Metamorphic Petrology GLY 262 Metamorphic reactions and isograds

Transcription

1 Metamorphic Petrology GLY 262 Metamorphic reactions and isograds

2 What do we mean by reaction?

3 Reaction: change in the nature or types of phases in a system=> formation of new mineral(s) ) which are stable under the new conditions. Reactions are written in the form: reactants = products A + B = C + D Or A = B + C + D Al-Silicates => Quartz + Corundum Al2O3 SiO2 => SiO2 + Al2O3 Stable under P1-T1 conditions Stable under P2-T2 conditions = Equilibrium

4 What is an Equilibrium?

5 Equilibrium If we subject a system to some specific conditions of P-T, and maintain these conditions unchanged for a sufficiently long time (t), the atoms/minerals in the system will group themselves into the most stable possible configuration and the system is then said to be in equilibrium. The most stable configuration is the one with the least free energy.

6 Energy States: Stability and Equilibrium example (A) Unstable: falling or rolling (B) Metastable: : in low-energy (C) Stable: at rest in lowest energy state

7 )) A system is said to be in Equilibrium when the result of forces acting on the system at rest is zero. )) Thus equilibrium is a state of no change The most stable state is the one with lowest free energy (G)

8 System is simply a group of atoms, minerals (phases), or rocks which are under consideration. A system is made up of one or more phases Ex: )) Quartzite = system,, made of Quartz = phase )) Gneiss = System), made of Biot, Qtz,, Sill, Kfs = phases

9 Phase (P) Each physically separable part (constituent) of the system. a a mineral a a liquid/melt a a vapor/fluid If: system = rock => phases = minerals, fluids, melt. Each phase is composed of a set of chemical component.

10 Chemical components (C) Each phase in a system = one or more components

11 How many phases can coexist in equilibrium in a system? -Willard Gibbs ) => rule which determines the number of degrees of freedom for a heterogeneous system in equilibrium : Phase Rule The number of degrees of freedom for a system is the number of intensive variables (often P, T, and X (composition)) that may be arbitrarily specified without changing the number of phases. If a system in equilibrium contains P phases and C components then the phase rule states that the number of degrees of freedom is given by : F = C + n - Φ F = number of variables that can change independently without destabilising the system (independent variables). C= number of component of the system n = number of variables: P, T P= number of phases in the system that coexist at different topological locations in a phase diagram. For an assemblage to be stable over a finite range of T and P: F 2, so P C

12 Phase Rule and Topologic Features Value of F Topologic feature < 0 The system contains more than one invariant point and is a multisystem 0 Invariant Point - No intensive variables may be changed 1 Univariant Line (reaction) - One intensive variable may be changed independently 2 Divariant Field or Divariant Phase assemblage - Two intensive variables may be changed independently 3 Trivariant field or Trivariant Phase Assemblage- Three intensive variables may be changed independently

13 C = 1 Al 2 SiO 5 Φ = 3 : Therefore F = 0, paragenesis is invariant: forms a point on ground surface and P-T plot. Φ = 2: Therefore F = 1, paragenesis is univariant: forms a line on the ground surface (isograd) and a line on the P-T plot. Φ = 1: Therefore F = 2, paragenesis is divariant: forms an

14 An isograd is a line in the field or on a map defined by the appearance or disappearance of a specific mineral, mineral composition or a group of minerals as a result of a specific reaction

15 Isograds are reactions therefore we can: Understand what physical variables might affect the location of a particular isograd We may also be able to estimate the P-T-X conditions that an isograd represents Metamorphic reactions are just chemical reactions Therefore we can use equilibrium thermodynamics to investigate the P-T-X conditions of any given chemical reaction

16 Chemographics refers to the graphical representation of the chemistry of mineral assemblages A simple example: the plagioclase system as a linear C = 2 plot: = 100 An/(An+Ab)

17 Chemographic Diagrams 3-C mineral compositions are plotted on a triangular chemographic diagram. They may form the hypothetical minerals x, y, z, xz, xyz, and x 2 z

18 Suppose that the rocks in our area have the following 5 assemblages: X+xy+x 2 z Xy+xyz+x 2 z Xy+xyz+y Xyz+z+x 2 z Y+z+xyz Three mineral phases stable together define a triangle Equilibrium mineral assemblages are indicated by tie-lines which connect co-existing minerals. Minerals not directly connected by tie lines cannot stably coexist under P-T conditions represented by the diagram.

19 Note that this subdivides the chemographic diagram into 5 sub-triangles, labeled (A)-(E) X+xy+x 2 z = (A) Xy+xyz+x 2 z = (B) Xy+xyz+y = (C) Xyz+z+x 2 z = (D) Y+z+xyz = (E) Each of these assemblages forms in rocks of different bulk composition, and they are called compatible assemblages. Any point within the diagram represents a rock of specific bulk composition.

20 We can use chemographics to infer reactions Per Fo En Qtz MgO SiO 2 Fo + Qtz = En En + Per = Fo Per + Qtz = Fo Per + Qtz = En If we know the chemographics we can determine that a reaction is possible (and can dispense with balancing the reaction)

21 What reaction is possible between A-B-D- W? A chemographic diagram for some metamorphic zone A + B + D = W W

22 Chemographic Diagrams 3-C X-Y-Z mineral compositions are plotted on a triangular or ternary diagram. Write reactions for the formation of minerals 1-3 expressed in terms of xyz? X + Y = XY 2. 2X + Z = X 2 Z 3. X + Y + Z = XYZ 1. 2.

23 What reaction is possible between A-B-C- D? A chemographic diagram for some metamorphic zone

24 Below the isograd A + B = C + D At the isograd Above the isograd This is called a tie-line flip, and results in new groupings in the next metamorphic zone

25 Two kinds of isograds: A) isograds caused by discontinuous reactions B) isograds caused by continuous reactions

26 A) Isograds caused by discontinuous reactions e.g. staurolite isograd grt + chl + mu ----> staur + bt + qtz + H 2 O Phase Rule: Φ + F = C + 2 C = 6 Φ = 7 F = 1 discontinuous reaction forms a line on P-T graph and a line in the field. Can be deduced by the breaking of a tie-line on a Δ diagram

27 staur A + muscovite + quartz + H 2 O staur A F grt x y z chlorite M F grt x y z chlorite M biotite biotite Discontinuous reaction all rocks in grt-chl-bt Δ show same reaction at same T regardless of small F/M/A variations

28 B) Isograds caused by continuous reactions e.g. garnet isograd chl 1 + mu + qtz ----> chl 2 + grt + bt + H 2 O Fe-rich Mg-rich Phase Rule: Φ + F = C + 2 C = 6 Φ = 6 F = 2 A continuous reaction forms an area on a P-T grid and area in the field. In this example tie-lines migrate to more Mg-rich comps.

29 A A grt chlorite grt chlorite F M F M biotite biotite Continuous reactions e.g. first appearance of garnet critically depends on the F/M ratio

30 Continuous reactions occur when F > 1, and the reactants and products coexist over a temperature interval Schematic isobaric T-X Mg diagram representing the simplified metamorphic reaction Chl + Qtz Grt + H 2 O. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

31 Isobaric T-X phase diagram at atmospheric pressure After Bowen and Shairer (1932), Amer. J. Sci. 5th Ser., 24, Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

32 Garnet appears at different grades in rocks of different composition 200m WHY? 600 o C Geologic map of a hypothetical field area in which metamorphosed pelitic sediments strike directly up metamorphic grade. Modified from Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. 550 o C

33 Two possible reasons for variation across the map: 1. Such contrasting composition that the garnet reaction is different Example: garnet in some pelites may be created by the reaction: Chl + Ms + Qtz Grt + Bt + H 2 O Whereas in more Fe-rich and K-poor pelites, garnet might be generated by an (unbalanced) reaction involving chloritoid: Chl + Ctd + Qtz Grt + H 2 O 2. The reaction on which the isograd is based is the

34 If Chl + Ms + Qtz Grt + Bt + H 2 O were a discontinuous reaction and is responsible for the formation of garnet in the map, the reaction should run to completion (when one of the reactants was consumed) at a single grade (P-T). If Chl + Ms + Qtz Grt + Bt + H 2 O were a continuous reaction, then we would find chlorite, muscovite, quartz, biotite, and garnet all together in the same rock over an interval of metamorphic grade above the garnet-in isograd

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36 Reactions can be deduced from compatibility diagrams in 4 ways

37 A + B = C + D 1. Crossing tie-lines

38 2. Within a triangle A + B + D = W W

39 3. Along a binary join A + B = C A C B

40 E D F 1. A + E = D 2. D + E + C = F 3. A + C = B 4. B + F = D + C 5. B + E = D + C 6. D + B = A + F 7. E + B = F A B C Deduce at least 7 reactions that can occur between these hypothetical minerals