Laboratory Manual for Introductory Geology 4e

FIGURE 7.2 Growth of metamorphic minerals.
(a) A huge, nearly perfect garnet crystal.
(b) Andalusite crystals.
surface have relatively open structures compatible with their low-pressure environment.
Under high pressure, those structures collapse, and the ions are rearranged
in more densely packed structures. Bury graphite, for example, deeply
enough in the mantle and it undergoes a phase change to diamond.
Both temperature and pressure increase with depth, so rocks must respond to
these agents simultaneously. The geothermal gradient varies, but is typically 30°C per
kilometer, whereas the geobaric gradient is about 0.33 kilobars (kbar) per kilometer
(1 kilobar 5 1,000 times atmospheric pressure). Thus, temperature-pressure conditions
10 km below the surface would be about 300°C and 3.3 kbar. At 25 km, temperature
would be about 750°C and pressure about 8.3 kbar. Therefore, as a rock is buried
deeper in the Earth, it must adjust to progressively increasing temperatures and pressures.
An assemblage of metamorphic minerals that formed at 10 km would have to
change to adjust to more intense conditions if it were buried more deeply at 25 km.
EXERCISE 7.1
Responses to Changing Metamorphic Conditions
Name:
Course:
The minerals andalusite, sillimanite, and kyanite are
polymorphs of aluminosilicate (Al 2
SiO 5
) that differ in
their internal structures and in the conditions under
which they are stable. The aluminosilicate phase
diagram on the right, which uses different colors to
delineate the stability zones of these polymorphs, provides
insight into how metamorphic minerals respond
to changes in temperature and pressure conditions.
Examine the phase diagram and answer the following
questions:
(a) Indicate the temperature-pressure (T-P) conditions
and estimated depths for rocks A, B, and C in the
following table. Assume average geothermal (30°C per
kilometer) and geobaric gradients (0.33 kbar per
kilometer).
Pressure (kbar)
10
8
6
4
2
A
0
400 500
Andalusite
Section:
Date:
Kyanite
B
Sillimanite
600 700 800 900 1,000
Temperature (ºC)
Phase diagram showing stability zones for three
aluminosilicate (Al 2
SiO 5
) polymorphs.
C
(continued)
178 CHAPTER 7 INTERPRETING METAMORPHIC ROCKS