Partial melting

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Partial melting

• 1. Binary and ternary phase diagrams melting of
  the mantle

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1 - C Systems
The system SiO2
After Swamy and Saxena (1994), J. Geophys. Res.,
99, 11,787-11,794. AGU
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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.
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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.
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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
Di - An Eutectic Di - Ab Eutectic Ab - An solid
solution
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C 3 Ternary SystemsExample 1 Ternary
EutecticDi - An - Fo
Anorthite

• Note three binary eutectics
• No solid solution
• Ternary eutectic M

M
T
Forsterite
Diopside
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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.
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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.
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Pressure effects
Figure 10-8 After Kushiro (1968), J. Geophys.
Res., 73, 619-634.
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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.
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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.
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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
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gt 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
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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.
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Source, melt and residuum
Tholeiitic basalt
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Partial Melting
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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.
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Lherzolite
Harzburgite
Residuum
Dunite
0
0.8
0.4
0.6
0.2
0.0
Wt. TiO2
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How does the mantle melt??

• 1) Increase the temperature

Figure 10-3
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• 2) Lower the pressure
• Adiabatic rise of mantle with no conductive heat
  loss
• Decompression melting could melt at least 30

Figure 10-4
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• 3) Add volatiles (especially H2O)

Figure 10-5
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Oblique View
Isothermal Section
Figure 7-8. Oblique view illustrating an
isothermal section through the diopside-albite-ano
rthite 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.