Slide 1Equilibrium Mineral AssemblagesPhase Rule in Metamorphic RocksSlide 4Slide 5Slide 6Slide 7Slide 8Chemographic DiagramsSlide 10Slide 11Chemographic Diagrams w/ Solid SolutionSlide 13Chemographic Diagrams ACF diagramSlide 15Chemographic Diagrams AKF diagramChemographic Diagrams Apical Phase ProjectionsSlide 18GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGYLecture 23Stable Mineral Assemblages in Metamorphic RocksMarch 30, 2009EQUILIBRIUM MINERAL ASSEMBLAGESEvidence 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 zoningPHASE RULE IN METAMORPHIC ROCKSPhase rule, as applied to systems at equilibrium:F = C -  + 2 = the number of phases in the systemC = the number of components: the minimum number of chemical constituents required to specify every phase in the systemF = 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 rulePHASE RULE IN METAMORPHIC ROCKS2)  < C Common with minerals that exhibit solid solution (e.g., Plagioclase - single mineral, but two components)Suppose we have determined C for a rockConsider the following three scenarios:1)  = CThe standard divariant situation of the Phase RuleThe rock probably represents an equilibrium mineral assemblage from within a metamorphic zonePHASE RULE IN METAMORPHIC ROCKS3)  > CA more interesting situation and at least one of three situations must be responsible:A)F < 2The sample is collected from a location right on a univariant reaction curve (isograd) or invariant pointB)Equilibrium has not been attainedC) The number of components were not properly chosenPHASE RULE IN METAMORPHIC ROCKSChoosing Components to define Metamorphic SystemsAs 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 2DAvoid: - Components that are major constituents of single phases (e.g., P2O5 – apatite, TiO2 – 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 (H2O, CO2, ...)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.PHASE RULE IN METAMORPHIC ROCKS“Perfectly Mobile” H2O or NotImplies 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 H2O fluid is presentRetrograde reaction requires H2O to be present to go, but it is not a component of this system (one comp – MgO)MgOMg(OH)2ProgradeRetrogradePHASE RULE IN METAMORPHIC ROCKSHow 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-H2O 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 H2O or CO2, in order to apply the phase rule correctlyCHEMOGRAPHIC DIAGRAMSChemographics refers to the graphical representation of the chemistry of mineral assemblagesA simple example: the plagioclase system as a linear C=2 plot: 3-C mineral compositions are plotted on a triangular chemographic diagramx, y, z, xz, xyz, and yz2CHEMOGRAPHIC DIAGRAMSDivariant EquilibriumDivariant EquilibriumMineral AssemblagesMineral Assemblages(A) x-xy-x(A) x-xy-x22zz(B) xy-xyz-x(B) xy-xyz-x22zz(C) xy-xyz-y(C) xy-xyz-y(D) xyz-z-x(D) xyz-z-x22zz(E) y-z-xyz(E) y-z-xyzCompatibility Diagrams determines the equilibrium mineral assemblage that should develop for a particular whole rock composition defined by three componentsCHEMOGRAPHIC DIAGRAMSValid 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 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)CHEMOGRAPHIC DIAGRAMSW/ SOLID SOLUTIONPhases with SS between Y and ZPhases with SS between Y, X. and ZCHEMOGRAPHIC DIAGRAMSW/ SOLID SOLUTIONTie lines link coexisting compositionsCHEMOGRAPHIC DIAGRAMSACF DIAGRAMFigure 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 = Al2O3 + Fe2O3 - Na2O - K2OC = CaO - 3.3 P2O5 F = FeO + MgO + MnOCHEMOGRAPHIC DIAGRAMSACF DIAGRAMWater is omitted under the assumption that it is perfectly mobileNote that SiO2 is simply ignored. We shall see that this is equivalent to projecting from quartzIn 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 FeldsparBy creating these three pseudo-components, Eskola reduced the number of components in mafic rocks from 8 to 3CHEMOGRAPHIC DIAGRAMSAKF DIAGRAMBest Suited to Pelitic (clay-rich) Sedimentary RocksA = Al2O3 + Fe2O3 - Na2O - K2O - CaOK =