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C = 3: Ternary Systems: Example 1: Ternary Eutectic Di - An - Fo
Note three binary eutectics No solid solution Ternary eutectic = M Anorthite As add components, becomes increasingly difficult to depict. 1-C: P - T diagrams easy 2-C: isobaric T-X, isothermal P-X… 3-C: ?? Still need T or P variable Projection? Hard to use as shown As add components, becomes increasingly difficult to dipict. 1-C: P - T diagrams easy 2-C: isobaric T-X, isothermal P-X… 3-C: ?? Still need T or P variable Project? Hard to use as shown M T Forsterite Diopside
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T - X Projection of Di - An - Fo
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. X-X diagram with T contours (P constant) Liquidus surface works like topographic map Red lines are ternary Cotectic troughs Run from binary eutectics down T to ternary eutectic M Separate fields labeled for phases in that field Cotectic: multiple solid phases crystallize from a single liquid X-X diagram with T contours (P constant) Liquidus surface works like topographic map Red lines are ternary cotectic troughs Run from binary eutectics down T to ternary eutectic M Separate fields labeled for liquidus phase in that field
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Crystallization Relationships
Cool composition a from 2000oC Still liquid At 2000oC: phi = 1 (liquid) F = C - phi + 1 = = 3 = T, X(An)Liq, X(Di)Liq, and X(Fo)Liq only 2 of 3 X’s are independent Next cool to 1700oC Intersects liquidus surface Solids form Cool composition a from 2000oC Still liquid At 2000oC: phi = 1 (liquid) F = C - phi + 1 = = 3 = T, X(An)Liq, X(Di)Liq, and X(Fo)Liq only 2 of 3 X’s are independent Next cool to 1700oC Intersect liquidus surface Solids form Crystallization Relationships
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Pressure fixed at 0.1 MPa Pure Fo forms Just as in binary f = ? F = ?
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Lever principle ® relative proportions of liquid & Fo
At 1500oC At any point can use the lever principle to determine the relative proportions of liquid and Fo
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What happens next at 1350oC , pt. b?
Pure diopside joins olivine + liquid phi = 3 F = = 1 (univariant at constant P) Xliq = F(T) only Liquid line of descent follows cotectic -> M
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At 1300oC liquid = x Imagine triangular plane X - Di - Fo balanced on bulk a Draw a working line from x through a to the opposite side, m = total solids Liq x a Di m Fo Liq/total solids = a-m/Liq-a total Di/Fo = m-Fo/Di-m Total amounts of three phases at any T can be determined by a modified lever principle: Total amounts of three phases at any T can be determined by a modified lever principle
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At 1270oC reach M the ternary eutectic
anorthite joins liquid + forsterite + diopside phi = 4 (3 solids 1 liq) F = = 0 (invariant) discontinuous reaction: Liq = Di + An + Fo stay at 1270oC until consume liquid Below 1270oC have all solid Fo + Di + An phi = 3 F = = 1 At 1270oC reach M the ternary eutectic anorthite joins liquid + forsterite + diopside phi = 4 F = = 0 (invariant) discontinuous reaction: Liq = Di + An + Fo stay at 1270oC until consume liquid Below 1270oC have all solid Fo + Di + An phi = 3 F = = 1
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Partial Melting (remove melt):
Suppose bulk = a First melt at M (1270oC) Stay at M until consume one phase (An) Only Di and Fo left Jump to Di-Fo binary No further melt until at 1387oC mp of Diopside Stay at 1387oC until consume one phase (Di) Only Fo left No further melt until 1890oC mp of Fo Try bulk = a First melt at M (1270oC) Stay at M until consume one phase (An) Why An? Only Di and Fo left Jump to Di-Fo binary No further melt until N at 1387oC Stay at N until consume one phase (Di) Only Fo left No further melt until 1890oC
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The Effect of Pressure Solid Liquid Pressure
Temperature Solid P1 P2 T1 T2 ~All solid - liquid (melting) reactions have a positive slope You recall the Clapeyron Equation if dG =0, then dP/dT = ΔS / ΔV OR dP ΔV = dT ΔS For dG to stay zero, higher pressure requires lower volume so dT ΔS stays constant which phase should be stable at high P? The one where volume decreases as pressure increases, a solid. Notice then that the melting point increases with temperature, i.e. solid is stable at higher temperatures. ~All solid - liquid (melting) reactions have a positive slope Increasing pressure (P1 -> P2) thus raises the melting point (T1 -> T2) of a solid
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The magnitude of the effect will vary for different minerals
Increasing pressure decreases volume, and the liquid state, which usually needs more volume, is less likely at high P, so the Melting Point occurs at Higher Temperature For example, in a eutectic system, increasing pressure will raise the eutectic melting point The magnitude of the effect will vary for different minerals Anorthite compresses less than Diopside Thus the elevation of the melting point is less for An than Di along the liquidus The only way to do that is to move the eutectic toward An as the liquidi rise. Change in An liquidus small Change in Diopside liquidus large In a eutectic system increasing pressure will raise the melting point (as predicted) The magnitude of the effect will vary for different minerals Anorthite compresses less than Diopside Thus the elevation of the m. p. is less for An than Di The eutectic thus shifts toward An Eutectic system Figure 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,
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The Effect of Water on Melting
Dry melting: solid ® liquid Add water- some water enters the melt Reaction becomes: solid + water = melt + water (right side is an aqueous solution) Dry melt reaction: solid -> liquid Add water- water enters the melt So the reaction becomes: solid + water = liq(aq) Thus adding water drives the reaction to the right. Liquid is stabilized at the expense of the solid. The result is to lower the melting point. Pressure is required to hold the water in the melt, so increased pressure allows more water to enter the melt and this increases the melting point depression Figure The effect of H2O saturation on the melting of Albite, from the experiments by Burnham and Davis (1974). A J Sci 274, The “dry” melting curve is from Boyd and England (1963). JGR 68,
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The Effect of Water on Melting
The reaction was Solid => Liquid. Adding water “drives the reaction to the right”, in the sense that it melts (goes to liquid) at a lower temp than it would if dry Liquid is stabilized at the expense of the solid. The result is to lower the melting point. Pressure is required to hold the water in the melt, so increased pressure allows more water to enter the melt and this increases the melting point depression THE SLOPE CHANGES TO NEGATIVE Dry melt reaction: solid -> liquid Add water- water enters the melt So the reaction becomes: solid + water = liq(aq) Thus adding water drives the reaction to the right. Liquid is stabilized at the expense of the solid. The result is to lower the melting point. Pressure is required to hold the water in the melt, so increased pressure allows more water to enter the melt and this increases the melting point depression Figure The effect of H2O saturation on the melting of albite, from the experiments by Burnham and Davis (1974). A J Sci 274, The “dry” melting curve is from Boyd and England (1963). JGR 68,
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Note the difference in the dry melting temperature and
the melting interval of a basalt as compared to the water- saturated equivalents The melting point depression is quite dramatic, especially at low to intermediate Pressures 1 – 1.5 GPa Note the difference in the dry melting temperature and the melting interval of a basalt as compared to the water- saturated equivalents The melting point depression is quite Dramatic, especially at low to intermediate pressures Figure Experimentally determined melting intervals of gabbro under H2O-free (“dry”), and H2O-saturated conditions. After Lambert and Wyllie (1972). J. Geol., 80,
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Dry and water-saturated solids for some common rock types
Granite is only a crustal rock (not found deeper than 2.2 GPa) ~80km MAX cont. crust The more mafic the rock the higher the melting point All solidi are greatly lowered by water Granite is only a crustal rock (not found deeper than 1 GPa) Figure 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).
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The solubility of water in a melt depends on the structure of
the melt (which reflects the structure of the mineralogical equivalent) Water dissolves more in polymerized melts (An > Di) Thus the melting point depression effect is greater for An than Di The eutectic moves toward An The solubility of water in a melt depends on the structure of the melt (which reflects the structure of the mineralogical equivalent) Water dissolves more in polymerized melts (An > Di) Thus the melting point depression effect is greater for An than Di The eutectic moves toward An Figure The effect of H2O on the diopside-anorthite liquidus. Dry and 1 atm from Figure 7-16, PH2O = Ptotal curve for 1 GPa from Yoder (1965). CIW Yb 64.
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Two principal types of basalt in the ocean basins
Tholeiitic Basalt and Alkaline Basalt Ocean Islands such as Hawaii have both Tholeitic AND Alkaline Basalts Table 10-1 Common petrographic differences between tholeiitic and alkaline basalts Subalkaline: Tholeiitic-type Basalt Alkaline Basalt Usually fine-grained, intergranular Usually fairly coarse, intergranular to ophitic Groundmass No olivine Olivine common Clinopyroxene = augite (plus possibly pigeonite) Titaniferous augite (reddish) Orthopyroxene (hypersthene) common, may rim ol. Orthopyroxene absent (a third, minor, one is hi-Al, or calc-alk basalt & will be discussed later) No alkali feldspar Interstitial alkali feldspar or feldspathoid may occur Interstitial glass and/or quartz common Interstitial glass rare, and quartz absent Olivine rare, unzoned, and may be partially resorbed Olivine common and zoned Phenocrysts or show reaction rims of orthopyroxene Orthopyroxene uncommon Orthopyroxene absent Early plagioclase common Plagioclase later in sequence, uncommon Clinopyroxene is pale brown augite Clinopyroxene is titaniferous augite, reddish rims after Hughes (1982) and McBirney (1993). A third is hi-Al, or calc-alkaline basalt, usually continental
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Effect of Pressure, Water, and CO2 on the position
of the eutectic in the basalt system Increased pressure moves the ternary eutectic (first melt) from silica-saturated to highly undersat. alkaline basalts Water moves the (2 GPa) eutectic toward higher silica, while CO2 moves it to more alkaline types Ne Fo En Ab SiO2 Oversaturated (quartz-bearing) tholeiitic basalts Highly undesaturated (nepheline-bearing) alkali olivine basalts Undersaturated 3GPa 2GPa 1GPa 1atm Volatile-free Ne Fo En Ab SiO2 Oversaturated (quartz-bearing) tholeiitic basalts Highly undesaturated (nepheline-bearing) alkali olivine basalts Undersaturated CO2 H2O dry P = 2 GPa Constant P H2O down CO2 up Increasing pressure From Tholeiitic to Alkaline Left: Effect of pressure on the ternary eutectic (minimum melt composition) in the system Fo-Ne-SiO2 (base of the “basalt tetrahedron”). From Kushiro (1968). JGR 73, Right: Figure Effect of volatiles on the ternary eutectic (minimum melt composition) in the system Fo-Ne-SiO2 (base of the “basalt tetrahedron”) at 2 GPa. Volatile-free from Kushiro (1968) JGR 73, , H2O-saturated curve from Kushiro (1972), J. Petrol., 13, , CO2–saturated curve from Eggler (1974) CIW Yb 74.
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