8 Degrees of Freedom Pressure Temperature PH2O PCO2 pH Oxygen fugacity Eh
9 Simplifying Degrees of Freedom Generally ignore pH, Eh, Oxygen fugacity for most rocksImportant for sulfide systemsUsually care about T,PAssume PH2O = Ptotal for silicates, PCO2 = 0Assume PCO2 = Ptotal for carbonates, PH2O = 0Assume PH2O = 0 and PCO2 = 0 in some cases
10 Simplifying Degrees of Freedom P + F = C + 2If F = 2, then P = CNumber of phases = number of componentsComponents = SiO2, Al2O3, Fe2O3, FeO, CaO, MgO, Na2O, K2O, TiO2, Cr2O3, MnO, BaO, SrO, P2O5, H2O, CO2, F, Cl
11 Simplifying Components We can plot a maximum of three components using triangle diagramsIgnore SiO2(excess), H2O (excess or 0), CO2 (excess or 0)Assume P2O5 goes into apatite, Na2O into albite, TiO2 into rutile or ilmenite, Cr2O3, into chromite, F into fluorite or apatite, Cl into haliteInclude MnO with FeO, BaO and SrO with CaO, Fe2O3 with Al2O3 or FeO, Cl, F with OHSubtract major elements as necessary
12 Simplifying Components Components reduced to Al2O3, FeO, CaO, MgO, K2OACF graphs Al2O3, CaO, (FeO+ MgO)A’KF graphs Al2O3, (FeO + MgO), K2OAFM graphs Al2O3, FeO, MgO, K2O in a tetrahedronSiO2 – CaO – MgO used for carbonate systemsPlot Molar amounts, not weights
13 Metamorphic Phase Diagrams Tie lines denote two coexisting phasesTriangular subfields denote three coexisting phasesPhase Diagram is snapshot of mineral combinations under given T,P conditionsEvolution tracked by changes in diagramsNot really interested in amounts
15 Metamorphic Phase Diagrams Normally F = 2, C = 3, P = 3On a reaction curve, F = 1, P = 4How to get 4 Phases Together:New Phase Appears in Middle of FieldNew Phase Appears on Tie LineTie Line Breaks and New One FormsChanges in MetamorphismNew Minerals AppearOld Minerals DisappearCompatibilities Shift
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