1. G EOL 2312 I GNEOUS AND M ETAMORPHIC P ETROLOGY Lecture 23 Stable Mineral Assemblages in Metamorphic Rocks March 30, 2009

2. E QUILIBRIUM M INERAL A SSEMBLAGES Evidence of Chemical Equilibrium - Lack of disequilibrium textures (replacement textures, corona, compositional zoning,....) - Each mineral type shares contacts every other mineral phase in the rock - Layers are rare or are homogeneous within the layers - Rocks are in textural equilibrium - Rocks conform to Gibbs Phase Rule - Minerals lack chemical zoning

3. P HASE R ULE IN M ETAMORPHIC R OCKS Phase rule, as applied to systems at equilibrium: F = C -  + 2  = the number of phases in the system C = the number of components: the minimum number of chemical constituents required to specify every phase in the system F = 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.) In natural systems, there are multiple compositional variables in addition to independent changes in P & T. If F  2 is the most common situation, then the phase rule may be adjusted accordingly: F = C -  + 2  2, or   C Goldschmidt’s mineralogical phase rule, or simply the mineralogical phase rule

4. P HASE R ULE IN M ETAMORPHIC R OCKS 2)  < C Common with minerals that exhibit solid solution (e.g., Plagioclase - single mineral, but two components) Suppose we have determined C for a rock Consider the following three scenarios: 1)  = C The standard divariant situation of the Phase Rule The rock probably represents an equilibrium mineral assemblage from within a metamorphic zone

5. P HASE R ULE IN M ETAMORPHIC R OCKS 3)  > C A more interesting situation and at least one of three situations must be responsible: A) F < 2 s The sample is collected from a location right on a univariant reaction curve (isograd) or invariant point B) Equilibrium has not been attained C) The number of components were not properly chosen

6. P HASE R ULE IN M ETAMORPHIC R OCKS Choosing Components to define Metamorphic Systems As with igneous rocks, it is not reasonable to choose every chemical constituent of a rock as a component. Stick to: - Essential Components that generate a new phase with a limited P&T range (garnet – yes; plagioclase – no) - Three Components (or component combinations) that can be graphically portrayed in 2D Avoid: - Components that are major constituents of single phases (e.g., P 2 O 5 – apatite, TiO 2 – ilmenite) - Components that substitute for other components (e.g. Ab-An, Fa-Fo, Mn for Fe, Al for Si, Na for K) - “Perfectly mobile” components (H 2 O, CO 2,...)

7. Winter (2001)Figure 24-1. P-T diagram for the reaction brucite = periclase + water. From Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. P HASE R ULE IN M ETAMORPHIC R OCKS “Perfectly Mobile” H 2 O or Not Implies that fluid may come and go based on external conditions not controlled by mineral reactions. It is typically not considered a component. It is added as needed and leaves when in excess. Prograde reaction will go regardless if H 2 O fluid is present Retrograde reaction requires H 2 O to be present to go, but it is not a component of this system (one comp – MgO) MgO Mg(OH) 2 Prograde Retrograde

8. P HASE R ULE IN M ETAMORPHIC R OCKS How do you know if you have chosen the proper components? 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-  assemblages (e.g. Per or Bru in the MgO-H 2 O system), then some components may be mobile  If assemblages have many phases in an area 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 H 2 O or CO 2, in order to apply the phase rule correctly

9. C HEMOGRAPHIC D IAGRAMS Chemographics refers to the graphical representation of the chemistry of mineral assemblages A simple example: the plagioclase system as a linear C = 2 plot: 3-C mineral compositions are plotted on a triangular chemographic diagram x, y, z, xz, xyz, and yz 2

10. C HEMOGRAPHIC D IAGRAMS Divariant Equilibrium Mineral Assemblages (A) x-xy-x 2 z (B) xy-xyz-x 2 z (C) xy-xyz-y (D) xyz-z-x 2 z (E) y-z-xyz Compatibility Diagrams determines the equilibrium mineral assemblage that should develop for a particular whole rock composition defined by three components

11. C HEMOGRAPHIC D IAGRAMS Valid compatibility diagram must be referenced 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  Previous diagram refers to a P-T range in which the fictitious minerals x, y, z, xy, xyz, and x 2 z 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)

12. C HEMOGRAPHIC D IAGRAMS W / S OLID S OLUTION Phases with SS between Y and Z Phases with SS between Y, X. and Z

13. C HEMOGRAPHIC D IAGRAMS W / S OLID S OLUTION Tie lines link coexisting compositions

14. C HEMOGRAPHIC D IAGRAMS ACF DIAGRAM Figure 24-4. After Ehlers and Blatt (1982). Petrology. Freeman. And Miyashiro (1994) Metamorphic Petrology. Oxford. Best Suited to Mafic Igneous Rocks and Sedimentary Rocks (Graywackes) The three pseudo-components are all calculated on an atomic basis: A = Al 2 O 3 + Fe 2 O 3 - Na 2 O - K 2 O C = CaO - 3.3 P 2 O 5 F = FeO + MgO + MnO

15. C HEMOGRAPHIC D IAGRAMS ACF DIAGRAM Water is omitted under the assumption that it is perfectly mobile Note that SiO 2 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 e.g. Alkali Feldspar By creating these three pseudo- components, Eskola reduced the number of components in mafic rocks from 8 to 3

16. C HEMOGRAPHIC D IAGRAMS AKF DIAGRAM Best Suited to Pelitic (clay-rich) Sedimentary Rocks A = Al 2 O 3 + Fe 2 O 3 - Na 2 O - K 2 O - CaO K = K 2 O F = FeO + MgO + MnO Projected From Quartz & Plagioclase

17. C HEMOGRAPHIC D IAGRAMS A PICAL P HASE P ROJECTIONS MgO SiO 2 FoEnDi'QPer Only valid if phase projecting from is present with all phases Mathematically, the same as ignoring CaO in the Di formula and normalizing MgO and SiO 2 to 100%

18. C HEMOGRAPHIC D IAGRAMS A PICAL P HASE P ROJECTIONS x = ABCQ y = A 2 B 2 CQ Possible Phase assemblages (q)-b-x-c(q)-a-x-y(q)-b-x-y(q)-a-b-y(q)-a-x-c Fig. 24-13 Mathematically the same as ignoring SiO 2 in the X & Y formulas and normalizing A, B, and C to 100%