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Partial melting 1. Binary and ternary phase diagrams; melting of the mantle 1 - C Systems The system SiO2 Stishovite Pressure (GPa) 10 After Swamy and Saxena (1994), J. Geophys. Res., 99, 11,787-11,794. AGU 8 6 Coesite 4 2 - quartz - quartz Liquid Cristobalite Tridymite 600 1000 1400 1800 2200 Temperature C o 2600 The Olivine System Fo - Fa (Mg2SiO4 - Fe2SiO4) also a solid-solution series 1900 1890 Liquid 1700 a b c Olivine plus T oC 1500 d Liquid Olivine 1300 1205 Fa 20 40 60 Wt.% Forsterite 80 Fo 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 1553 Liquid 1500 T oC 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 M T Forsterite Diopside T - X Projection of Di - An - Fo Liquid a An + Liq Di + Liq Di + 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. A Melting in a ternary • Consider a composition close to the Fo apex and with Di>An (mantle-like) Effect of pressure Figure 7-16. Effect of lithostatic pressure on the liquidus and eutectic composition in the diopsideanorthite system. 1 GPa data from Presnall et al. (1978). Contr. Min. Pet., 66, 203-220. Pressure effects: Ne E 3GPa Volatile-free E 2Gpa E 1GPa Ab Figure 10-8 After Kushiro (1968), J. Geophys. Res., 73, 619-634. E 1atm Oversaturated (quartz-bearing) tholeiitic basalts Fo En SiO2 NB • Do you remember – alkaline vs. Subalkaline series? 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. 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 Ne Volatile-free 3GPa P = 2 GPa CO2 2GPa dry 1GPa Highly undesaturated (nepheline-bearing) alkali olivine basalts Ab Highly undesaturated (nepheline-bearing) alkali olivine basalts 1atm En Ab Oversaturated (quartz-bearing) tholeiitic basalts Oversaturated (quartz-bearing) tholeiitic basalts Fo H2O SiO2 Fo En SiO2 > 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 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: Tholeiitic basalt 15 10 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 Dunite 0 0.0 0.2 Residuum 0.4 Wt.% TiO2 0.6 0.8 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. 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! • Slides not used 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 postorogenic magmatism with both mafic mantle and silicic crustal melts. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Partial melting 3. Migmatites and melt extraction Partially molten rocks MELANOSOME = Residue LEUCOSOME = Liquid = granitic magma MESOSOME = Not melted Qz KF Biot Plg Qz Biot KF 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 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 • Slides not used