Metamorphic Petrology GLY 262Lecture 3: An introduction to metamorphism (II)

Metamorphic processes• Metamorphism is very complex and involves a 

large number of chemical and physical processes  occurring episodically at various scales 

• Metamorphic processes can be viewed as a  combination of;

– chemical reactions between minerals and  between minerals and gasses, liquids and 

fluids (mainly H2

O) and,

– transport and exchange of substances and  heat between domains where such reactions 

take place

Metamorphic Textures• Textures of a metamorphic rock evolve through the 

interaction of:(A) Mechanical (destructive) processes that are due 

to deviatoric

stresses.(B) Thermal (constructive) processes due to the input 

of heat.• The texture of a metamorphic rock depends on the 

metamorphic environment:‐

Regional Metamorphism

‐

Contact Metamorphism‐

Dynamic Metamorphism

Regional Metamorphism & Deformation

• Most regional metamorphism occurs in a  deviatoric

stress field.

• The main effect is to produce a tectonic  foliation and/or lineation by:

(1)

Oriented growth of new minerals(2)

Reorientation by crenulation

(3)

Pressure solution(4)

Elongation by plastic deformation

Foliations

Foliations may be produced through the rotation of pre-existingminerals into parallelismorthe growth of new minerals in a preferred orientation

Foliations may record poly-metamorphic events resultingin the development of a crenulation cleavage

Analysis of Deformed RocksAnalysis of Deformed Rocks

Pressure solution

Grains may develop a weak shape fabric that defines the foliation.

Pressure solution induces mass transfer.

Intra‐granular textures• Deformation results in an increase in lattice strain 

energy

Strained grains show UNDULOSE extinction e.g. quartz

Strain promotes recrystallization through the formation of sub-grains and the migration of grain boundaries.

Relative timing of metamorphism &  deformation

PrePre--kinematic kinematic crystalscrystals

a.a. Bent crystal with Bent crystal with unduloseundulose extinctionextinction

b.b. Foliation Foliation wrapped around wrapped around a porphyroblasta porphyroblast

c.c. Pressure shadow Pressure shadow or fringeor fringe

d.d. Kink bands or Kink bands or foldsfolds

e.e. MicroboudinageMicroboudinagef.f. Deformation Deformation

twins twins

Syn‐kinematic porphyroblast

From Yardley (1989) An Introduction to Metamorphic Petrology. Longman.

Post-kinematic

Pre-kinematic

Syn-kinematic

Contact Metamorphism

• Occurs in the absence of deviatoric

stress• Thermal energy input induces recrystallization:(i) Grain growth & increase in grain size(ii) Reducing surface energy by forming flat grain 

boundaries with equilibrium shapes.

120°

represent a minimumsurface energy state

Granoblastic grainboundaries:

Progressive thermal metamorphism of slate. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

Progressive thermal metamorphism of slate. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

Rapid growth inducedby local heat sourceleads to poikiloblastdevelopment.

Pre-existing fabricse.g. bedding or tectonicfoliation are progressivelyremoved due to recrystallization

Progressive thermal metamorphism of slate. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

Porphyroblasts and/orpoikiloblasts arerandomly oriented

Most Most EuhedralEuhedralTitaniteTitanite, , rutilerutile, pyrite, , pyrite, spinelspinel

Garnet, Garnet, sillimanitesillimanite, staurolite, , staurolite, tourmalinetourmaline

EpidoteEpidote, magnetite, ilmenite, magnetite, ilmenite

AndalusiteAndalusite, pyroxene, amphibole, pyroxene, amphibole

Mica, chlorite, dolomite, Mica, chlorite, dolomite, kyanitekyanite

Calcite, Feldspar, quartz, Calcite, Feldspar, quartz, cordieritecordierite

Least Least EuhedralEuhedral

Differences in development of crystal form among some metamorphic minerals. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

Crystalloblastic series

Reaction Textures

Depletion haloesDepletion haloes

Progressive development of a depletion halo about a growing porphyroblast. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

Figure 23-13. Light colored depletion haloes around cm-sized garnets in amphibolite. Fe and Mg were less plentiful, so that hornblende was consumed to a greater extent than was plagioclase as the garnets grew, leaving hornblende-depleted zones. Sample courtesy of Peter Misch. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Depletion halo around garnet porphyroblast. Boehls Butte area, Idaho

Fault Rock Textures• The processes that occur in relatively localized zones of 

deformation (fault or shear zones) are traditionally  recognised

as Dynamic metamorphism.

• In general deformation in fault zones (high strain) involves  grain size reduction.

Metamorphic ReactionsReactions are responsible for introducing or consuming minerals during metamorphism.By looking at the reactions we can:

Understand what physical variables might  affect the location of a particular reaction

We may also be able to estimate the P‐T‐X  conditions that a reaction represents

1.  Phase TransformationsIsochemical phase transformations (the polymorphs of SiO2 or Al2SiO5 or graphite‐diamond or calcite‐aragonite are in many ways the simplest to deal with)

The transformations depend on temperature and pressure only

1.  Phase Transformations

1. Phase Transformations

2. Solid‐Solid Net‐Transfer  Reactions

Involve solids onlyDiffer from polymorphic transformations: involve solids of differing composition, and thus material must diffuse from one site to another for the reaction to proceed

2. Solid‐Solid Net‐Transfer  Reactions

Examples:NaAlSi2

O6

+ SiO2

=  NaAlSi3

O8

Jadeite QtzAlbite

MgSiO3

+ CaAl2

Si2

O8

=  CaMgSi2

O6

+ Al2

SiO5

Enstatite

Anorthite Diopside

Andalusite

4 (Mg,Fe)SiO3

+ CaAl2

Si2

O8

= Opx

Anorthite

(Mg,Fe)3

Al2

Si3

O12

+ Ca(Mg,Fe)Si2

O6

+ SiO2Garnet

Cpx

Qtz

2. Solid‐Solid Net‐Transfer  Reactions

If minerals contain volatiles, the volatiles must be conserved in the reaction so that no fluid phase is generated or consumedFor example, the reaction:

Mg3

Si4

O10

(OH)2

+ 4 MgSiO3

=  Mg7

Si8

O22

(OH)2Tlc

En

Ath

involves hydrous phases, but conserves H2

O

It may therefore be treated as a solid‐solid net‐ transfer reaction

3. Devolatilization ReactionsAmong the most common metamorphic reactionsH2O‐CO2 systems are most common, but the principles are the same for any reaction involving volatiles Reactions are dependent not only upon temperature and pressure, but also upon the partial pressure of the volatile species

3. Devolatilization Reactions

The equilibrium curve  represents equilibrium  between the reactants and 

products under water‐ saturated conditions

KAl2

Si3

AlO10

(OH)2

+ SiO2

=  KAlSi3

O8

+ Al2

SiO5

+ H2

O Ms                Qtz

Kfs

Sill

W

Suppose H2O is withdrawn from the system at some point on the water‐saturated equilibrium curve: pH2O < PlithostaticAccording to Le Châtelier’s Principle, removing water at equilibrium will be compensated by the reaction running to the right, thereby producing more water

This has the effect of stabilizing the right side of the reaction at the expense of the left side

So as water is withdrawn the Kfs + Sill + H2O field expands slightly at the expense of the Mu + Qtz field, and the reaction curve shifts toward lower temperature

4. Devolatilization ReactionspH2O can become less than PLith by either of two ways

Pfluid

< PLith

by drying out the rock and reducing the fluid  content

Pfluid

= PLith

, but the water in the fluid can become  diluted by adding another fluid component, such as 

CO2

or some other volatile phase

3. Devolatilization ReactionsAn important point arising is thus

The temperature of an isograd/reaction based on a devolatilization reaction is sensitive to the partial pressure of the volatile species involved

An alternative: T‐Xfluid phase diagram

Because H2

O and CO2

are by far the most common  metamorphic volatiles, the X in T‐X diagrams is  usually the mole fraction of CO2

(or H2

O) in H2

O‐CO2

mixtures

Because pressure is also a common variable, a T‐Xfluid diagram must be created for a specified pressure

3. Devolatilization Reactions

3. Devolatilization ReactionsDecarbonation reactions may be treated in an identical fashionFor example, the reaction:

CaCO3

+ SiO2

=  CaSiO3

+ CO2

(26‐6)Cal

Qtz

Wo

Can also be shown on a T‐XCO2

diagram

3. Devolatilization Reactions

Figure 26-1. A portion of the equilibrium boundary for the calcite- aragonite phase transformation in the CaCO3 system. After Johannes and Puhan (1971), Contrib. Mineral. Petrol., 31, 28-38. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Figure 26-5. T-XCO2 phase diagram for the reaction Cal + Qtz = Wo + CO2 at 0.5 GPa assuming ideal H2 O-CO2 mixing, calculated using the program TWQ by Berman (1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

4. Ion Exchange Reactions

• Exchange of components between 2 or  more minerals

–Annite

+ Pyrope

= Phlogopite

+  Almandine

• Expressed as pure end‐members, but really  involves Mg‐Fe (or other) exchange 

between intermediate solutions

• Basis for many geothermobarometers

5. Redox

Reactions• Involves a change in oxidation state of an element

– 6 Fe2

O3

= 4 Fe3

O4

+ O2

– 2 Fe3

O4

+ 3 SiO2

= 3 Fe2

SiO4

+ O2

6. Reactions Involving Dissolved Species

• Minerals plus ions neutral molecules dissolved in a fluid• One example is hydrolysis:• 2 KAlSi3

O8

+ 2 H+

+ H2

O = Al2

Si2

O5

(OH)4

+ SiO2

+ 2 K+Kfs

aq. species      kaolinite

qtz

aq.species