Page 267 - Microtectonics

9.9 · Temperature Gauges 259
ary diffusion rates are fully temperature dependent,
which is directly reflected by the spacing of the symplec-
tite (Shewmon 1969). Since the exsolved volume percent-
age of mineral B depends on the original composition of
mineral A, it is the combined width of a pair of lamellae
of A' and B that is critical, not their individual widths
(Fig. 9.9; van Roermund 1992). Good examples of exso-
lution symplectites can be found in retrogressed eclogites
in which the Na-bearing clinopyroxene becomes replaced
by clinopyroxene-plagioclase symplectites (Boland and
van Roermund 1983; van Roermund and Boland 1983).
Development of phase transformations such as exso-
lution structures can be shown in a TTT diagram (time-
temperature-transformation) as pairs of curves, one for
the time and temperature when a new phase is first de-
tectable, the second for conditions when the reaction is
completed (Putnis and McConnell 1980). At high tem-
perature, approaching the critical temperature below
which exsolution is possible, diffusion rates are high but
nucleation rates are low; at low temperature, the reverse
is true. Consequently, the curves in a TTT diagram show
a minimum time for nucleation at some intermediate
temperature.
Fig. 9.8. Diagram showing the development trend of twins with increas- Figure 9.9 shows a TTT diagram with a scheme of
ing strain (arrows): at low temperature, more twins of Type I tend to be
generated; at higher temperature, wider twins form which increase in possible geometries of exsolved plagioclase lamellae in
width, but not in number with increasing strain. (After Ferrill et al. 2004) clinopyroxene. At high temperature, lamellae form ho-

Fig. 9.9. Schematic TTT diagram showing the relative position of start and finish curves for inter- and intracrystalline exsolution of plagioclase
from omphacite, after van Roermund (1992). Straight dotted arrows show isothermal trajectories of symplectite growth. Insets show schematic
details of the developing microstructure along a grain boundary between two omphacite grains with identical chemistry but different orienta-
tion, indicated by striping. At high temperature, plagioclase nucleates inside the omphacite crystals. At lower temperature, exsolution symplec-
tites are formed when the grain boundary migrates into one of the crystals. Small arrows over insets indicate movement direction of the grain
boundary. The spacing of symplectite lamellae pairs decreases with decreasing temperature, as indicated. If temperature decreases signifi-
cantly with time (curved paths), complex internal structures are formed. Such microstructures can be used to reconstruct cooling paths