metamorphic petrology and strontium - The Department of Geology
Metamorphic Petrology GLY 262 - Geology papers€¦ · Igneous and Metamorphic Petrology. W. H....
Transcript of Metamorphic Petrology GLY 262 - Geology papers€¦ · Igneous and Metamorphic Petrology. W. H....
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