Partial melting

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

1 - C Systems1 - C SystemsThe system SiOThe system SiO22

Stishovite

Coesite

- quartz

- quartz

Liquid

TridymiteCristobalite

600 1000 1400 1800 2200 2600

2

4

6

8

10P

ress

ure

(GP

a)

Temperature oC

After Swamy and Saxena (1994), J. Geophys. Res., 99, 11,787-11,794. AGU

The Olivine SystemThe Olivine SystemFo - FaFo - Fa (Mg (Mg22SiOSiO44 - Fe - Fe22SiOSiO44))

also a solid-solution seriesalso 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.

Fo20 40 60 80Fa

1300

1500

1700

1890

1205

T oC

Olivine

Liquid

Liquid

plus

1900

a

b c

d

Wt.% Forsterite

Olivine

2-C Eutectic Systems2-C Eutectic Systems Example: Diopside - AnorthiteExample: Diopside - Anorthite

No solid solutionNo solid solution

1274

Di 20 40 60 80 An

1200

1300

1400

1500

1600

T oC

Anorthite + Liquid

Liquid Liquidus

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.

C = 3: Ternary Systems:Example 1: Ternary Eutectic

Di - An - Fo

TT

MM

AnorthiteAnorthite

ForsteriteForsterite

DiopsideDiopside

Note three binary Note three binary eutecticseutectics

No solid solutionNo solid solution

Ternary eutectic = MTernary eutectic = M

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)(1994), Basalts and , Basalts and Phase Diagrams. Phase Diagrams. Krieger Publishers.Krieger Publishers.

An + Liq

Liquid

Di + Liq

Di + An

aAn

Effect of pressureEffect 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.

Pressure effects:

Figure 10-8 Figure 10-8 After Kushiro (1968), After Kushiro (1968), J. Geophys. Res.J. Geophys. Res., , 7373, 619-634., 619-634.

NeNe

FoFo EnEn

AbAb

SiOSiO22

Oversaturated(quartz-bearing)tholeiitic basalts

Highly undesaturated

(nepheline - b

earing)

alkali basalts

Undersaturated

tholeiitic basalts

EE3GPa3GPa

EE 2Gpa2Gpa

EE1GPa1GPa

EE 1atm1atm

Volatile-freeVolatile-free

Effect of waterEffect 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.

Figure 7-20.Figure 7-20. Experimentally determined melting intervals of gabbro under H Experimentally determined melting intervals of gabbro under H22O-free (“dry”), and O-free (“dry”), and

HH22O-saturated conditions. After Lambert and Wyllie (1972).O-saturated conditions. After Lambert and Wyllie (1972). J. Geol., 80, 693-708. J. Geol., 80, 693-708.

Ne

Fo En

Ab

SiO2

Oversaturated(quartz-bearing)tholeiitic basalts

Highly undesaturated(nepheline-bearing)

alkali olivinebasalts

Undersaturated

tholeiitic basalts

3GPa2GPa

1GPa

1atm

Volatile-free

Ne

Fo En

Ab

SiO2

Oversaturated(quartz-bearing)tholeiitic basalts

Highly undesaturated(nepheline-bearing)

alkali olivinebasalts

Undersaturated

tholeiitic basalts

CO2

H2Odry

P = 2 GPa

Effect of Pressure, Water, and COEffect of Pressure, Water, and CO2 2 on the positionon the position

of the eutectic in the basalt systemof the eutectic in the basalt systemIncreased pressure moves theIncreased pressure moves theternary eutectic (first melt) fromternary eutectic (first melt) fromsilica-saturated to highly undersat.silica-saturated to highly undersat.alkaline basaltsalkaline basalts

Water moves the (2 Gpa) eutecticWater moves the (2 Gpa) eutectictoward higher silica, while COtoward higher silica, while CO22

moves it to more alkaline typesmoves it to more alkaline types

> 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

Experiments on melting mantle samples:

• Tholeiite easily created

by 10-30% PM

• More silica saturated

at lower P

• Grades toward alkalic

at higher PFigure 10-17a. Figure 10-17a. After Jaques and Green (1980).After Jaques and Green (1980). Contrib. Mineral. Petrol., 73, 287-310.Contrib. Mineral. Petrol., 73, 287-310.

15

10

5

00.0 0.2 0.4 0.6 0.8

Wt.

% A

l 2O3

Wt.% TiO2

DuniteHarzburgite

Lherzolite

Tholeiitic basalt

Partia

l Melt

ing

Residuum

Source, melt and residuum:Source, melt and residuum:

Figure 10-1 Figure 10-1 Brown and Brown and Mussett, A. E. (1993), Mussett, A. E. (1993), The Inaccessible Earth: The Inaccessible Earth: An Integrated View of Its An Integrated View of Its Structure and Structure and Composition. Composition. Chapman Chapman & Hall/Kluwer.& Hall/Kluwer.

How does the mantle melt??1) Increase the temperature

Figure 10-3Figure 10-3

2) Lower the pressure– Adiabatic rise of mantle with no conductive heat

loss– Decompression melting could melt at least 30%

Figure 10-4Figure 10-4

3) Add volatiles (especially H2O)

Figure 10-5Figure 10-5

ObliqueObliqueViewView

IsothermalIsothermalSectionSection

Figure 7-8.Figure 7-8. Oblique view illustrating an isothermal section through the diopside-albite-anorthite Oblique view illustrating an isothermal section through the diopside-albite-anorthite system. system. Figure 7-9.Figure 7-9. Isothermal section at 1250 Isothermal section at 1250ooC (and 0.1 MPa) in the system Di-An-Ab. Both from C (and 0.1 MPa) in the system Di-An-Ab. Both from Morse (1994)Morse (1994), Basalts and Phase Diagrams. Krieger Publishers., Basalts and Phase Diagrams. Krieger Publishers.