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.

Diopside-Albite-Anorthite Figure 7-5. Isobaric diagram illustrating the liquidus temperatures in the system diopside-anorthite-albite at atmospheric pressure (0.1 MPa). After Morse (1994), Basalts and Phase Diagrams. Krieger Publushers Oblique View Cotectic trough slopes down continuously from Di - An (1274oC) to Di - Ab (1133oC) Di - An Eutectic Di - Ab Eutectic Ab - An solid solution

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

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

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.

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

How does the mantle melt?? 1) Increase the temperature No realistic mechanism for the general case Local hot spots OK very limited area Figure 10-3

2) Lower the pressure Adiabatic rise of mantle with no conductive heat loss Decompression melting could melt at least 30% Adiabatic rise of mantle with no conductive heat loss Steeper than solidus Intersects solidus D slope = heat of fusion as mantle melts Decompression melting could melt at least 30% Figure 10-4

3) Add volatiles (especially H2O) Remember solid + water = liq(aq) and LeChatelier Dramatic lowering of melting point of peridotite Figure 10-5

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.