D. Control of grain size & Control of chemical environment
As discussed above, the control of grain size is an important
issue for high pressure and temperature in-situ deformation experiments
). We will incorporate our knowledge on grain-growth kinetics
(e.g., (KARATO, 1989b; NISHIHARA et al., 2006b; YAMAZAKI et al.,
2005; YAMAZAKI et al., 1996)) to control the grain size in this
range. We anticipate that although a small grain size could be
maintained at “dry” (water-free) conditions (NISHIHARA
et al., 2006b), it would be difficult to maintain a small grain
size at “wet” (water-rich) conditions. In these cases,
we may need to add a small amount of a secondary phase particles
to stabilize the grain size as has been done by (GORDON and TERWILLINGER,
1972). Grain size may be controlled automatically during dislocation
creep at large strains. If this is the case, then the stress level
needs to be chosen carefully to make sure that grain size is small
enough for high-resolution x-ray powder diffraction can be made.
Plastic deformation and hence deformation microstructures are
sensitive to chemical environment. Among the parameters in chemical
environment that are known to affect the plastic deformation include
water fugacity, oxygen fugacity and oxide activity (e.g., BAI
et al., 1991; KARATO and JUNG, 2003; MEI and KOHLSTEDT, 2000a;
MEI and KOHLSTEDT, 2000b). The control of oxygen fugacity and
oxide activity under high-pressure (and temperature) environment
is straightforward (e.g., NISHIHARA et al., 2006b; RUBIE et al.,
1993). In contrast, the control of water fugacity is more difficult,
but is more important than oxygen fugacity and oxide activity.
The challenges here include: (i) to find a way to maintain roughly
a same water fugacity through a deformation experiment, and (ii)
to find a suitable method to buffer water fugacity. Recent findings
of the stability of hydrous minerals indicates that there are
several hydrous minerals that can survive to relatively high temperatures
(~1500K) under high pressures (~10-20 GPa) (e.g., (OHTANI et al.,2001).
We will explore the use of some of the chemical reactions involving
hydrous minerals such as MgO + H2O = Mg(OH)2 to buffer the water
fugacity. We will determine the water content of a sample both
before and after deformation experiment. This will assure, at
the minimum, that water content of a sample is bracketed in a
certain range.
