Geol 2312 Igneous and Metamorphic Petrology

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Presentation transcript:

Geol 2312 Igneous and Metamorphic Petrology Lecture 8 Phase Diagrams for Three-Component Systems February 9, 2009

Three Component System with Eutectics Olivine-Plagioclase-Pyroxene (Fo-An-Di) Liquidus surface showing temperature contours F = 3 – 2 + 1 = 2 Liquidus surface showing temperature contours F = 3 – 2 + 1 = 2 (divariant field) Cotectic (or binary eutectic) F = 3 – 3 + 1 = 1 (univariant line) Ternary eutectic F = 3 – 4 + 1 = 0 (invariant point) Pressure = 0.1 MPa Winter (2001) Figure 7-2. Isobaric diagram illustrating the liquidus temperatures in the Di-An-Fo system at atmospheric pressure (0.1 MPa). After Bowen (1915), A. J. Sci., and Morse (1994), Basalts and Phase Diagrams. Krieger Publishers.

Three Component System with Eutectics Olivine-Plagioclase-Pyroxene (Fo-An-Di) X – starting magma composition S – starting bulk solid composition X’ – magma composition when plagioclase become saturated along with olivine S’ – bulk solid composition when magma at X’ (olivine composes ~ 30% of system) Equilibrium Crystallization Pressure = 0.1 MPa X” – magma comp at 50% crystallized (based on lever rule of tie-line through X) S” – bulk solid comp when magma at X” (composed of 68%Ol & 32%Pl) X’ X’’ X*=M – magma reached ternary eutectic at 65% crystallized S* – bulk solid comp when magma reaches ternary eutectic (composed of 60%Ol & 40%Pl) X* Sf S* X S’’ X*=M – magma comp fixed at ternary eutectic until 100% crystallized Sf – final bulk solid = X-starting liquid comp X* S S’

Three Component System with Eutectics Olivine-Plagioclase-Pyroxene (Fo-An-Di) Fractional Crystallization PCO Pressure = 0.1 MPa PO X’ X’’ O X* Sf S* X S’’ X* S S’

Three Component System with a Peritectic Plagioclase-Olivine-(Orthopyroxene)-Quartz (An-Fo-(En-)SiO2) 3 binary systems: Fo-An eutectic An-SiO2 eutectic Fo-SiO2 peritectic Liquidus contours not shown to reduce clutter Winter (2001) Figure 7-4. Isobaric diagram illustrating the cotectic and peritectic curves in the system forsterite-anorthite-silica at 0.1 MPa. After Anderson (1915) A. J. Sci., and Irvine (1975) CIW Yearb. 74. Pressure = 0.1 MPa

Three Component System with a Peritectic Plagioclase-Olivine-(Orthopyroxene)-Quartz (An-Fo-(En-)SiO2) LIQUID PATH SOLID PATH a – Starting Liquid Comp a-b – liquid path due to Fo crystallization b- En crystallization (and partial replacement of Fo) begins b-c – liquid path due to Fo+En crystallization c – An joins En and Fo as crystallizing phases; last liquid comp for equilibrium crystallization AB – Fo only crystallization drives liquid from ab AB-C – En crystallization (and replacement) enriches bulk solid in En to C (where liquid reaches c) C-F – when liquid reaches c, An is added to bulk solid and is 100% crystallized when reach starting composition F F AB C

Three Component System with a Peritectic Plagioclase-Olivine-(Orthopyroxene)-Quartz (An-Fo-(En-)SiO2) g-d leg only possible with fractional crystallization final rock is En + An under equilibrium crystallization All Ol is consumed

Three Component System with Solid Solution Plagioclase-Pyroxene (An-Ab-Di) Winter (2001) Figure 7-5. Isobaric diagram illustrating the liquidus temperatures in the system diopside-anorthite-albite at atmospheric pressure (0.1 MPa). After Morse (1994), Basalts and Phase Diagrams. Krieger Publishers. Liquid – An content Tie-lines

Three Component System with Solid Solution Plagioclase-Pyroxene (An-Ab-Di) Starting Liquid Composition Last Liquid (EC) Final Plag (EC) First Plag

Three Component System with Solid Solution Plagioclase-Pyroxene (An-Ab-Di) Liquid composition arcs due to decreasing An content of plagioclase Last Liquid (EC) Starting Liquid Composition First Plag Final Plag (EC)

Four Component Systems Fo-Di-An-Ab Becoming difficult to visualize Time to revert to multi-dimensional mathematical models Winter (2001) Figure 7-12. The system diopside-anorthite-albite-forsterite. After Yoder and Tilley (1962). J. Petrol.

Back to Thermodynamics Think about Gibbs Free Energy again dG = VdP – SdT at a given P (0.1 MPa) and T (1300ºC) Low Volume, trumps Entropy  Minerals more stable High Entropy trumps volume  Liquid more stable Liq