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

Transcription

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

2 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 H 2 O) and, transport and exchange of substances and heat between domains where such reactions take place

3 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

4 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

5 Foliations Foliations may be produced through the rotation of pre-existing minerals into parallelism or the growth of new minerals in a preferred orientation

6 Foliations may record poly-metamorphic events resulting in the development of a crenulation cleavage

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8 Analysis of Deformed Rocks

9 Pressure solution Grains may develop a weak shape fabric that defines the foliation. Pressure solution induces mass transfer.

10 Intra granular textures Deformation results in an increase in lattice strain energy Strained grains show UNDULOSE extinction e.g. quartz

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

12 Relative timing of metamorphism & deformation Pre-kinematic crystals a. Bent crystal with undulose extinction b. Foliation wrapped around a porphyroblast c. Pressure shadow or fringe d. Kink bands or folds e. Microboudinage f. Deformation twins

13 Syn kinematic porphyroblast

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16 Post-kinematic Pre-kinematic Syn-kinematic From Yardley (1989) An Introduction to Metamorphic Petrology. Longman.

17 Contact Metamorphism Granoblastic grain boundaries: 120 represent a minimum surface energy state 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.

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

19 Pre-existing fabrics e.g. bedding or tectonic foliation are progressively removed due to recrystallization Rapid growth induced by local heat source leads to poikiloblast development. Progressive thermal metamorphism of slate. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

20 Porphyroblasts and/or poikiloblasts are randomly oriented Progressive thermal metamorphism of slate. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

21 Crystalloblastic series Differences in development of crystal form among some metamorphic minerals. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco. Most Euhedral Titanite, rutile,, pyrite, spinel Garnet, sillimanite,, staurolite, tourmaline Epidote,, magnetite, ilmenite Andalusite,, pyroxene, amphibole Mica, chlorite, dolomite, kyanite Calcite, Feldspar, quartz, cordierite Least Euhedral

22 Reaction Textures

23 Depletion haloes Progressive development of a depletion halo about a growing porphyroblast. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco. Figure 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 Depletion halo around garnet Igneous porphyroblast. and Metamorphic Petrology. Boehls Butte Prentice area, Hall. Idaho

24 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.

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26 Metamorphic Reactions Reactions 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

27 1. Phase Transformations Isochemical phase transformations (the polymorphs of SiO 2 or Al 2 SiO 5 or graphite diamond or calcite aragonite are in many ways the simplest to deal with) The transformations depend on temperature and pressure only

28 1. Phase Transformations

29 1. Phase Transformations

30 2. Solid Solid Net Transfer Reactions 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

31 2. Solid Solid Net Transfer Examples: Reactions NaAlSi 2 O 6 + SiO 2 = NaAlSi 3 O 8 Jadeite Qtz Albite MgSiO 3 + CaAl 2 Si 2 O 8 = CaMgSi 2 O 6 + Al 2 SiO 5 Enstatite Anorthite Diopside Andalusite 4 (Mg,Fe)SiO 3 + CaAl 2 Si 2 O 8 = Opx Anorthite (Mg,Fe) 3 Al 2 Si 3 O 12 + Ca(Mg,Fe)Si 2 O 6 + SiO 2 Garnet Cpx Qtz

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33 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 consumed For example, the reaction: Mg 3 Si 4 O 10 (OH) MgSiO 3 = Mg 7 Si 8 O 22 (OH) 2 Tlc En Ath involves hydrous phases, but conserves H 2 O It may therefore be treated as a solid solid nettransfer reaction

34 3. Devolatilization Reactions Among the most common metamorphic reactions H 2 O CO 2 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

35 3. Devolatilization Reactions The equilibrium curve represents equilibrium between the reactants and products under watersaturated conditions

36 KAl 2 Si 3 AlO 10 (OH) 2 + SiO 2 = KAlSi 3 O 8 + Al 2 SiO 5 + H 2 O Ms Qtz Kfs Sill W Suppose H 2 O is withdrawn from the system at some point on the water saturated equilibrium curve: p H2O < P lithostatic According 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 + H 2 O field expands slightly at the expense of the Mu + Qtz field, and the reaction curve shifts toward lower temperature

37 4. Devolatilization Reactions p H2O can become less than P Lith by either of two ways P fluid < P Lith by drying out the rock and reducing the fluid content P fluid = P Lith, but the water in the fluid can become diluted by adding another fluid component, such as CO 2 or some other volatile phase

38 3. Devolatilization Reactions An 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 X fluid phase diagram Because H 2 O and CO 2 are by far the most common metamorphic volatiles, the X in T X diagrams is usually the mole fraction of CO 2 (or H 2 O) in H 2 O CO 2 mixtures Because pressure is also a common variable, a T X fluid diagram must be created for a specified pressure

39 3. Devolatilization Reactions

40 3. Devolatilization Reactions Decarbonation reactions may be treated in an identical fashion For example, the reaction: CaCO 3 + SiO 2 = CaSiO 3 + CO 2 (26 6) Cal Qtz Wo Can also be shown on a T X CO2 diagram

41 3. Devolatilization Reactions Figure A portion of the equilibrium boundary for the calcitearagonite phase transformation in the CaCO 3 system. After Johannes and Puhan (1971), Contrib. Mineral. Petrol., 31, Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Figure T-X CO2 phase diagram for the reaction Cal + Qtz = Wo + CO 2 at 0.5 GPa assuming ideal H 2 O-CO 2 mixing, calculated using the program TWQ by Berman (1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

42 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

43 5. Redox Reactions Involves a change in oxidation state of an element 6 Fe 2 O 3 = 4 Fe 3 O 4 + O 2 2 Fe 3 O SiO 2 = 3 Fe 2 SiO 4 + O 2

44 6. Reactions Involving Dissolved Species Minerals plus ions neutral molecules dissolved in a fluid One example is hydrolysis: 2 KAlSi 3 O H + + H 2 O = Al 2 Si 2 O 5 (OH) 4 + SiO K + Kfs aq. species kaolinite qtz aq.species