Geoscience Reference
In-Depth Information
Recognizing these limitations, a group of
scientists initiated a coordinated effort to
develop new techniques of quantitative studies
of plastic properties at high pressures exceeding
∼
Deformation experiments at much higher pres-
sures were conducted using a diamond anvil cell
(DAC) (e.g., Meade & Jeanloz, 1988; Sung
et al
.,
1977; Kinsland &Bassett, 1977; Wenk
et al
., 2004;
Merkel
et al
., 2006, 2007; Miyagi
et al
., 2011).
However, in most studies with DAC, temper-
ature was low (some are at room temperature)
and in all cases, strain-rates are unknown. Con-
sequently, the applicability of these results to
deformation in the Earth's hot interior is highly
questionable.
It must be remembered that simply conducting
some poorly characterized deformation experi-
ments under high pressures does not help our
understanding of rheological properties of the
deep interior of the Earth. Many issues learned by
low-pressure experiments (e.g., control of water
content and grain-size, identification of defor-
mation mechanisms to justify the extrapolation
in strain-rate) need to be carefully examined
in order to obtain results that can be applied
to the Earth's deep interior. The lack of such
an analysis is the main source of confusions
as shown by (Karato, 2010b) for olivine. A
combination of careful experimentation with
technical developments toward high-pressure
studies is essential to make further progress in
this area.
Figure 4.7 shows a range of pressure and tem-
perature conditions in which deformation exper-
iments were performed using various apparatus.
10GPa (Karato & Weidner, 2008). These new
developments include the design of new types
of deformation apparatus (Figure 4.6c,d) and the
use of synchrotron X-ray facilities to measure
the stress and strain
in-situ
(e.g., Weidner, 1998;
Karato & Weidner, 2008). In these new methods,
stress is not measured by a load cell as has been
the case of low-pressure apparatuses but by X-ray
diffraction.
Theories of stress measurements using X-ray
diffraction were developed by Singh (1993) and
Karato (2009). Singh (1993) considered only
elastic deformation, and in this theory, the
anisotropy in lattice strain (strain in crystalline
lattice) is caused by the anisotropy in the elastic
constants. But this theory does not explain
observed highly anisotropic lattice strain in
plastically deformed materials (e.g., Weidner
et al
., 2004; Chen
et al
., 2006a). Karato (2009)
developed a new theory in which the influence of
plastic deformation to redistribute stress among
grains is included for nonlinear constitutive
relationship. In this theory, anisotropy in lattice
strain is caused both by elastic and plastic
anisotropy. Plastic anisotropy is usually much
larger than elastic anisotropy and this theory
explains the observed large anisotropy in lattice
strain. When elastic anisotropy is known, this
theory provides a way to infer plastic anisotropy
from observed anisotropy in lattice strain.
At the time of this writing (February 2012),
quantitative deformation experiments have been
conducted to P
4.4 Basic Experimental Observations
4.4.1 Influence of temperature, stress and
grain-size
2200 K using
the rotational Drickamer apparatus (RDA) with
the resolution of stress measurements of
∼
23GPa and T
∼
Figure 4.8 illustrates some of the experimental
data on plastic deformation of some minerals
showing the evidence of various deformation
mechanisms discussed above. Transition from
diffusion to dislocation creep as stress increases
(or grain-size increases) is well documented
(Figure 4.8b), and nonlinear flow law is also
well established at relatively high stresses. Most
experimental results show strongly temperature-
dependent rheology. However, there are limited
10MPa
(Hustoft
et al
., 2013; Weidner
et al
., 2010) (to
∼
∼
1700K with D-DIA (Kawazoe
et al
.,
2011)). The resolution of stress measurements
is not as good as that in the gas-medium,
low-pressure apparatus, but this resolution is
high enough to characterize rheological prop-
erties in most cases (the resolution of stress is
∼
15GPa,
∼
10MPa).
