binary phase diagrams.pdf

Binary Phase Diagrams Binary systems can be defined using only 2 components The number of phases varies depending on the specific system considered, but is usually small (2, 3, 4...) In this class, we wi...

Binary Phase Diagrams Binary systems can be defined using only 2 components The number of phases varies depending on the specific system considered, but is usually small (2, 3, 4...) In this class, we will consider only binary systems that are relevant to crystallisation from a magma Because C = 2, the variance can be as high as 3 in one-phase systems, requiring three-dimensional diagrams for illustration. However, for simplification, we keep the pressure constant and change the temperature and composition As pressure is constant, the phase rule becomes F = C - P + 1  Binary Phase Diagrams Binary systems can be defined using only 2 components The number of phases varies depending on the specific system considered, but is usually small (2, 3, 4...) These systems consist of some combination of melt (L or M) and crystals (Xls) The phase diagram axes are generally T and X; diagrams are constructed for a specified (constant) P Form of diagram depends on whether or not the phases show significant solid solution  Binary Phase Diagrams This is an example of a 2-component system with 2 phases: (note that one of the phases shows complete solid solution) the plagioclase system components? P constant phases? Complete solid solution, in T (C) which both components mix completely with each other: Na+ + Si4+ = Ca2+ + Al3+ X (mol % An) Nesse, Fig. 5.10a this diagram can be used to explain why plagioclase in igneous rocks is generally zoned but first, some more definitions...... Introduction to mineralogy: William Nesse  Binary Phase Diagrams components: Ab (NaAlSi3O8), An (CaAl2Si2O8) phases: melt, plagss the plagioclase system At each end of the horizontal axis: One component system melt Pure end-member, pure albite on the left, and pure anorthite on the right Pure systems behave like a typical isobaric one-component system Solids melt at a single fixed plagss temperature (P=2, C=1) F=1 - 2+1 =0 Ab An liquidus – (P)-T-X curve (surface) where melt = melt + Xls at higher T: melt only at lower T: melt + Xls solidus – (P)-T-X curve (surface) where melt + Xls = Xls Introduction to mineralogy: at higher T: melt + Xls at lower T: Xls only William Nesse  Binary Phase Diagrams components: Ab (NaAlSi3O8), An (CaAl2Si2O8) phases: melt, plagss for systems with the plagioclase system solid solution, melt comp (An65) compositions of melt coexisting melt 1150oC and crystals plag comp at a given T (An85) are determined by corresponding plagss liquidus and solidus compositions at that T Ab An liquidus – (P)-T-X curve (surface) where melt = melt + Xls at higher T: melt only at lower T: melt + Xls solidus – (P)-T-X curve (surface) where melt + Xls = Xls Introduction to mineralogy: at higher T: melt + Xls at lower T: Xls only William Nesse  Binary Phase Diagrams what happens in this system during crystallisation of magma? the plagioclase system Tinitial phase(s) present at Tinitial: ? ? phase(s) present at Tfinal: Tfinal An75 for perfect equilibrium crystallisation, composition of solid products at Tfinal = composition of melt at Tinitial Introduction to mineralogy: but what happens in between? William Nesse  Binary Phase Diagrams what happens in this system during crystallisation of magma? the plagioclase system phases present and compositions: Tinitial Tliq Tinitial: melt (An75) Tliq/sol Tsol Tliquidus: Tliq/sol: Tsolidus: Tfinal Tfinal: plag (An75) An75 what is composition of crystals in equilibrium with melt at Tliquidus? what is composition of melt in equilibrium with crystals at Tsolidus? Introduction to mineralogy: William Nesse  Binary Phase Diagrams what happens in this system during crystallisation of magma? the plagioclase system phases present and compositions: Tinitial Tliq Tinitial: melt (An75) Tliq/sol Tsol Tliquidus: melt (An75) plag (An90) Tliq/sol: melt (An65) plag (An85) Tsolidus: melt (An50) plag (An75) Tfinal Tfinal: plag (An75) An75 what is composition of crystals in equilibrium with melt at Tliquidus? what is composition of melt in equilibrium with crystals at Tsolidus? Introduction to mineralogy: plagss is always more calcic than coexisting melt!! William Nesse Binary Phase Diagrams what happens in this system during crystallisation of magma? the plagioclase system phases present lever rule: and compositions: melt bulk Xls Tinitial Tliq Tinitial: melt (An75) Tliq/sol % total distance Tsol Tliquidus: melt (An75) % Xls % melt plag (An90) Tliq/sol: melt (An65) plag (An85) Tsolidus: melt (An50) plag (An75) Tfinal Tfinal: plag (An75) An75 % melt vs % Xls can be determined by “lever rule” what is composition of crystals in equilibrium with melt at Tliquidus? what is composition of melt in equilibrium with crystals at Tsolidus? plagss is always more calcic than coexisting melt!! Introduction to mineralogy: William Nesse Introduction to igneous and metamorphic petrology: J.D Winter Binary Phase Diagrams Fractions of A and B in any composition f are determined from the relative lengths of the lines fB and fA, respectively. Fraction of A is given by the ratio fB/AB, and the fraction of B, by the ratio fA/AB. Thus, the longer the line fB, the greater the amount of A present. Introduction to mineralogy: William Nesse Introduction to igneous and metamorphic petrology: J.D Winter  Binary Phase Diagrams “EQUILIBRIUM VS FRACTIONAL CRYSTALLISATION” the plagioclase system phases present and compositions: Tinitial Tliq Tinitial: melt (An75) Tliq/sol Tsol Tliquidus: melt (An75) plag (An90) Tliq/sol: melt (An65) plag (An85) Tsolidus: melt (An50) plag (An75) Tfinal Tfinal: plag (An75) An75 plagss is always more calcic than coexisting melt!! equilibrium crystallisation: plag continuously reacts with melt so that its composition is always in equilibrium with melt Introduction to mineralogy: final plag composition = An75; melt solidifies at ~1100oC William Nesse  Binary Phase Diagrams EQUILIBRIUM VS FRACTIONAL CRYSTALLISATION composition and appearance of plagioclase crystals An77 An68 An55 Nesse, Fig. 5.10b continuous reaction plag melt: plag homogeneous; final plag comp = bulk comp = initial melt comp Introduction to mineralogy: melt solidifies at T where plag comp = bulk comp William Nesse  Binary Phase Diagrams EQUILIBRIUM VS FRACTIONAL CRYSTALLISATION the plagioclase system phases present and compositions: Tinitial Tinitial: melt (An75) Tliquidus: melt (An75) plag (An90) Tliq/sol: melt (An75  Ano) plag (An90  Ano) Tfinal: plag (An0) Tfinal plag is zoned!!! plagss is always more calcic than coexisting melt!! fractional crystallisation: plagioclase does not react with melt; both plag and melt composition change continuously with decreasing T Introduction to mineralogy: final plag composition = An0; melt solidifies at ~750oC William Nesse  Binary Phase Diagrams equilibrium vs fractional crystallisation composition and appearance of plagioclase crystals An77 An68 An77 plag is zoned!!! Nesse, Fig. 5.10c little or no reaction plag melt: plag zoned; both plag and melt evolve toward Na-rich compositions Introduction to mineralogy: melt solidifies at minimum T on liquidus curve William Nesse  Binary Phase Diagrams EQUILIBRIUM VS FRACTIONAL CRYSTALLISATION Igneous rocks and processes: Robin Gill  Zoned Crystals “normal” zoning progressive cooling (in plag, from and crystallisation Ca-rich core  An30 An20 An5 Na-rich rim) implies continuous growth during cooling zoned plagioclase in a granite, OM-12 (4 mm, XN) zoning during growth from a melt can now be understood in terms of evolving crystal and melt compositions as magma cools  Zoned Crystals “reverse” zoning (in plag, from progressive cooling Na-rich core  and crystallisation Ca-rich rim) An30 An40 An50 implies continuous growth during heating (e.g. in xenocryst or xenolith) zoned plagioclase in a granite, OM-12 (4 mm, XN) zoning during growth from a melt can now be understood in terms of evolving crystal and melt compositions as magma cools  Zoned Crystals variable extinction angle (grey level) reflects variable composition “oscillatory” zoning (in plag, fluctuations in An / Ab from core to rim) implies.......? Some other parameters must change during cooling (X or P) zoned plagioclase in andesite, OM-17 (4 mm, XN) zoning during growth from a melt can now be understood in terms of evolving crystal and melt compositions as magma cools but how do we explain reversals or fluctuations?