1, Carmen Gaina1 and Laurie L. Brown2
1811-5209/09/0005-0241$2.50 DOI: 10.2113/gselements.5.4.241
M
agnetic anomalies are deviations from an internal planetary magnetic
field produced by crustal materials. Crustal anomalies, measured over
a wide range of vertical distances, from near-surface to satellites, are
caused by magnetic minerals that respond to the changing planetary field.
Previously, magnetism of continental crust was described in terms of the bulk
ferrimagnetism of crustal minerals, which is mostly due to induced magnetization.
The recent discovery of lamellar magnetism, a new interface-based remanence type,
has changed our thinking about the contribution of remanent magnetization.
Lamellar magnetism may also be an important contributor to deep-seated
anomalies in the crust of the Earth and in other planets with highly magnetic
crusts, like Mars.
Leonhardt and Fabian 2007; Aubert
et al. 2008). However, the revolutionary fact of global polarity reversal,
only demonstrated with certainty
in the early 1960s, led directly to
the even more revolutionary
modern theory of plate tectonics,
which now governs all aspects of
the study of Earth history.
INDUCED AND REMANENT
MAGNETIZATION
Rocks carry two types of magnetization: (1) magnetization induced
(Mind) by an external magnetic field
Keywords : magnetic anomalies, remanent magnetism, lamellar magnetism,
and proportional to the rocks’
hematite, ilmenite, crustal magnetism, Mars
magnetic susceptibility, χ, and to the
BASIC GEOMAGNETISM
field, H; and (2) remanent magnetization (Mr), which in igneous rocks is typically locked in
Geomagnetism is one of the oldest natural sciences, and
during cooling. While the induced magnetization is a response
historians consider the book De Magnete, Magneticisque
Corporibus, et de Magno Magnete Tellure (On the Magnet and to the present magnetic field, the remanent component
Magnetic Bodies, and on That Great Magnet the Earth),
published by William Gilbert in 1600, as the first modern
scientific treatise. While Gilbert envisaged the Earth as a
large magnet, we now know that the geomagnetic field is
a direct signal from Earth’s outer core, propagating nearly
undistorted to the surface (Merrill et al. 1996). The geomagnetic field has been recorded by magnetic minerals in
surface rocks since formation of the solid crust. The mineralmagnetic record of the paleomagnetic field is an archive
of the history of magnetic-field generation in the outer
core, where liquid iron is convecting at a speed of up to
5 m/h. On Earth, records of magnetic-field variation are
preserved in many rock types and throughout time, starting
with rocks magnetized nearly 3.7 billion years ago and
continuing to today’s sea floor and active volcanoes.
What Gilbert found was that the geometry of the geomagnetic field closely resembles that of a magnetized sphere.
The predominant dipole component is equivalent to the field
of a bar magnet mounted at the Earth’s centre and nearly
aligned with the rotation axis (Fig. 1). Magnetic-field intensity decreases from about 60 microteslas (μT) at the poles
to ~30 μT at the geomagnetic equator. Paleomagnetic studies
around the world indicate that the field geometry has been
an axial dipole throughout most of Earth history and that
it has reversed its polarity at numerous and irregular intervals
through time (Merrill et al. 1996). What exactly happens
during a polarity reversal is a special field of research (e.g.
1 Geological Survey of Norway, N-7491 Trondheim, Norway
2 Department of Geoscience, University of Massachusetts
Amherst, MA 01103, USA
E-mail: [email protected]
E lements , V ol . 5,
pp.
241–24 6
A
B
C
(A) William Gilbert stated in 1600 that the Earth itself is
a large magnet. Although, in fact, the Earth’s magnetic
field is created by a dynamo process driven by extremely fast convection
(averaging ~1 m/h) of highly conductive liquid iron in the outer core,
its geometry at and above the surface closely resembles the field that
a dipole at the Earth's centre would generate. (B) Today, the magnetic
south pole nearly coincides with the geographic north pole, but the
field axis is slightly tilted. The field strength at the geomagnetic poles
is twice the strength at the geomagnetic equator. (C) If a remanent
magnetization formed 1 billion years ago in the southern hemisphere
parallel to a normal field and later was transported by plate motion
to the north, it could create a negative remanent anomaly.
241
Figure 1
A ugus t 2009
may have formed a billion years ago in the past, on the
other side of the Earth, and been transported and rotated
into its current direction by plate motion (Fig.1c). When
carrying out a magnetic survey, it is essential to recognize
that the total magnetization measured is a vector sum:
Mtot = Mind + Mr = χH + Mr
Thus, the Koenigsberger ratio (Q = Mr / Mind) is a key characteristic for a correct interpretation of magnetic signals
from the crust (Fig. 2). The total magnetization of samples
with low Q values (<0.5) is dominated by the induced
magnetization (Mind). Their “magnetic anomaly”, which is
the deviation between the expected dipolar and the measured
field value at that location, will map primarily as a magnetic
high (positive) compared to non-magnetic crust. The
amplitude of an induced anomaly increases with magnetic
mineral content, which makes it valuable for ore exploration. The natural remanent magnetization (NRM) vector,
Mr, becomes important for high Q, i.e. values around 2 (Fig. 2),
when the direction and intensity of the NRM must be
known to model the anomaly. When Q reaches 5, the dominant remanence can lead to large negative anomalies when
the NRM vector is at an obtuse angle to the inducing field.
When the NRM direction is close to the inducing field,
one finds large positive anomalies, with the remanent vector
adding to the induced vector. In the following, we will
discuss the mineralogical sources for anomalies with high
Q values. In quickly cooled continental or oceanic lavas,
fine-grained (titano-)magnetite is known to carry a sufficiently strong remanence to produce moderate to high Q
values. Much of the continental crust contains coarser magnetite that produces induced anomalies. Yet, some coarsely
crystalline rocks still show large remanent anomalies.
Recently, our interest has been to understand the mineralogical carriers of this hard, commonly billion-years-old
magnetization.
OCEANIC CRUST
The global oceans are floored by a dense crust less than
10 km thick, produced continuously at the mid-ocean ridges
by magma rising, solidifying and cooling. Rocks acquire a
thermoremanent magnetization aligned with the Earth’s
magnetic field, which episodically reverses its polarity.
Because the ages of polarity reversal are now well established
from records in lavas and sediments, the oceanic remanent
magnetic anomalies can be directly transformed into a map
of the age of global oceanic floor (Fig. 3). Oceanic anomalies
dominated by remanence are commonly due to members of
the magnetite–ulvöspinel solid solution or oxidized equivalents.
A fascinating observation is that, worldwide, the magneticsignal amplitude systematically varies from high to low and
then back to high amplitude when moving away from the
mid-ocean ridges (Bleil and Petersen 1983). Because continental basalts of similar age do not show this effect, this
amplitude variation reflects, at least partly, progressive lowtemperature oxidation of titanomagnetite in the uppermost
oceanic crustal layer (Matzka et al. 2003). Which signal
fraction arises from geomagnetic-field intensity variations
is a yet-unsolved problem of paleomagnetism (Valet 2003).
A
B
C
(A) The age of global ocean floor determined from the
geomagnetic reversal chronology (after Müller et al. 2008).
Warm colours indicate young crust, cold colours older crust. (B) Inset
from A displaying in detail the age of the southern North Atlantic Ocean.
Shaded relief (dark = normal polarity; light = reversed polarity) shows
the normal and reversed polarities mapped by magnetic surveys (Maus
et al. 2007). (C) Magnetic anomaly data profile (white lines shown in
A and B) acquired by the ODP cruise 150 in the southern North Atlantic
Ocean. The magnetic anomaly is calculated by subtracting the long
wavelength of the magnetic reference field (IGRF). It results from the
varying remanent magnetization of the ocean floor created during
the last 120 million years. A model of the oceanic crust, magnetized
with alternating polarities and subsiding as it cools, is shown below
the magnetic profile. The light-green draped profile shows the true
bathymetry (from ETOPO2). High amplitudes of the magnetic anomaly for the young oceanic crust (<30 Ma, red band) contrast with
decreased amplitudes for the next 10 million years (light blue bands).
Figure 3
The total magnetization, M tot, of a rock is the sum of
the induced magnetization, M ind , which is a response
to the external field, H, and the remanent magnetization, Mr , carried by
magnetic minerals. In general, the magnetic anomaly related to a rock
is positive (+) or negative (-) according to the sign of the projection of
M tot onto H. Induced anomalies, where Q = M r/M ind << 1, are always
positive, while remanent anomalies, where Q > 1, can have either sign,
depending on their remanence direction.
Figure 2
E lements
242
A ugus t 2009
CONTINENTAL CRUST
The continental crust is mineralogically diverse, and remanence-dominated anomalies have various origins. Here we
focus on a newly discovered source of anomalies related
to the hematite–ilmenite series. We discuss the mineralogical
background of this series and present three case studies to
demonstrate its abundance and magnetic properties.
Hematite–Ilmenite Series
All minerals in the hematite–ilmenite series have rhombohedral
structure involving layers of octahedrally coordinated cations.
Hematite is a canted antiferromagnet (CAF) (see Harrison
and Feinberg 2009 this issue) with a room-temperature
magnetization of 2.1 kA/m (~0.4% that of magnetite) parallel
to the (0001) basal plane. Ilmenite is paramagnetic (PM)
above 57 K and therefore carries no magnetization on Earth
at geologically relevant temperatures. The complexity of the
phase diagram (Fig. 4) results from the interplay between cation
ordering, magnetic ordering and exsolution processes.
A
B
C
Hematite–ilmenite phase diagram at 1 atmosphere
showing composition–temperature relations involving
Fe–Ti (long-dashed line) and magnetic (short-dashed line) ordering,
miscibility gaps, and two ‘tricritical points’. A tricritical point on the
Fe–Ti ordering curve at ≈57 mol% FeTiO3 and 1050 K creates the twophase region disordered paramagnetic PM R3̄c + ordered PM R3̄. The
second tricritical point, on the magnetic ordering curve at ≈15 mol%
FeTiO3 and 815 K, creates the two-phase region CAF R3̄c + PM R3̄c.
Limbs of the PM R3̄c region converge at the eutectoid (E, ≈800 K) and
the stable low-T assemblage CAF R3̄c hematite + PM R3̄ ilmenite. For
compositions richer in hematite than E, titanohematite can magnetize
at T > 800 K. For compositions richer in ilmenite than E, magnetization
occurs only at <800 K, with exsolution of CAF hematite (a chemical
reaction) from a host richer in ilmenite. Insets illustrate atomic configurations
for ≈80 mol% FeTiO3 above (A) and below (B) the Fe–Ti ordering
reaction, and for the fully exsolved case (C) with a CAF hematite
lamella (HEM) and two contact layers(CL) in PM ilmenite (ILM).
Figure 4
A
Exsolution occurs at intermediate to low temperature
(Burton 1991; Harrison 2006; Ghiorso and Evans 2008) and
leads to the development of fine-scale intergrowths of CAF
hematite and PM ilmenite (“hemo-ilmenite”) in slowly cooled
rocks. Remarkably, these intergrowths are several times more
magnetic than can be explained by the CAF moment of
the hematite contained within them. Examination of the
intergrowths using scanning electron and transmission electron microscopy (Fig. 5) shows that exsolution has taken
place in diffusion-controlled steps. Commonly in the last
stages, under slow cooling, final crops of lamellae are
produced as thin as 1–3 nm, equivalent in thickness to just
1–2 unit cells. Recent work, described below, has demonstrated
that the excess magnetization of these fine-scale intergrowths
comes from the phase interfaces between nanoscale CAF
hematite and PM ilmenite lamellae.
Lamellar Magnetism
Several case studies indicate that rocks containing finely
exsolved hematite–ilmenite microstructures, with abundant
(0001) nanoscale phase interfaces, create large remanencedominated anomalies (McEnroe and Brown 2000; McEnroe
et al. 2001, 2002). Based on these observations, the theory
of “lamellar magnetism” or chemical interface magnetization
has been developed (Harrison and Becker 2001; Robinson
et al. 2002, 2004). This theory claims that mixed-valence
contact layers form at the interface between CAF hematite
and PM ilmenite to reduce local ionic charge imbalance
(Robinson et al. 2006) (Fig. 4). This hypothesis was further
confirmed by ab initio calculations (Pentcheva and Nabi
2008). A contact layer carries an average magnetization of
~4.5 µB (where μB is the Bohr magneton, a measure of the
electron magnetic dipole moment). The two contact layers
on either side of a lamella are magnetized parallel to each
other, yielding a total of 2 × 4.5 ≈ 9 µ B. This moment is
counterbalanced by the opposite moment of one hematite
layer at ~5 µ B, giving a net unbalanced “lamellar” moment
of 2 × 4.5 – 5 ≈ 4 µ B. When the moments of individual
lamellae are magnetically aligned with each other, the intensity of lamellar magnetism is proportional to the density
of lamellae and the surface area of contact layers. Parallel
alignment is most strongly favoured in crystals with (0001)
parallel to the magnetizing field. Lamellar magnetism is
thermally very stable and extremely resistant to demagnetization in alternating fields commonly 1000 times that of
the Earth’s field. This makes it an ideal candidate for the
generation of ancient crustal magnetic anomalies.
Exchange Bias Proves Lamellar Magnetism
Areas of negative remanent magnetization near Modum,
Norway, contain metamorphic titanohematite with abundant
nanoscale exsolution of ilmenite ("ilmeno-hematite").
Low-temperature magnetic measurements provide direct
proof that the magnetic anomalies result from lamellar magne-
B
C
(A) Electron backscatter image of an intergrowth of exsolved hematite and ilmenite, and minor magnetite, from pyroxene granulite,
SW Sweden. (B) Electron backscatter image of hemo-ilmenite; hematite lamellae are white and ilmenite gray. (C) High-resolution TEM
image of hemo-ilmenite, courtesy of Falko L angenhorst
Figure 5
E lements
243
A ugus t 2009
tism (Fig. 6). Cooling the titanohematite and its NRM below
the ordering temperature of ilmenite leads to a large shift
of the low-temperature hysteresis loop (exchange bias).
This shift can only be explained by an interaction between
spins across the hematite–ilmenite interfaces, proving that
the spins that generate the NRM must reside in the interfaces and cannot be due to defect moments or stressinduced moments, which occur in normal bulk hematite
(Fabian et al. 2008). Exchange bias is an extremely important property for magnetic recording media like hard disks
and solid-state memories. Despite intense research in
magnetic material science, the exchange bias observed in
these natural intergrowths is larger than any known in
synthetic technical materials (McEnroe et al. 2007; Harrison
et al. 2007). Thus, the same magnetic properties arising
from the microstructures in the oxides which produced
remanent crustal anomalies due to billion-year-old “memories”
are templates for future storage devices.
LAMELLAR MAGNETISM: A COMMON SOURCE
OF CRUSTAL REMANENT ANOMALIES?
Having established the physical mechanism of lamellar
magnetism, it is important to understand its abundance
and conditions of formation. Here we present three geographically and geologically different settings where lamellar
magnetism contributes significantly to the magnetic anomaly.
Anorthosites and a Layered Intrusion,
Rogaland, Norway
The Rogaland (~930 Ma) intrusive province is known for
massif anorthosites, hemo-ilmenite ores, and the BjerkreimSokndal (BKS) layered intrusion. Figure 7a is part of a
regional aeromagnetic map, in which magnetic highs (redviolet) of over 2500 nT are separated by prominent magnetic
lows (blue) down to -3500 nT. Figure 7b is a perspective
from the east from a high-resolution helicopter survey.
Highs are due to induced magnetization of coarse-grained
magnetite; lows are primarily due to the reversed NRM of
hemo-ilmenite. The anorthosite bodies contain finely
exsolved hemo-ilmenite +/- magnetite and have negative
anomalies. The NRM acquired ~930 million years ago is
steeply inclined. High Q values, from 6 to 26, reflect abundance of hemo-ilmenite. Within the Åna-Sira Anorthosite,
with Mr = 4 A/m and Mind of only 0.25 A/m, lies the Tellnes
Norite, which, with ~30% hemo-ilmenite, is the world’s
second-largest bedrock source of ilmenite. It also has a
negative remanent anomaly. The Håland-Helleren
Anorthosite has an even larger Mr (6 A/m), but also a larger
Mind (1 A/m) due to its additional magnetite content. These
anorthosites have NRM intensities similar to or higher than
those of Tertiary basalts (Brown and McEnroe 2008), but
lower susceptibilities, a hallmark of lamellar magnetism.
The 7 km thick BKS layered intrusion is in a doubly
plunging syncline. The southern Sokndal lobe is mostly
marked by the occurrence of magnetite-bearing rocks. The
northern Bjerkreim lobe is outlined by the huge curving
negative magnetic anomaly in the northern part of Figure
7a. The BKS intrusion has 5 megacyclic units. Each began
with the intrusion of a new, slightly more primitive magma
into the bottom of the chamber; this magma mixed with
previous magma, and then it returned to a more normal
path of fractional crystallization (Wilson et al. 1996). In
terms of oxides, the most primitive magmas locally crystallized a small amount of Cr-rich spinel, then ferri-ilmenite
with gradually declining hematite content until the melts
reached saturation with magnetite. Further crystallization
led to ilmenite with lower hematite content without hematite exsolution. The progression from ferri-ilmenite precipitation, with hemo-ilmenite crystallization upon cooling,
to magnetite precipitation accompanied by ferri-ilmenite,
and finally to precipitation of near-end-member ilmenite
has profound effects on magnetic properties and the
magnetic map (McEnroe et al. 2001, 2008). Magnetic highs
correspond to magnetite with end-member ilmenite horizons. Remanent magnetic lows (blue in Fig. 7a) outline the
hemo-ilmenite-bearing horizons in which lamellar magnetism controls the magnetic properties. The BKS intrusion
is of economic interest for potential vanadium-magnetite,
apatite and ilmenite deposits.
A
B
Exchange bias in a rock from Norway, creates an
asymmetric hysteresis loop of magnetization, m, versus
applied field, B, after cooling the initial NRM in zero-field to 5 K. The
difference between the upper and lower branches (orange) is bimodal,
while saturation magnetization, Ms, is symmetric (after Fabian et al.
2008). The exchange bias originates from uncompensated moments
of ilmenite lamellae interacting with the CAF hematite matrix.
Figure 6
E lements
(A) Regional aeromagnetic map covering part of
Rogaland, Norway (modified from McEnroe et al. 2008).
H indicates the southern end of the Heskestad anomaly; Å-S: Åna-Sira;
HH: Håland Helleren anorthosites; G: Garsaknatt leuconorite; S and
BK: Sokndal and Bjerkreim lobes of BKS intrusion. (B) Shaded 3D
oblique perspective view from the east from a helicopter survey
showing the Heskestad anomaly, the Sokndal lobe of the BKS layered
intrusion and the Åna-Sira Anorthosite
Figure 7
244
A ugus t 2009
The striking anomaly at Heskestad (H in Fig. 7), centred
over a norite layer, continues for 25 km to the northwest.
Near Heskestad, the magma chamber was expanding laterally southward during crystallization. Along one groundmagnetic profile in this region, the intensity changes from
a high of 53,000 nT to a low of 23,000 nT over a distance
of 200 m, canceling more than 50% of the local geomagnetic field of ~50,000 nT. The source of the anomaly is a
norite (McEnroe et al. 2004), which has a huge Mr (25 A/m)
and a large Mind (4 A/m), resulting in a high Q (6). The two
likely candidates for the remanence are 2% hemo-ilmenite,
and orthopyroxenes with hemo-ilmenite exsolution. Both
cases provide lamellar magnetism. A possible explanation
combines two factors: (1) strong alignment of the pyroxenes
containing oriented hemo-ilmenite lamellae in a favourable
orientation with respect to the Proterozoic magnetizing
field, and (2) acquisition of an inverse induced magnetization by magnetite from the local inverse stray field, therefore adding to the remanent magnetic vector.
The Rogaland anomalies are not a unique case of lamellar
remanence in Scandinavia. A larger effect of ilmeno-hematite and hemo-ilmenite with nanoscale exsolution lamellae
occurs over granulites in southwest Sweden (McEnroe et al.
2001). The granulitic rocks have an average NRM of 5 A/m
and only 1.4 A/m of induced magnetization, resulting in
a high Q value of 3.6. Again, lamellar magnetism provides
high coercivity and thermal stability of the NRM, making
these rocks excellent carriers of the ancient magnetic field.
Metamorphosed Altered Volcanics,
Adirondack Mountains, USA
Remanence-dominated anomalies are common where the
crust is oxidized. The rocks will have high NRMs, low susceptibility and high Q values. An example is the microcline–
sillimanite gneiss of the Russell Belt (Fig. 8) in the Adirondack
Mountains, USA, where Balsley and Buddington (1958)
made a classic study of chemistry, petrology and magnetic
anomalies. The anomaly is 2250 nT below background in
the airborne survey and registers -5000 nT at ground level
(McEnroe and Brown 2000). These rocks have an induced
magnetization of only 0.1 A/m, compared to NRMs of 3.5 A/m,
resulting in an extreme Q of 35. The magnetic mineralogy
is dominated by titanohematite with ilmenite exsolution
(ilmeno-hematite) with minor rutile and pyrophanite
(MnTiO3) lamellae. Multiple generations of lamellae, from
the micron scale to unit-cell size, are common (Kasama et
al. 2004). Here, lamellar magnetism is responsible for the
large NRMs, as well as the high coercivity needed to
preserve the magnetization for nearly 1 billion years. Samples
do not demagnetize in alternating fields of 100 mT, more
than 1000 times the present-day field strength. Their magnetic
memories easily survived the thousands of magnetic-field
reversals on Earth since Mesoproterozoic time.
Metamorphic Hematite, South Australia:
A New Type of Lamellar Magnetism?
The last case study concerns a positive remanence-controlled
anomaly at the Peculiar Knob (PK) deposit, South Australia,
and widens the scope of possible sources of lamellar magnetism.
PK is a metamorphosed iron formation occurring under
25 m of sediment, within the Paleoproterozoic basement
of the Mount Woods Inlier, northern Gawler Craton. It was
discovered by an airborne-magnetic survey, and later groundmagnetic and gravity surveys showed extreme anomalies.
The ground-level magnetic anomaly is 30,000 nT above
background and is dominated by remanent magnetization
acquired during cooling from Mesoproterozoic metamorphic conditions (Schmidt et al. 2007). Though first thought
to be a massive magnetite deposit that created a very large
induced anomaly, exploratory drill cores intersecting the
E lements
Geological and aeromagnetic map of part of the Russell
Belt. Stippled unit is microcline granite gneiss with
associated metasedimentary layers; gagn, alaskite gneiss; gba,
anorthosite gabbro gneiss; ghgn, hornblende-microcline-oligoclase
granite gneiss; ms, metasedimentary rocks. Magnetic intervals of 500
nT are coloured. The inset shows the location of the Russell Belt (small
box) within the Adirondack Mountains, USA (stippled area). Modified
from M cEnroe and B rown (2000) and Balsley and B uddington (1958)
Figure 8
body were dominated by coarsely crystalline end-member
hematite. The question was: Did the drilling miss the
magnetite deposit or were the drill cores representative?
The magnetic properties from the 4 drill holes yielded
upward-directed NRMs of from 36 to 227 A/m, averaging
190 A/m. The anomaly was then successfully modeled using
a large NRM of 120 A/m. As a result, the “magnetite
deposit” vanished. Why did these rocks have such large
NRMs? In reflected light at high magnification, and only
in oil, some coarse hematite crystals showed a very fine
texture. Microprobe traverses across coarse crystals yielded
sums consistently too high for pure hematite, and the
samples also had unusually large coercivities. These observations led to the interpretation that some hematite contained
fine intergrowths of magnetite and/or maghemite on their
(0001) planes, which would increase the coercivity, stabilize the NRM and result in a larger remanence-dominated
anomaly than would be produced by pure hematite.
Current studies are targeted on whether a new variety of
lamellar (interface) magnetism can explain the magnetic
properties at PK. This is of major importance for exploration, because many of the magnetic target rocks of the
Mount Woods Inlier are associated with base metal and
Cu–Au (± U and Ag) mineralization. The nearby giant
Olympic Dam hydrothermal Cu–U–Au–Ag deposit occurs
in an enormous volume of semi-massive hematite.
RAMIFICATIONS
These case studies indicate that lamellar magnetism is not
exotic, but a common cause of remanent magnetic anomalies.
Temperature–pressure experiments at 10 kb and 580 o C
show that lamellae are stable and not resorbed (McEnroe
et al. 2004), as would be predicted from Figure 4, thus
indicating that lamellar magnetism of considerable strength
245
A ugus t 2009
may persist deep in the crust. The other major source for
remanent anomalies, fine-grained magnetite, is coarsened
and demagnetized under such conditions. Are deep crustal
magnetic anomalies due to lamellar magnetism? If this is
the case, a main paradigm of crustal magnetism must be
reconsidered: the paradigm that there are no crustal
magnetic rocks below the depth at which the geotherm
crosses magnetite’s Curie-temperature of 580 oC.
By understanding the mineral-magnetic properties of rocks
which create large remanent anomalies on Earth, we gain
insight into possible mechanisms for generating magnetic
signals from other planets and moons of the solar system.
On Mars, active magnetic field generation ceased billions of
years ago, and satellites detect only the residual remanent
magnetization of magnetic minerals in crustal rocks. Retention
of strong remanence over such time spans requires good
magnetic stability. Lamellar magnetism can provide these
properties. These unusual properties are also of interest for
technological applications. The unbalanced interface
REFERENCES
Aubert J, Aurnou J, Wicht J (2008) The
magnetic structure of convection-driven
numerical dynamos. Geophysical Journal
International 172: 945-956
Balsley JR, Buddington AF (1958) Irontitanium oxide minerals, rocks, and aeromagnetic anomalies of the Adirondack area,
New York. Economic Geology 53: 777-805
Bleil U, Petersen N (1983) Variations in
magnetization intensity and low-temperature titanomagnetite oxidation of ocean
floor basalts. Nature 301: 384-388
Brown LL, McEnroe SA (2008) Magnetic
properties of anorthosites: A forgotten
source for planetary magnetic anomalies?
Geophysical Research Letters 35: L02305,
doi:10.1029/2007GL032522
moments cannot be thermally flipped at ambient temperature, even though they reside in nanometre-sized lamellae.
Therefore, lamellar magnetism may be useful for designing
extremely stable high-density storage devices. The newly
discovered giant exchange bias related to nanoscale lamellae
is of interest for future applications in nanomagnetics and
spintronics. Thus, research on Earth’s oldest memory system
has led to discoveries that could become templates for new
technology. Geomagnetism, the topic of the first modern
scientific treatise and the driving force of the plate-tectonic
revolution, may still be good for future surprises.
Acknowledgments
Constructive reviews were provided by Mike Jackson,
Jurgen Matzka and Richard Harrison. Harrison also provided
the colourful insets in Figure 4. Falko Langenhorst kindly
provided Figure 5c. Bjørg Svendgård helped with figures.
This work was supported by grants from the Research Council
of Norway Petromaks and Nanomat Programs.
Kasama T, McEnroe SA, Ozaki N, Kogure T,
Putnis A (2004) Effects of nanoscale
exsolution in hematite–ilmenite on the
acquisition of stable natural remanent
magnetization. Earth and Planetary
Science Letters 224: 461-475
McEnroe SA, Brown LL, Robinson P (2008)
Remanent and induced magnetic anomalies over a layered intrusion: Effects from
crystal fractionation and magma
recharge. Tectonophysics doi: 10.1016/j.
tecto.2008.11.021
Leonhardt R, Fabian K (2007) Paleomagnetic
reconstruction of the global geomagnetic field
evolution during the Matuyama/Brunhes
transition: Iterative Bayesian inversion
and independent verification. Earth and
Planetary Science Letters 253: 172-195
Merrill RT, McElhinny MW, McFadden PL
(1996) The Magnetic Field of the Earth:
Paleomagnetism, the Core and the Deep
Mantle. Academic Press, London, 531 pp
Matzka, J, Krása D, Kunzmann T, Schult A,
Petersen N (2003) Magnetic state of
10–40 Ma old ocean basalts and its
implications for natural remanent
magnetization. Earth and Planetary
Science Letters 206: 541-553
Burton BP (1991) The interplay of chemical
and magnetic ordering. In: Lindsley DH
(ed) Oxide Minerals: Petrologic and
Magnetic Significance. Reviews in
Mineralogy 25: 303-321
Maus S, Sazonova T, Hemant K, Fairhead
JD, Ravat D (2007) National Geophysical
Data Center candidate for the World Digital
Magnetic Anomaly Map. Geochemistry,
Geophysics, Geosystems 8: Q06017,
doi: 10.1029/2007GC001643
Fabian K, McEnroe SA, Robinson P,
Shcherbakov VP (2008) Exchange bias
identifies lamellar magnetism as the origin
of the natural remanent magnetization
in titanohematite from ilmenite exsolution,
Modum, Norway. Earth and Planetary
Science Letters 268: 339-353
McEnroe SA, Brown LL (2000) A closer look
at remanence-dominated anomalies: Rock
magnetic properties and magnetic mineralogy
of the Russell Belt microcline-sillmanite
gneiss, northwest Adirondack Mountains,
New York. Journal of Geophysical Research
105(B7): 16437-16456
Ghiorso MS, Evans BW (2008)
Thermodynamics of rhombohedral oxide
solid solutions and a revision of the Fe-Ti
two-oxide geothermometer and oxygenbarometer. American Journal of Science
308: 957-1039
McEnroe SA, Harrison RJ, Robinson P,
Golla U, Jercinovic MJ (2001) Effect of
fine-scale microstructures in titanohematite on the acquisition and stability of
natural remanent magnetization in granulite facies metamorphic rocks, southwest
Sweden: Implications for crustal magnetism. Journal of Geophysical Research
106(B12): 30523-30546
Harrison RJ (2006) Microstructure and
magnetism in the ilmenite-hematite solid
solution: a Monte Carlo simulation study.
American Mineralogist 91: 1006-1024
Harrison RJ, Becker U (2001) Magnetic
ordering in solid solutions. European
Mineralogical Union Notes in
Mineralogy 3: 349-383
Harrison RJ, Feinberg JM (2009) Mineral
magnetism: Providing new insights into
geoscience processes. Elements 5: 209-215
Harrison RJ, McEnroe SA, Robinson P,
Carter-Stiglitz B, Palin EJ, Kasama T (2007)
Low-temperature exchange coupling
between Fe2O3 and FeTiO3 : Insight into
the mechanism of giant exchange bias in
a natural nanoscale intergrowth. Physical
Review B 76: 174436, 10 pp
E lements
McEnroe SA, Harrison RJ, Robinson P,
Langenhorst F (2002) Nanoscale haematiteilmenite lamellae in massive ilmenite
rock: an example of lamellar magnetism
with implications for planetary magnetic
anomalies. Geophysical Journal
International 151: 890-912
McEnroe SA, Langenhorst F, Robinson P,
Bromiley GD, Shaw CSJ (2004) What is
magnetic in the lower crust? Earth and
Planetary Science Letters 226: 175-192
McEnroe SA, Carter-Stiglitz B, Harrison RJ,
Robinson P, Fabian K, McCammon C
(2007) Magnetic exchange bias of more
than 1 Tesla in a natural mineral intergrowth. Nature Nanotechnology 2: 631-634
246
Müller RD, Sdrolias M, Gaina C, Roest WR
(2008) Age, spreading rates, and spreading
asymmetry of the world’s ocean crust.
Geochemistry, Geophysics, Geosystems
9: Q04006, doi: 10.10129/2007GC001743
Pentcheva R, Nabi H, (2008) Interface
magnetism in Fe2O3/FeTiO3 heterostructures. Physical Review B 77: doi: 10.1103/
PhysRevB.77.172405
Robinson P, Harrison RJ, McEnroe SA,
Hargraves RB (2002) Lamellar magnetism
in the haematite–ilmenite series as an
explanation for strong remanent magnetization. Nature 418: 517-520
Robinson P, Harrison RJ, McEnroe SA,
Hargraves RB (2004) Nature and origin
of lamellar magnetism in the hematiteilmenite series. American Mineralogist
89: 725-747
Robinson P, Harrison RJ, McEnroe SA
(2006) Fe2+ / Fe3+ charge ordering in
contact layers of lamellar magnetism:
bond valence arguments. American
Mineralogist 91: 67-72
Schmidt PW, McEnroe SA, Clark DA,
Robinson P (2007) Magnetic properties
and potential field modeling of the
Peculiar Knob metamorphosed iron
formation, South Australia: An analog for
the source of the intense Martian
magnetic anomalies? Journal of
Geophysical Research 112: B03102,
doi:10.1029/2006JB004495
Valet J-P (2003) Time variations in geomagnetic intensity. Reviews in Geophysics 41:
1004, doi: 10.1029/2001RG000104
Wilson JR, Robins B, Neilsen F, Duchesne JC,
Vander Auwera J (1996) The Bjerkreim-Sokndal
layered intrusion, southwest Norway. In:
Cawthorn RG (ed) Layered Intrusions. Elsevier,
Amsterdam, pp 231-256 A ugus t 2009