Geoscience Reference
In-Depth Information
tend to have lower resolution in mechanical
measurements.
Let us consider a case of high-resolution,
low-pressure deformation apparatus versus high-
pressure, low-resolution deformation apparatus.
In the late 1960s to early 1970s, a large number of
pioneering experimental studies were conducted
at UCLA using a solid-medium deformation
apparatus designed by David Griggs (Griggs,
1967). Most of the basic concepts on the rheo-
logical properties of rocks such as (i) nonlinear
constitutive relationship, (ii) water weakening,
(iii) development of lattice-preferred orientation
(and its implications for seismic anisotropy) and
(iv) dynamic recrystallization (and its possible
implications for shear localization) were estab-
lished by their classic studies (for a summary
of these studies see Heard et al ., 1972). With
this apparatus, one can conduct deformation
experiments to P
Olgaard (2000) developed a torsion apparatus that
can be operated to
0.3 GPa. Subsequently, these
methods were used extensively in a few groups
(e.g., Mei & Kohlstedt, 2000a,2000b; Hirth &
Kohlstedt, 1995a,1995b; Holtzman et al ., 2003b;
Rybacki & Dresen, 2004; Rybacki et al ., 2006;
Bystricky et al ., 2001). The applications of these
low-pressure, high-resolution apparatus under
carefully controlled chemical environment were
critical in establishing the rigorous bases of
mineral and rock deformation studies (for an
excellent review on the low-pressure studies see
Kohlstedt, 2009).
However, the maximum pressure of experi-
mentation with this apparatus is low, P < 0 . 5GPa
(corresponding to a depth of
15 km) (most ex-
periments were conducted at P
0.3GPa). This
pressure range is small compared to the actual
pressures in the Earth's mantle (
2-10GPa
1600K), but the
uncertainties in stress measurements are large
due to the large influence of friction (errors in
stress measurements sometimes exceed 100%
(Gleason & Tullis, 1993)).
In the 1960s-80s, Mervyn Paterson at the
Australia National University (ANU) developed
a gas-medium deformation apparatus (Paterson,
1970, 1990; Chopra & Paterson, 1981) with which
high-resolution mechanical tests can be made
to P
2GPa(Tto
in the asthenosphere,
13GPa at 410 km,
24GPa at 660 km). Therefore applications of
low-pressure data need a large extrapolation in
pressure. For such an extrapolation, pressure
dependence of creep strength needs to be known
precisely. However, it is difficult to determine
the pressure dependence of deformation from
low-pressure experiments. In addition, many
minerals are stable only under high-pressure con-
ditions (e.g., orthopyroxene (13GPa > P > 1GPa),
wadsleyite (17GPa > P > 14GPa), perovskite
(120GPa > P > 24GPa)). Rheological properties
of these minerals cannot be studied using
these low-pressure apparatus. Furthermore,
the functional form of pressure dependence of
deformation likely changes at around P
0.3GPa and T
1600K (stress resolution
1MPa). A key element in this apparatus is
the use of an internal load cell for load (stress)
measurements. Because the load cell is located
in the pressure vessel, there is no need for
the correction for friction and the load (stress)
measurements can be made as precisely as at
room pressure. This apparatus was used exten-
sively in the Paterson's lab at ANU (e.g., Chopra
& Paterson, 1981, 1984; Karato et al ., 1986;
Mackwell et al ., 1985). In particular, Karato et al .
(1986) introduced a new method of deformation
experiments where high-resolution mechanical
tests are conducted on synthetic samples with
controlled grain-size and water content. Also
Zhang and Karato (1995) developed a shear
deformation technique using the high-resolution
gas-medium apparatus. Similarly Paterson and
0.5GPa
when water is present in the system (Karato,
2008). This means that the results on the
influence of water obtained below P
0.5GPa
cannot be extrapolated to higher pressures even
qualitatively (see the later section on the water
effect). Consequently, results from low-pressure
( < 0.5GPa) experiments have limited applicabil-
ity to the regions deeper than
20 km (in the
Earth). Consequently, the rheological properties
of more than 99% of the mantle cannot be
investigated by these low-pressure studies.
Search WWH ::




Custom Search