1. Binary and ternary phase diagrams; melting of the mantle Partial melting 1. Binary and ternary phase diagrams; melting of the mantle

1 - C Systems The system SiO2 After Swamy and Saxena (1994), J. Geophys. Res., 99, 11,787-11,794. AGU

The Olivine System Fo - Fa (Mg2SiO4 - Fe2SiO4) also a solid-solution series Fo 20 40 60 80 Fa 1300 1500 1700 1890 1205 T oC Olivine Liquid plus 1900 a b c d Wt.% Forsterite Isobaric T-X phase diagram at atmospheric pressure (After Bowen and Shairer (1932), Amer. J. Sci. 5th Ser., 24, 177-213.

2-C Eutectic Systems Example: Diopside - Anorthite No solid solution 1600 Liquid 1553 Liquidus 1500 T o C 1400 Anorthite + Liquid 1392 1300 Diopside + Liquid 1274 1200 Diopside + Anorthite Di 20 40 60 80 An Wt.% Anorthite Isobaric T-X phase diagram at atmospheric pressure (After Bowen (1915), Amer. J. Sci. 40, 161-185.

Melting in a binary system An-rich composition (right of the eutectic) Di-rich composition

C = 3: Ternary Systems: Example 1: Ternary Eutectic Di - An - Fo Anorthite Note three binary eutectics No solid solution Ternary eutectic = M 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

T - X Projection of Di - An - Fo An + Liq Liquid Di + Liq Di + An a An 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 liquidus phase in that field

Melting in a ternary Consider a composition close to the Fo apex and with Di>An (mantle-like)

Effect of pressure 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 Figure 7-16. 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.

Pressure effects: Ne Fo En Ab SiO2 Oversaturated (quartz-bearing) tholeiitic basalts Highly undesaturated (nepheline - bearing) alkali basalts Undersaturated E 3GPa 2Gpa 1GPa 1atm Volatile-free Figure 10-8 After Kushiro (1968), J. Geophys. Res., 73, 619-634. Increased pressure moves the ternary eutectic minimum from the oversaturated tholeiite field to the under-saturated alkaline basalt field Alkaline basalts are thus favored by greater depth of melting

NB Do you remember – alkaline vs. Sub-alkaline series?

Effect of water 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 7-25. 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.

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 7-20. Experimentally determined melting intervals of gabbro under H2O-free (“dry”), and H2O-saturated conditions. After Lambert and Wyllie (1972). J. Geol., 80, 693-708.

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 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, 619-634. Right: Figure 7-27. 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, 619-634, H2O-saturated curve from Kushiro (1972), J. Petrol., 13, 311-334, CO2–saturated curve from Eggler (1974) CIW Yb 74.

> 4 Components May as well melt real rocks On right is a P-T diagram for the melting of a Snake River basalt. Each curve represents the loss of a phase as heat system. - Compare to simpler systems Note pressure effects Ol - Plag - Cpx at low P Garnet at high P (eclogite) Figure 7-13. 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

Experiments on melting mantle samples: Tholeiite easily created by 10-30% PM More silica saturated at lower P Grades toward alkalic at higher P Figure 10-17a. After Jaques and Green (1980). Contrib. Mineral. Petrol., 73, 287-310.

Figures not used

Source, melt and residuum: 15 Tholeiitic basalt 10 Partial Melting Wt.% Al2O3 Figure 10-1 Brown and Mussett, A. E. (1993), The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall/Kluwer. 5 Lherzolite Harzburgite Residuum Dunite 0.0 0.2 0.4 0.6 0.8 Wt.% TiO2

Oblique View Isothermal Section Figure 7-8. Oblique view illustrating an isothermal section through the diopside-albite-anorthite system. Figure 7-9. Isothermal section at 1250oC (and 0.1 MPa) in the system Di-An-Ab. Both from Morse (1994), Basalts and Phase Diagrams. Krieger Publishers.

2. Melting reactions, experimental petrology Melting of the crust Partial melting 2. Melting reactions, experimental petrology Melting of the crust

Qz-Ab-Or + H2O At 1 kbar (supersolvus) At 5 kbar (subsolvus)

Chapter 18: Granitoid Rocks Figure 18-3. The Ab-Or-Qtz system with the ternary cotectic curves and eutectic minima from 0.1 to 3 GPa. Included is the locus of most granite compositions from Figure 11-2 (shaded) and the plotted positions of the norms from the analyses in Table 18-2. Note the effects of increasing pressure and the An, B, and F contents on the position of the thermal minima. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

5um powder 12.7mm

Incongruent melting reactions (Limpopo SMZ, Ga-Mathule village, E. of Bandelierkop)

Chapter 18: Granitoid Rocks Figure 18-5. a. Simplified P-T phase diagram and b. quantity of melt generated during the melting of muscovite-biotite-bearing crustal source rocks, after Clarke (1992) Granitoid Rocks. Chapman Hall, London; and Vielzeuf and Holloway (1988) Contrib. Mineral. Petrol., 98, 257-276. Shaded areas in (a) indicate melt generation. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

2 generations of melt in a single outcrop ? M1: Bt still stable (Q+KSp+Ph+H2O=M) M2: incongruent melting yielding crd (Velay dome, french hercynian belt)

Melting of an heterogeneous crust Orthogneiss: Qz-Pg-Bt Paragneiss: Or-Ab-Qz-Bt-AlS Shear zone: add water to the above What will melt, at what temperature, with which melting reaction? NB- this is a simplified model!

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Figure 18-8. Schematic models for the uplift and extensional collapse of orogenically thickened continental crust. Subduction leads to thickened crust by either continental collision (a1) or compression of the continental arc (a2), each with its characteristic orogenic magmatism. Both mechanisms lead to a thickened crust, and probably thickened mechanical and thermal boundary layers (“MBL” and “TBL”) as in (b) Following the stable situation in (b), either compression ceases (c1) or the thick dense thermal boundary layer is removed by delamination or convective erosion (c2). The result is extension and collapse of the crust, thinning of the lithosphere, and rise of hot asthenosphere (d). The increased heat flux in (d), plus the decompression melting of the rising asthenosphere, results in bimodal post-orogenic magmatism with both mafic mantle and silicic crustal melts. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

3. Migmatites and melt extraction Partial melting 3. Migmatites and melt extraction

Partially molten rocks

MELANOSOME = Residue LEUCOSOME = Liquid = granitic magma MESOSOME = Not melted

Qz KF Biot Plg Qz KF Biot Plg

Metatexites Diatexites « Dirty » granites

Rheology of partially molten systems

Melt extraction

Brown, 1994

Melt depletion

An experimental study of melt extraction J. Barraud, PhD 2000

Films Exp39.avi

(Films) Exp32.avi Exp43.avi

Evolution de la déformation 1) 5% shortening 2) 22% shortening

3) 30% shortening 4) 36% shortening Zone de cisaillement Asymetrical fold; shear zone on one flank 4) 36% shortening Bande de cisaillement Strain localization in liquid patches.

Melt extraction: role of deformation

Melting and migmatitic domes – migmatites in orogenic belts The Velay dome Burg et al., 1994

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