What Happens To Our PROTOLITH When Acted On by AGENTS OF CHANGE??
What Happens To Our PROTOLITH When Acted On by AGENTS OF CHANGE??
What Happens To Our PROTOLITH When Acted On by AGENTS OF CHANGE??
Hunk o’ rock
Thermodynamics Primer
• Thermodynamics describes IF a reaction CAN
occur at some condition (T, P, composition
typically)
• Second Law of thermodynamics:
• G=H – TS
– Where G, Gibb’s free energy determines IF the
REACTION will go forward (-G=spontaneous)
– H is enthalpy – has to do with heat…
– S is entropy – has to do with bonds and order…
Thermodynamics vs. Kinetics
• Thermodynamics – comparing the potential
ENERGY of things what is more stable?
Will a reaction occur at some T,P, soln, melt
composition go or Not?
• Kinetics IF thermodynamics says YES,
the reaction should occur (always toward
lower energy!) kinetics determines how fast
• Minerals out of equilibrium pass the
thermodynamic test but the kinetics of their
reaction is very slow…
Phase diagrams
• Tool for ‘seeing’ phase transitions
• H2Oice H2Oliquid
• Reaction (line) governed
by G=H – TS
• Phase Rule:
– P+F=C+2
– Phases coexisting + degrees of freedom = number
of components + 2
– Degree of freedom 2= either axis can change
and the phase stays the same where??
Phase diagrams
• Let’s think about what
happens to water as
conditions change…
A B
• P+F=C+2
C
• Point A?
• Point B?
• Point C?
Mineral Assemblages in
Metamorphic Rocks
T
Yellow = (OH) O
- vdw
S = serpentine T = talc
Nagby and Faust (1956) Am.
Mineralogist 41, 817-836.
“Pyralspites” - B = Al
Pyrope: Mg3 Al2 [SiO4]3
Almandine: Fe3 Al2 [SiO4]3
Spessartine: Mn3 Al2 [SiO4]3
“Ugrandites” - A = Ca
Uvarovite: Ca3 Cr2 [SiO4]3
Grossularite: Ca3 Al2 [SiO4]3
Andradite: Ca3 Fe2 [SiO4]3
Occurrence:
Mostly metamorphic
Some high-Al igneous
Also in some mantle peridotites
Garnet (001) view blue = Si purple = A turquoise = B
Staurolite
• Aluminosilicate - Fe2Al9Si4O22(OH)2
• Similar structure to kyanite with tetrahedrally
coordinated Fe2+ easily replaced by Zn2+ and Mg2+
• Medium-grade metamorphic mineral, typically
forms around 400-500 C
– chloritoid + quartz = staurolite + garnet
– chloritoid + chlorite + muscovite = staurolite + biotite +
quartz + water
• Degrades to almandine (garnet at higher T)
– staurolite + muscovite + quartz = almandine +
aluminosilicate + biotite + water
Actinolite
Metamorphic Facies
• Where do we find
these regimes of P-
T ‘off’ of the typical
continental
isotherms??
• How is the
environment that
forms a blueschist
facies rock different
from one forming a
hornfels?
Metamorphic Facies
• Table 25-1. The definitive mineral assemblages
that characterize each facies (for mafic rocks).
Facies Series
• Miyashiro (1961) initially proposed five facies series,
most of them named for a specific representative
“type locality” The series were:
1. Contact Facies Series (very low-P)
2. Buchan or Abukuma Facies Series (low-P
regional)
3. Barrovian Facies Series (medium-P
regional)
4. Sanbagawa Facies Series (high-P,
moderate-T)
5. Franciscan Facies Series (high-P, low T)
Fig. 25-3.
Temperature-
pressure diagram
showing the three
major types of
metamorphic
facies series
proposed by
Miyashiro (1973,
1994). Winter
(2001) An
Introduction to
Igneous and
Metamorphic
Petrology.
Prentice Hall.
Isograds
• Lines (on a map) or Surfaces (in the 3D
world) marking the appearance or
disappearance of the Index minerals in
rocks of appropriate composition
e.g. the ‘garnet-in isograd’; the ‘staurolite-
out isograd’
Complicated by the fact that most of these
minerals are solid solutions
• Isograds for a
single shale unit
in southern
Vermont
• Which side
reflects a higher
grade, or higher
P/T environment?