g314 06 w4 l1 partial melting 1

8/2/2019 G314 06 W4 L1 Partial Melting 1

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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


Highly undesaturated(nepheline-bearing)

alkali olivinebasalts

3GPa2GPa

1GPa

1atm

Volatile-free

Ne

Fo En

Ab

SiO2


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

g314 06 w4 l1 partial melting 1

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