Understanding Metamorphic Assemblages: A Comprehensive Overview

1. Introduction to Metamorphism 2 IN THIS LECTURE • Importance of Understanding Metamorphic Mineral Assemblages • Progressive Nature of Metamorphism • Stable Mineral Assemblages • The Phase Rule in Metamorphic Systems • The MgO-H2O system

2. What’s so Important About Metamorphic Mineral Assemblages • Equilibrium mineral assemblages can tell us about P-T conditions • P-T conditions tell us about tectonic environments • P-T conditions combined with geochronology tells us about how tectonic environments evolve through time • Optical Mineralogy is the starting point!!

3. The Progressive Nature of Metamorphism • A rock at a high metamorphic grade probably progressed through a sequence of mineral assemblages rather than hopping directly from an unmetamorphosed rock to the metamorphic rock that we find today • All rocks that we now find must also have cooled to surface conditions. Therefore, at what point on its cyclic P-T-t path did its present mineral assemblage last equilibrate? • The preserved zonal distribution of metamorphic rocks suggests that each rock preserves the conditions of the maximum metamorphic grade (temperature)

4. The Progressive Nature of Metamorphism • Prograde reactions are endothermic and easily driven by increasing T • Devolatilization reactions are easier than reintroducing the volatiles • Geothermometry indicates that the mineral compositions commonly preserve the maximum temperature

5. 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

6. The Phase Rule in Metamorphic Systems • Phase rule, as applied to systems at equilibrium: F = C - P + 2 the phase rule P 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.) • Remember we’ve seen this already with igneous systems

7. The Phase Rule in Metamorphic Systems • 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

8. Remember this diagram?

9. The Phase Rule in Metamorphic Systems • If F  2 is the most common situation, then the phase rule may be adjusted accordingly such that F = C - P + 2  2 and therefore P  C • This is Goldschmidt’s mineralogical phase rule, or simply the mineralogical phase rule

10. The Phase Rule in Metamorphic Systems If C has been determined for a particular rock then there are three potential situations according to the phase rule • P=C • This is the standard divariant situation in metamorphic rocks • The rock probably represents an equilibrium mineral assemblage from within a metamorphic zone • P<C • A situation that commonly arises in systems that display solid solution. • We’ve seen this already with the binary phase diagrams for the albite-anorthite system

11. Albite-Anorthite Phase Diagram

12. The Phase Rule in Metamorphic Systems If C has been determined for a particular rock then there are three potential situations according to the phase rule 1. P = C • This is the standard divariant situation in metamorphic rocks • The rock probably represents an equilibrium mineral assemblage from within a metamorphic zone 2. P < C • A situation that commonly arises in systems that display solid solution. • We’ve seen this already with the binary phase diagrams for the albite-anorthite system 3. P > C • A more interesting situation, and at least one of three situations must be responsible

13. The Phase Rule in Metamorphic Systems For P > C then the following three situations could apply • F < 2 • Equilibrium not attained • Choice of C not correct If (1) applies then the sample was collected from a location right on a univariant reaction curve or invariant point 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.

14. The Phase Rule in Metamorphic Systems 2. Equilibrium is not attained • The situation with (2) is more important in terms of optical mineralogy. • The phase rule only applies to systems that are in equilibrium. • If equilibrium is not attained or maintained then there could be any number of minerals co-existing • Unfortunately, this is often the case, especially in rocks that have been partially retrogressed or rocks in the blueschist and eclogite facies. • Optical Mineralogy is the main tool for decided which minerals represent the equilibrium mineral assemblage

15. The Phase Rule in Metamorphic Systems 3. The number of components was not correct • 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 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

16. The Phase Rule in Metamorphic Systems 3. The number of components was not correct (cont.) 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

17. 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)2 Per + Fluid = Bru • As written this is aretrograde reaction (occurs as the rock cools and hydrates)

18. The System MgO-H2O Cool to the temperature of the reaction curve, periclase reacts with water to form brucite: MgO + H2O  Mg(OH)2

19. The System MgO-H2O Reaction: periclase coexists with brucite: P = C + 1 F = 1 (2nd reason to violate the mineralogical 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

20. The Phase Rule in Metamorphic Systems Once the water is gone, the excess periclase remains stable as conditions change into the brucite stability field Thus 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

21. 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 P = brucite + periclase below the reaction curve (if water is limited), or periclase + water above the curve

22. 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-P 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