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Chapter 24. Stable Mineral Assemblages in Metamorphic Rocks

Chapter 24. Stable 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 “Mineral paragenesis” refers to such an equilibrium mineral assemblage

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Chapter 24. Stable Mineral Assemblages in Metamorphic Rocks

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  1. Chapter 24. Stable 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 • “Mineral paragenesis” refers to such an equilibrium mineral assemblage • Relict minerals or later alteration products are thereby excluded from consideration unless specifically stated

  2. The Phase Rule in Metamorphic Systems • Phase rule, as applied to systems at equilibrium: F = C - f + 2 the phase rule (6-1) f 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.)

  3. The Phase Rule in Metamorphic Systems • A typical sample from a metamorphic terrane • Likely select a sample from within a zone, and not from right on an isograd • Alternatively, pick a random point anywhere on a phase diagram • Likely point will be within a divariant field and not on a univariant curve or invariant point • The most common situation is divariant (F = 2), meaning that P and T are independently variable without affecting the mineral assemblage

  4. The Phase Rule in Metamorphic Systems If F  2 is the most common situation, then the phase rule may be adjusted accordingly: F = C - f + 2  2 f C (24-1) • Goldschmidt’s mineralogical phase rule, or simply the mineralogical phase rule

  5. The Phase Rule in Metamorphic Systems Suppose we have determined C for a rock Consider the following three scenarios: a) f = C • The standard divariant situation in metamorphic rocks • The rock probably represents an equilibrium mineral assemblage from within a metamorphic zone

  6. Liquid Plagioclase plus Liquid Plagioclase The Phase Rule in Metamorphic Systems b) f < C • Common with mineral systems that exhibit solid solution

  7. The Phase Rule in Metamorphic Systems c) f > C • A more interesting situation, and at least one of three situations must be responsible: 1)  F < 2 • The sample is collected from a location right on a univariant reaction curve (isograd) or invariant point

  8. The Phase Rule in Metamorphic Systems C = 1 • f = 1 common • f = 2 rare • f = 3 only at the specific P-T conditions of the invariant point (~ 0.37 GPa and 500oC) Consider the following three scenarios: 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.

  9. The Phase Rule in Metamorphic Systems 2)  Equilibrium has not been attained • The phase rule applies only to systems at equilibrium, and there could be any number of minerals coexisting if equilibrium is not attained

  10. The Phase Rule in Metamorphic Systems 3)  We didn’t choose the # of components correctly • Some guidelines for an appropriate choice of C • Begin with a 1-component system, such as CaAl2Si2O8 (anorthite), there are 3 common types of major/minor components that we can add a)  Components that generate a new phase • Adding a component such as CaMgSi2O6 (diopside), results in an additional phase: in the binary Di-An system diopside coexists with anorthite below the solidus

  11. The Phase Rule in Metamorphic Systems 3)  We didn’t choose the # of components correctly b) Components that substitute for other components • Adding a component such as NaAlSi3O8 (albite) to the 1-C anorthite system would dissolve in the anorthite structure, resulting in a single solid-solution mineral (plagioclase) below the solidus • Fe and Mn commonly substitute for Mg • Al may substitute for Si • Na may substitute for K

  12. The Phase Rule in Metamorphic Systems 3)  We didn’t choose the # of components correctly c) “Perfectly mobile” components • Either a freely mobile fluid component or a component that dissolves in a fluid phase and can be transported easily • The chemical activity of such components is commonly controlled by factors external to the local rock system • They are commonly ignored in deriving C for metamorphic systems

  13. The Phase Rule in Metamorphic Systems Consider the very simple metamorphic system, MgO-H2O • Possible natural phases in this system are periclase (MgO), aqueous fluid (H2O), and brucite (Mg(OH)2) • How we deal with H2O depends upon whether water is perfectly mobile or not • A reaction can occur between the potential phases in this system: MgO + H2O  Mg(OH)2Per + Fluid = Bru retrograde reaction as written (occurs as the rock cools and hydrates)

  14. The Phase Rule in Metamorphic Systems Cool to the temperature of the reaction curve, periclase reacts with water to form brucite: MgO + H2O  Mg(OH)2 The System MgO-H2O Figure 24-1. P-T phase diagram illustrating the reaction brucite = periclase + water, calculated using the program TWQ of Berman (1988, 1990, 1991). From Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  15. The Phase Rule in Metamorphic Systems Reaction: periclase coexists with brucite: f = C + 1 F = 1 (#2 reason to violate the mineral-ogical phase rule) To leave the curve, all the periclase must be consumed by the reaction, and brucite is the solitary remaining phase f = 1 and C = 1 again The System MgO-H2O Figure 24-1. P-T phase diagram illustrating the reaction brucite = periclase + water. From Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  16. The Phase Rule in Metamorphic Systems Periclase + H2O react to form brucite MgO + H2O  Mg(OH)2 The System MgO-H2O Figure 24-1. P-T phase diagram illustrating the reaction brucite = periclase + water. From Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  17. The Phase Rule in Metamorphic Systems Once the water is gone, the excess periclase remains stable as conditions change into the brucite stability field The System MgO-H2O Figure 24-1. P-T phase diagram illustrating the reaction brucite = periclase + water. From Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  18. The Phase Rule in Metamorphic Systems We can now conclude that periclase can be stable anywhere on the whole diagram, if water is present in insufficient quantities to permit the reaction to brucite to go to completion Figure 24-1. P-T phase diagram illustrating the reaction brucite = periclase + water. From Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  19. The Phase Rule in Metamorphic Systems At any point (other than on the univariant curve itself) we would expect to find two phases, not one f = brucite + periclase below the reaction curve (if water is limited), or periclase + water above the curve Figure 24-1. P-T phase diagram illustrating the reaction brucite = periclase + water. From Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  20. The Phase Rule in Metamorphic Systems How do you know which way is correct? • The rocks should tell you • The phase rule is an interpretive tool, not a predictive tool, and does not tell the rocks how to behave • If you only see low-f assemblages (e.g. Per or Bru in the MgO-H2O system), then some components may be mobile • If you often observe assemblages that have many phases in an area (e.g. periclase + brucite), it is unlikely that so much of the area is right on a univariant curve, and may require the number of components to include otherwise mobile phases, such as H2O or CO2, in order to apply the phase rule correctly

  21. Chemographic Diagrams Chemographics refers to the graphical representation of the chemistry of mineral assemblages A simple example: the olivine system as a linear C = 2 plot: = Fe/(Mg+Fe)

  22. Chemographic Diagrams 3-C mineral compositions are plotted on a triangular chemographic diagram as shown in Fig. 24-2 x, y, z, xz, xyz, and yz2

  23. Suppose that the rocks in our area have the following 5 assemblages: • x-xy-x2z • xy-xyz-x2z • xy-xyz-y • xyz-z-x2z • y-z-xyz Figure 24-2. Hypothetical three-component chemographic compatibility diagram illustrating the positions of various stable minerals. Minerals that coexist compatibly under the range of P-T conditions specific to the diagram are connected by tie-lines. After Best (1982) Igneous and Metamorphic Petrology. W. H. Freeman.

  24. Xbulk point within the sub-triangle (B), the corresponding mineral assemblage corresponds to the corners = xy - xyz - x2z A diagram like this is a compatibility diagram, a type of phase diagram commonly employed by metamorphic petrologists Note that this subdivides the chemographic diagram into 5 sub-triangles, labeled (A)-(E)

  25. Any common point corresponds to 3 phases, thus f = C, in accordance with the common case for the mineralogical phase rule Figure 24-2. Hypothetical three-component chemographic compatibility diagram illustrating the positions of various stable minerals. Minerals that coexist compatibly under the range of P-T conditions specific to the diagram are connected by tie-lines. After Best (1982) Igneous and Metamorphic Petrology. W. H. Freeman.

  26. What happens if you pick a composition that falls directly on a tie-line, such as point (f)? Figure 24-2. Hypothetical three-component chemographic compatibility diagram illustrating the positions of various stable minerals. Minerals that coexist compatibly under the range of P-T conditions specific to the diagram are connected by tie-lines. After Best (1982) Igneous and Metamorphic Petrology. W. H. Freeman.

  27. In the unlikely event that the bulk composition equals that of a single mineral, such as xyz, then f = 1, but C = 1 as well Such special situations, requiring fewer components than normal, have been described by the intriguing term compositionally degenerate

  28. Chemographic Diagrams Valid compatibility diagram must refer to a specific range of P-T conditions, such as a zone in some metamorphic terrane, because the stability of the minerals and their groupings vary as P and T vary • The previous diagram refers to a P-T range in which the fictitious minerals x, y, z, xy, xyz, and x2z are all stable and occur in the groups shown • At different grades the diagrams change • Other minerals become stable • Different arrangements of the same minerals (different tie-lines connect different coexisting phases)

  29. Some minerals exhibit solid solution Figure 24-2. Hypothetical three-component chemographic compatibility diagram illustrating the positions of various stable minerals, many of which exhibit solid solution. After Best (1982) Igneous and Metamorphic Petrology. W. H. Freeman.

  30. f = 1, but the system is not degenerate For bulk rock composition is in field of the mineral (xyz)ss

  31. Xbulk (f) = yellow spot on a tie-line. f = 2 and C is still 3, F = 3 - 2 + 2 = 3 Figure 24-2. Hypothetical three-component chemographic compatibility diagram illustrating the positions of various stable minerals, many of which exhibit solid solution. After Best (1982) Igneous and Metamorphic Petrology. W. H. Freeman.

  32. Figure 24-2. Hypothetical three-component chemographic compatibility diagram illustrating the positions of various stable minerals, many of which exhibit solid solution. After Best (1982) Igneous and Metamorphic Petrology. W. H. Freeman.

  33. Chemographic Diagrams for Metamorphic Rocks • Most common natural rocks contain the major elements: SiO2, Al2O3, K2O, CaO, Na2O, FeO, MgO, MnO and H2O such that C = 9 • Three components is the maximum number that we can easily deal with in two dimensions • What is the “right” choice of components? • We turn to the following simplifying methods:

  34. 1) Simply “ignore” components • Trace elements • Elements that enter only a single phase (we can drop both the component and the phase without violating the phase rule) • Perfectly mobile components

  35. 2) Combine components • Components that substitute for one another in a solid solution: (Fe + Mg) 3) Limit the types of rocks to be shown • Only deal with a sub-set of rock types for which a simplified system works 4) Use projections • I’ll explain this shortly

  36. A triangular diagram thus applies rigorously only to true (but rare) 3-component systems • If drop components and phases, combine components, or project from phases, we face the same dilemma we faced using simplified systems in Chapters 6 and 7 • Gain by being able to graphically display the simplified system, and many aspects of the system’s behavior become apparent • Lose a rigorous correlation between the behavior of the simplified system and reality The phase rule and compatibility diagrams are rigorously correct when applied to complete systems

  37. Illustrate metamorphic mineral assemblages in mafic rocks on a simplified 3-C triangular diagram • Concentrate only on the minerals that appeared or disappeared during metamorphism, thus acting as indicators of metamorphic grade The ACF Diagram

  38. Figure 24-4. After Ehlers and Blatt (1982). Petrology. Freeman. And Miyashiro (1994) Metamorphic Petrology. Oxford.

  39. The three pseudo-components are all calculated on an atomic basis: A = Al2O3 + Fe2O3 - Na2O - K2O C = CaO - 3.3 P2O5 F = FeO + MgO + MnO The ACF Diagram

  40. A = Al2O3 + Fe2O3 - Na2O - K2O Why the subtraction? The ACF Diagram • Na and K in mafic rocks are typically combined with Al to produce Kfs and Albite • In the ACF diagram, interested only in other K-bearing metamorphic minerals, and thus only in the amount of Al2O3 that occurs in excess of that combined with Na2O and K2O (in albite and K-feldspar) • Since the ratio of Al2O3 to Na2O or K2O in feldspars is 1:1, we subtract from Al2O3an amount equivalent to Na2O and K2O in the same 1:1 ratio

  41. The ACF Diagram C = CaO - 3.3 P2O5 F = FeO + MgO + MnO

  42. By creating these three pseudo-components, Eskola reduced the number of components in mafic rocks from 8 to 3 The ACF Diagram • Water is omitted under the assumption that it is perfectly mobile • Note that SiO2 is simply ignored • We shall see that this is equivalent to projecting from quartz • In order for a projected phase diagram to be truly valid, the phase from which it is projected must be present in the mineral assemblages represented

  43. An example: The ACF Diagram • Anorthite CaAl2Si2O8 • A = 1 + 0 - 0 - 0 = 1, C = 1 - 0 = 1, and F = 0 • Provisional values sum to 2, so we can normalize to 1.0 by multiplying each value by ½, resulting in A = 0.5 C = 0.5 F = 0

  44. Figure 24-4. After Ehlers and Blatt (1982). Petrology. Freeman. And Miyashiro (1994) Metamorphic Petrology. Oxford.

  45. A typical ACF compatibility diagram, referring to a specific range of P and T (the kyanite zone in the Scottish Highlands) Figure 24-5. After Turner (1981). Metamorphic Petrology. McGraw Hill.

  46. Because pelitic sediments are high in Al2O3 and K2O, and low in CaO, Eskola proposed a different diagram that included K2O to depict the mineral assemblages that develop in them The AKF Diagram • In the AKF diagram, the pseudo-components are: A = Al2O3 + Fe2O3 - Na2O - K2O - CaO K = K2O F = FeO + MgO + MnO

  47. Figure 24-6. After Ehlers and Blatt (1982). Petrology. Freeman.

  48. Figure 24-7. After Eskola (1915) and Turner (1981) Metamorphic Petrology. McGraw Hill. AKF compatibility diagram (Eskola, 1915) illustrating paragenesis of pelitic hornfelses, Orijärvi region Finland

  49. Andalusite and muscovite plot as the same point in the ACF diagram, and microcline wouldn’t plot at all, making the ACF diagram much less useful for pelitic rocks that are rich in K and Al Notice that three of the most common minerals in metapelites andalusite, muscovite, and microcline, all plot as distinct points in the AKF diagram Figure 24-7. After Ehlers and Blatt (1982). Petrology. Freeman.

  50. When we explore the methods of chemographic projection we will discover: Projections in Chemographic Diagrams • Why we ignored SiO2 in the ACF and AKF diagrams • What that subtraction was all about in calculating A and C • It will also help you to better understand the AFM diagram in the next section and some of the shortcomings of projected metamorphic phase diagrams

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