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Partial melting

Partial melting. 1. Binary and ternary phase diagrams; melting of the mantle. 1 - C Systems. The system SiO 2. After Swamy and Saxena (1994) , J. Geophys. Res., 99 , 11,787-11,794 . AGU. 1900. 1890. Liquid. a. 1700. b. c. Olivine. T o C. plus. Liquid. d. 1500. Olivine. 1300.

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Partial melting

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

  2. 1 - C Systems The system SiO2 After Swamy and Saxena (1994), J. Geophys. Res., 99, 11,787-11,794. AGU

  3. 1900 1890 Liquid a 1700 b c Olivine T oC plus Liquid d 1500 Olivine 1300 1205 Fa Fo 60 20 40 80 Wt.% Forsterite The Olivine System Fo - Fa (Mg2SiO4 - Fe2SiO4) also a solid-solution series Isobaric T-X phase diagram at atmospheric pressure (After Bowen and Shairer (1932), Amer. J. Sci. 5th Ser., 24, 177-213.

  4. 2-C Eutectic Systems Example: Diopside - Anorthite No solid solution 1600 1553 Liquid Liquidus 1500 T C o 1400 Anorthite + Liquid 1392 Diopside + Liquid 1300 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.

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

  6. C = 3: Ternary Systems:Example 1: Ternary EutecticDi - An - Fo Anorthite Note three binary eutectics No solid solution Ternary eutectic = M M T Forsterite Diopside

  7. a Liquid An An + Liq Di + Liq Di + An 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.

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

  9. Effect of pressure 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.

  10. Ne Volatile-free E 3GPa E 2Gpa E 1GPa Ab Highly undesaturated (nepheline - bearing) alkali basalts E 1atm Oversaturated (quartz-bearing) Undersaturated tholeiitic basalts tholeiitic basalts Fo En SiO2 Pressure effects: Figure 10-8 After Kushiro (1968), J. Geophys. Res., 73, 619-634.

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

  12. Effect of water 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.

  13. 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.

  14. Ne Ne Volatile-free P = 2 GPa 3GPa CO2 2GPa H2O dry 1GPa Ab Ab Highly undesaturated Highly undesaturated (nepheline-bearing) (nepheline-bearing) 1atm alkali olivine alkali olivine basalts basalts Oversaturated Oversaturated (quartz-bearing) (quartz-bearing) Undersaturated Undersaturated tholeiitic basalts tholeiitic basalts tholeiitic basalts tholeiitic basalts Fo En Fo SiO2 En SiO2 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

  15. > 4 Components 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

  16. 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.

  17. Figures not used

  18. Source, melt and residuum: Tholeiitic basalt 15 Partial Melting 10 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 0.8 0.4 0.6 0.2 0.0 Wt.% TiO2

  19. 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.

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

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

  22. 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.

  23. 5um powder 12.7mm

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

  25. 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.

  26. 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)

  27. 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!

  28. Slides not used

  29. 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.

  30. Partial melting 3. Migmatites and melt extraction

  31. Partially molten rocks

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

  33. Qz KF Biot Plg Qz KF Biot Plg

  34. Metatexites Diatexites « Dirty » granites

  35. Rheology of partially molten systems

  36. Melt extraction

  37. Brown, 1994

  38. Melt depletion

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