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

4. 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,

5. 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

6. 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

7. 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

8. 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 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,

9. 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,
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

10. 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 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.

11. 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,

12. 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,
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.

13. > 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 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

14. 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,

15. 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

16. 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

17. 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

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

23. 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.