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Figure 1. Image
of the magnetization states of individual magnetite blocks in a natural intergrowth
of magnetite and ulvöspinel. Figure courtesy of Richard Harrison.
Seismic-frequency anelasticity in perovskite
As seismologists develop a sharper picture of seismic attenuation in the lower
mantle, there is now a clear need to understand the mineralogical and rock-mechanical
processes responsible for the attenuation of seismic waves (B. Romanowicz and
J.J. Durek, Earth's Deep Interior - Mineral Physics and Tomography from the
Atomic to the Global Scale, American Geophysical Union, 2000). To investigate
the microscopic origin of anelasticity in minerals it is necessary to determine
their mechanical response in the low-frequency seismic range (millihertz-hertz)
rather than the high-frequency ultrasonic range (Megahertz-Gigahertz) more conventionally
used to measure the elastic properties of minerals. This is achieved using the
technique of dynamical mechanical analysis, employed in either forced-torsional-oscillation
or three-point-bend geometry.
Much recent work in this field has concentrated on the role of grain-boundary
viscoelastic processes (grain-boundary sliding) as a source of seismic attenuation
(for example, Webb and others, Physics of the Earth and Planetary Interiors,
v. 115, p. 259-291).
Harrison and co-workers considered an alternative mechanism related to the motion
of transformation domain walls in perovskite (for example, Physics of the
Earth and Planetary Interiors, v. 134, p. 253-272). They investigated the
seismic-frequency mechanical behaviour of single-crystal lanthanum aluminate,
a structural analogue of lower-mantle magnesium silicate perovskite. A structural
phase transition at 550 degrees Celsius leads to a loss of symmetry from cubic
to rhombohedral, accompanied by a spontaneous shear lattice strain. A choice
of energetically equivalent crystallographic orientations for the spontaneous
strain leads to the formation of transformation twins separated by domain walls.
When a dynamic stress in the seismic-frequency range is applied, the mechanical
strain is accommodated by the lateral displacement of domain walls.
The result is a dramatic mechanical softening and a large relaxational peak
in the mechanical loss spectrum. The frequency and temperature dependence of
mechanical loss yields an activation energy for domain wall motion of 0.72 electron
volts (72 kilojoules/mole), close to that observed for the diffusion of oxygen
vacancies in perovskite. Given reasonable estimates for the activation volume
of oxygen vacancy diffusion, it is possible that domain walls play a significant
role in determining the seismic properties of perovskite in the lower mantle.
Nanoscale magnetic interactions
in minerals
Since the first discovery of magnetic anomalies in the Martian crust (for example,
Acuña and colleagues, Science, v. 284, p. 790-793), a debate has
raged as to the mineralogical nature of the magnetic carriers (for example,
G. Kletetschka and others, Physics of the Earth and Planetary Interiors,
v. 119, p. 259-267. The anomalies were created more than 4 billion years ago,
at a time when Mars still generated its own magnetic field. Now that the Martian
geodynamo has stopped, the anomalies can only be explained by a remanent magnetization
carried by iron-bearing minerals in the crust. The remanent anomalies are around
20 times stronger than those observed on Earth and, given their age, are extremely
stable.
This combination of magnetic properties is difficult to generate in nature without
the aid of fine-scale microstructures. It has long been proposed that solid-state
processes such as subsolvus exsolution could generate stable single-domain (SD)
magnetite particles from multi-domain (MD) grains via processes of grain subdivision.
Harrison and co-workers (Proceedings of the National Academy of Sciences,
v. 99, p. 16556-16561) used off-axis electron holography (a TEM technique that
yields a two-dimensional vector map of the magnetic fields in a sample with
nanometer resolution) to image the magnetization states of individual magnetite
blocks in a natural intergrowth of magnetite and ulvöspinel (Fig. 1). Both
single-domain and vortex states in individual magnetite blocks could be imaged
directly, and detailed information about magnetostatic interaction fields was
obtained. The magnetic microstructure was found to be dominated by the shape
anisotropies of individual blocks and by magnetostatic interactions between
neighboring blocks.
Lamellar magnetism
Arguably, the terrestrial magnetic anomalies most closely resembling those in
the Martian crust are observed in igneous and metamorphic provinces in Sweden,
Norway, Canada and the United States (for example, McEnroe and others, Journal
of Geophysical Research, v. 106, p. 30523-30546), characterized by large
remanence-dominated magnetic anomalies up to 1 billion years old. The rocks
have high coercivities and high thermal stabilities, making them excellent analogies
of the magnetic anomalies on Mars. This intriguing possibility has led to a
comprehensive multidisciplinary study of their magnetic mineralogy (Robinson
and others, Nature, v. 418, p. 517-520).
Common to all the rocks studied are grains of ilmenite-hematite containing multiple
generations of exsolution lamellae, with thickness ranging down to unit-cell
scale (1-2 nanometers). These minerals consist of canted antiferromagnetic hematite
intergrown with paramagnetic ilmenite. The hematite-rich phase has a high Néel
temperature but is only weakly magnetic. The ilmenite-rich phase has the potential
to be strongly ferrimagnetic, but its Néel temperature is well below
room temperature. The observed magnetic properties of the intergrowth present
something of a paradox, since they exhibit both large magnetization and high
Néel temperature.
This paradox has been resolved with the aid of Monte Carlo computer simulations
(for example, Harrison and Becker, European Mineralogical Society Notes in
Mineralogy, v. 3). Their work has laid the basis for the theory of "lamellar
magnetism," in which a stable ferrimagnetic substructure is created by
the characteristic arrangement of cations and spins at the coherent interfaces
between hematite and ilmenite exsolution lamellae, a concept that has revolutionized
thinking about the possible magnetic moments in exsolved phases.
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