Ternary eutectic at point M
Systems with
more than 2 components
(Chapter 7)
last update:09/20/06
Ternary systems
Diopside-Forsterite-Anorthite eutectic system
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3-dimensional portrayal of the liquidus
surfaces of the Di-Fo-An system. A system of 3 binary eutectics with no
solid solution.
Ternary eutectic at point M |
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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. |
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Equilibrium crystallization
P = 2 (Fo + Liq) F = 3 - 2 + 1 = 2 If on liquidus, need to specify only 2 intensive variables to determine the system: T and XAnliq or XAnliq and XFoliq X of pure Fo is fixed |
| Lever
principle gives relative proportions of liquid & Fo
At 1500oC
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New continuous reaction as liquid follows
cotectic:
LiqA = LiqB + Fo + Di Bulk solid extract Di/Fo in bulk solid extract using lever principle |
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Crystallization proceeds down T along the cotectic
crystallizing Fo + Di until the ternary eutectic (M) is reached and An
joins the crystals.
Equilibrium melting is the same as equilibrium crystallization, but in reverse.
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| Fractional crystallization (instant
removal of crystal) will produce a layer of Fo, then a layer of Fo + Di
(in proportions defined by the cotectic) and then a layer with eutectic
proportions of Fo + Di + An.
Partial (fractional) melting initially generates a melt of eutectic composition at 1270 C until the An component is used up (producing a binary Fo-Di mixture), then there is a melting hiatus until 1387 when more melt is generated until the Di component is used up, then there is a second hiatus until 1890 C when pure Fo melts. |
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Ternary feldspars system
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Trace of solvus at three
temperature intervals
Triangle shows coexisting feldspars and liquid at 900oC Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. |
Systems with more than 4 components
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Pressure-temperature phase diagram for the melting of a Snake River (Idaho, USA) tholeiitic basalt under anhydrous conditions. After Thompson (1972). Carnegie Inst. Wash Yb. 71 |
The effects of Pressure
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Effect of lithostatic pressure on the liquidus and eutectic composition in the diopside-anorthite system. 1 GPa data from Presnall et al. (1978). Contr. Min. Pet., 66, 203-220. |
The effect of H2O on melting
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Dry melting: solid = liquid
Add water- water enters the melt and the reaction becomes: solid + water = liq(aq)The effect of H2O saturation on the melting of albite, from the experiments by Burnham and Davis (1974). A J Sci 274, 902-940. The "dry" melting curve is from Boyd and England (1963). JGR 68, 311-323. |
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Experimentally determined melting intervals of gabbro under H2O-free ("dry"), and H2O-saturated conditions. After Lambert and Wyllie (1972). J. Geol., 80, 693-708 |
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Dry and water-saturated
solidi for some common rock types
The more mafic the rock the higher the melting point All solidi are greatly lowered by water H2O-saturated (solid) and H2O-free (dashed) solidi (beginning of melting) for granodiorite (Robertson and Wyllie, 1971), gabbro (Lambert and Wyllie, 1972) and peridotite (H2O-saturated: Kushiro et al., 1968; dry: Ito and Kennedy, 1967). |
We know the behavior of water-free and water-saturated melting by experiments, which are easy to control by performing them in dry and wet sealed vessels
What about real rocks?
Some may be dry, some saturated, but most are more likely to be in between these extremes
fixed water content < saturation levels
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Melting of Albite with a fixed activity
of H2O
Fluid may be a CO2-H2O mixture with Pf = PTotal
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Fluid may be a CO2-H2O mixture with Pf = PTotal |