Transcript Lecture 31- Metamorphic Processes II

What happens to our PROTOLITH when
acted on by AGENTS OF CHANGE??
• Agents of Change  T, P, fluids, stress, strain
• Metamorphic Reactions!!!!
–
–
–
–
–
–
Solid-solid phase transformation
Solid-solid net-transfer
Dehydration
Hydration
Decarbonation
Carbonation
Solid-solid phase transformation
• Polymorphic reaction  a mineral reacts
to form a polymorph of that mineral
• No transfer of matter, only a rearrangment
of the mineral structure
• Example:
– Andalusite  Sillimanite
Al2SiO5
Al2SiO5
Solid-solid net-transfer
• Involve solids only
• Differ from polymorphic transformations: involve
solids of differing composition, and thus material
must diffuse from one site to another for the
reaction to proceed
• Examples:
• NaAlSi2O6 + SiO2 = NaAlSi3O8
Jd
Qtz
Ab
• MgSiO3 + CaAl2Si2O8 = CaMgSi2O6 + Al2SiO5
En
An
Di
And
Solid-Solid Net-Transfer II
• If minerals contain volatiles, the volatiles must
be conserved in the reaction so that no fluid
phase is generated or consumed
• For example, the reaction:
Mg3Si4O10(OH)2 + 4 MgSiO3 = Mg7Si8O22(OH)2
Talc
Enstatite
Anthophyllite
involves hydrous phases, but conserves H2O
It may therefore be treated as a solid-solid
net-transfer reaction
Hydration/ Dehydration Reactions
• Metamorphic reactions involving the
expulsion or incorporation of water (H2O)
• Example:
– Al2Si4O10(OH)2 <=> Al2SiO5 + 3SiO2 + H2O
Pyrophyllite
And/Ky
Quartz
water
Carbonation / Decarbonation
Reactions
• Reactions that involve the evolution or
consumption of CO2
• CaCO3 + SiO2 = CaSiO3 + CO2
calcite
quartz
wollastonite
Reactions involving gas phases are also
known as volatilization or devoltilization
reactions
These reactions can also occur with other
gases such as CH4 (methane), H2, H2S, O2,
NH4+ (ammonia) – but they are not as
common
Systems
• Rock made of different minerals
• Metamorphic agents of change beat on it 
metamorphic reactions occur
• A closed system does not gain or lose
material of any kind
• An open system can lose stuff – liquids,
gases especially
Outside
world
Hunk o’ rock
Thermodynamics Primer
• Thermodynamics describes IF a reaction CAN
occur at some condition (T, P, composition
typically)
• Second Law of thermodynamics:
• DG=DH – TDS
– Where G, Gibb’s free energy determines IF the
REACTION will go forward (-DG=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 DG=DH – TDS
• 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…
• P+F=C+2
A
C
• Point A?
• Point B?
• Point C?
B
Mineral Assemblages in
Metamorphic Rocks
• Equilibrium Mineral Assemblages
• At equilibrium, the mineralogy (and the
composition of each mineral) is determined
by T, P, and X
• Relict minerals or later alteration products
are thereby excluded from consideration
unless specifically stated
The Phase Rule in Metamorphic
Systems
• Phase rule, as applied to systems at equilibrium:
F=C-P+2
the phase rule
P is the number of phases in the system
C is the number of components: the minimum
number of chemical constituents required to
specify every phase in the system
F is the number of degrees of freedom: the
number of independently variable intensive
parameters of state (such as temperature,
pressure, the composition of each phase,
etc.)
The Phase Rule in Metamorphic Systems
Consider the following three scenarios:
C = 1 (Al2SiO5)
 F = 1 common
 F = 2 rare
 F = 3 only at the
specific P-T
conditions of the
invariant point
(~ 0.37 GPa and
500oC)
Figure 21-9. The P-T phase diagram for the system Al2SiO5
calculated using the program TWQ (Berman, 1988, 1990,
1991). Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
Metamorphic facies
• P-T conditions, presence of fluids induces
different metamorphic mineral assemblages
(governed by thermodynamics/ kinetics)
• These assemblages are lumped into
metamorphic facies (or grades)
Aluminosilicate Minerals
• SILLIMANITE: Orthorhombic: Octahedral Al chains (6-fold) are
crosslinked by both Si and Al tetrahedra (4-fold).
• ANDALUSITE: Orthorhombic: 5-coordinated Al; Same octahedral (6fold) chains.
• KYANITE: Triclinic: All the Al is octahedrally coordinated (6- and 6fold).
Andalusite
Kyanite
Sillimanite
•Clearly, changes in structure are in response to changing P and T. Result is
changes in Al coordination.
•Phase transformations require rebonding of Al. Reconstructive polymorphism
requires more energy than do displacive transformations. Metastability of these 3
are therefore important (Kinetic factors limit equilibrium attainment).
•All 3 are VERY important metamorphic index minerals.
Aluminosilicate Minerals
• 3 polymorphs of Al2SiO5 are important
metamorphic minerals
Andalusite
Kyanite
Sillimanite
Topaz
• Aluminosilicate mineral as well, one oxygen
substituted with OH, F
• Al2SiO4(F,OH)2
• Where do you think Topaz forms??
Serpentine Minerals
• Mg3Si2O5(OH)4 minerals (principally as
antigorite, lizardite, chrysotile polymorphs)
• Forms from hydration reaction of magnesium
silicates
– Mg2SiO4 + 3 H2O  Mg3Si2O5(OH)4 + Mg(OH)2
forsterite
serpentine
brucite
• Asbestosform variety is chrysotile (accounts
for 95% of world’s asbestos production 
MUCH LESS DANGEROUS than crocidolite)
Phyllosilicates
Yellow = (OH)
Serpentine: Mg3 [Si2O5] (OH)4
T-layers and triocathedral (Mg2+) layers
(OH) at center of T-rings and fill base of VI layer 
weak van der Waals bonds between T-O groups
T
O
T
O
T
O
vdw
vdw
Serpentine
Antigorite maintains a
sheet-like form by
alternating segments of
opposite curvature
Chrysotile does not do this
and tends to roll into tubes
Octahedra are a bit larger than tetrahedral
match, so they cause bending of the T-O
layers (after Klein and Hurlbut, 1999).
Serpentine
Nagby and Faust (1956) Am.
Mineralogist 41, 817-836.
Veblen and Busek, 1979,
Science 206, 1398-1400.
S = serpentine T = talc
The rolled tubes in chrysotile resolves the apparent
paradox of asbestosform sheet silicates
Chlorite
• Another phyllosilicate, a group of
difficult to distinguish minerals
• Typically green, and the dominant
and characteristic mineral of
greenschist facies rocks
• Forms from the alteration of Mg-Fe
silicates (pyroxenes, amphiboles,
biotite, garnets)
Prehnite-Pumpellyite
• Minerals related to chlorite, form at slightly
lower P-T conditions
• Prehnite is also green, pumpellyite
Micas
• Biotite and Muscovite are also important
metamorphic minerals (muscovite often the
principle component of schists)
• Phlogopite – similar to biotite, but has little
iron, forms from Mg-rich carbonate deposits
and a common mineral in kimberlites
(diamond-bearing material)
• Sericite – white mica (similar to muscovite) –
common product of plagioclase feldspar
alteration at low grades
Zeolites
• Diverse group of minerals forming at lower
metamorphic grades
• Framework silicas, but characteristically
containing large voids and highly variable
amounts of H2O
– Name is from the greek – meaning to boil stone
as the water can de driven off with heat
– Voids can acts as molecular sieves and traps for
many molecules
– Diversity of minerals in this group makes a for a
wide variety of sieve and trapping properties
selective for different molecules
Epidote Group
• Sorosilicates (paired silicate tetrahedra)
• Include the mineral Epidote Ca2FeAl2Si3O12(OH),
Zoisite (Ca2Al3Si3O12(OH) and clinozoisite
(polymorph)
Garnets
Garnet: A2+3 B3+2 [SiO4]3
“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
thatTable
characterize
eachAssemblages
facies (for
mafic Facies
rocks).
25-1. Definitive Mineral
of Metamorphic
Facies
Zeolite
Definitive Mineral Assemblage in Mafic Rocks
zeolites: especially laumontite, wairakite, analcime
Prehnite-Pumpellyite
prehnite + pumpellyite (+ chlorite + albite)
Greenschist
chlorite + albite + epidote (or zoisite) + quartz ± actinolite
Amphibolite
hornblende + plagioclase (oligoclase-andesine) ± garnet
Granulite
orthopyroxene (+ clinopyrixene + plagioclase ± garnet ±
hornblende)
Blueschist
glaucophane + lawsonite or epidote (+albite ± chlorite)
Eclogite
pyrope garnet + omphacitic pyroxene (± kyanite)
Contact Facies
After Spear (1993)
Mineral assemblages in mafic rocks of the facies of contact metamorphism do not differ substantially from that of the corresponding
regional facies at higher pressure.
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.
Temperaturepressure 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 ‘stauroliteout 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?