Equilibrium Thermodynamics in Petrology

Geological materials are not the result of well controlled chemical experi-
ments. For example, during the metamorphism of a pelite, the original
clay or silt follows some temperature-pressure (P- T) path through time
(r) as a result of burial and heating followed by uplift and cooling. Can we
talk about equilibrium in the metamorphic process? This will depend
almost completely on the rate at which equilibrium is attained along the
P-T-t path. If the rate of equilibration is fast compared to the rate at
which P and T are changing with time, then the rock will maintain
equilibrium throughout its P-T path and would reach the Earth's surface
as some low temperature assemblage of clay minerals, etc. This does not
usually happen, the rock reaches the Earth's surface with a high P-T
assemblage. A reasonable interpretation is that the rate of attainment of
equilibrium became much slower than the rate of change of P-T with

time during cooling, so that the mineralogy of the rock records some high

P-T equilibrium attained before substantial cooling and uplift took place.The equilibrium represented in the mineralogy has been frozen in.


Metamorphic rocks are often more complicated, for example, the miner-
als may be zoned, the rock may have been metamorphosed several times,
or late-stage alteration minerals may be developed. Zoning can be
interpreted in several ways, but primarily it indicates that the rate of
formation of unzoned grains was slower than the rate of change of P-T
with time. For example, a zoned garnet in a metapelite may have formed
as the result of growth of the garnet over a section of the P-T-t path,
with each new zone of garnet attaining equilibrium with the rest of the
minerals while the rate of change of composition of previously-formed
zones in the garnet was insufficient to form homogeneous grains. The
presence of late-stage alteration minerals suggests that the high P-T
assemblage attempted to equilibrate at new lower P-T conditions under
certain favourable circumstances.

In general this book is tailor made for the newcomers in the realm of thermodynamics

with applications in petrology.