g314 06 w4 l1 partial melting 1
TRANSCRIPT
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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
Stishovite
Coesite
- quartz
- quartz
Liquid
Tridymite
Cristobalite
600 1000 1400 1800 2200 2600
2
4
6
8
10
Pressure(GPa)
Temperature oC
After Swamy andSaxena (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 phasediagram at atmosphericpressure (After Bowen
and Shairer (1932),Amer. J. Sci. 5th Ser.,24, 177-213.
Fo20 40 60 80Fa
1300
1500
1700
1890
1205
T oC
Olivine
Liquid
Liquid
plus
1900
a
bc
d
Wt.% Forsterite
Olivine
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2-C Eutectic Systems
Example: Diopside - Anorthite
No solid solution
1274
Di 20 40 60 80 An
1200
1300
1400
1500
1600
T oC
Anorthite + Liquid
Liquid
Diopside + Liquid
Diopside + Anorthite
1553
1392
Wt.% Anorthite
Isobaric T-X phase diagram at atmospheric pressure (After Bowen (1915), Amer. J. Sci.40, 161-185.
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C = 3: Ternary Systems:Example 1: Ternary Eutectic
Di - An - Fo
T
M
Anorthite
Forsterite
Diopside
Note three binary eutectics
No solid solution
Ternary eutectic = M
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T - X Projection of Di - An - Fo
Figure 7-2. Isobaric
diagram illustratingthe 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.
An + Liq
Liquid
Di + Liq
Di + An
a
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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.
Ne
Fo En
Ab
SiO2
Oversaturated(quartz-bearing)tholeiitic basalts
E3GPa
E2Gpa
E1GPa
E1atm
Volatile-free
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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), andH2O-saturated conditions. After Lambert and Wyllie (1972). J. Geol., 80, 693-708.
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Ne
Fo En
Ab
SiO2
Oversaturated(quartz-bearing)tholeiitic basalts
Highly undesaturated(nepheline-bearing)
alkali olivinebasalts
3GPa2GPa
1GPa
1atm
Volatile-free
Ne
Fo En
Ab
SiO2
Oversaturated(quartz-bearing)tholeiitic basalts
Highly undesaturated(nepheline-bearing)
alkali olivinebasalts
CO2
H2Odry
P = 2 GPa
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) eutectictoward higher silica, while CO2
moves it to more alkaline types
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> 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). CarnegieInst. Wash Yb. 71
E i l i l
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Experiments on melting mantle
samples:
Tholeiite easilycreated
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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15
10
5
0
0.0 0.2 0.4 0.6 0.8
Wt.% TiO2
Dunite
Harzburgite
Lherzolite
Tholeiitic basalt
Residuum
Source, melt and residuum:
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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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
ViewIsothermal
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 fromMorse (1994) Basalts and Phase Diagrams Krieger Publishers