Download LECTURE W3-L7-9 - Partial Melting

Partial
melting
1. Binary and ternary phase
diagrams; melting of the mantle
1 - C Systems
The system SiO2
Stishovite
Pressure (GPa)
10
After Swamy and
Saxena (1994), J.
Geophys. Res., 99,
11,787-11,794. AGU
8
6
Coesite
4
2
- quartz
- quartz
Liquid
Cristobalite
Tridymite
600
1000
1400
1800
2200
Temperature C
o
2600
The Olivine System
Fo - Fa (Mg2SiO4 - Fe2SiO4)
also a solid-solution series
1900
1890
Liquid
1700
a
b
c
Olivine
plus
T oC
1500
d
Liquid
Olivine
1300
1205
Fa
20
40
60
Wt.% Forsterite
80
Fo
Isobaric T-X phase
diagram at atmospheric
pressure (After Bowen
and Shairer (1932),
Amer. J. Sci. 5th Ser.,
24, 177-213.
2-C Eutectic Systems
Example: Diopside - Anorthite
No solid solution
1600
1553
Liquid
1500
T oC
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, 161-185.
Melting in a binary system
• An-rich composition (right of the eutectic)
• Di-rich composition
C = 3: Ternary Systems:
Example 1: Ternary Eutectic
Di - An - Fo
Anorthite
Note three binary eutectics
No solid solution
Ternary eutectic = M
M
T
Forsterite
Diopside
T - X Projection of Di - An - Fo
Liquid
a
An + Liq
Di + Liq
Di + 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.
A
Melting in a ternary
• Consider a composition close to the Fo
apex and with Di>An (mantle-like)
Effect of pressure
Figure 7-16. Effect of lithostatic pressure on the liquidus and eutectic composition in the diopsideanorthite system. 1 GPa data from Presnall et al. (1978). Contr. Min. Pet., 66, 203-220.
Pressure effects:
Ne
E 3GPa
Volatile-free
E 2Gpa
E 1GPa
Ab
Figure 10-8 After Kushiro (1968),
J. Geophys. Res., 73, 619-634.
E 1atm
Oversaturated
(quartz-bearing)
tholeiitic basalts
Fo
En
SiO2
NB
• Do you remember – alkaline vs. Subalkaline series?
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.
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.
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
Ne
Volatile-free
3GPa
P = 2 GPa
CO2
2GPa
dry
1GPa
Highly undesaturated
(nepheline-bearing)
alkali olivine
basalts
Ab
Highly undesaturated
(nepheline-bearing)
alkali olivine
basalts
1atm
En
Ab
Oversaturated
(quartz-bearing)
tholeiitic basalts
Oversaturated
(quartz-bearing)
tholeiitic basalts
Fo
H2O
SiO2
Fo
En
SiO2
> 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 P
Figure 10-17a. After Jaques and Green (1980).
Contrib. Mineral. Petrol., 73, 287-310.
• Figures not used
Source, melt and residuum:
Tholeiitic basalt
15
10
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
Dunite
0
0.0
0.2
Residuum
0.4
Wt.% TiO2
0.6
0.8
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.
Partial
melting
2. Melting reactions, experimental
petrology
Melting of the crust
Qz-Ab-Or + H2O
At 1 kbar (supersolvus)
At 5 kbar (subsolvus)
Chapter 18: Granitoid Rocks
Figure 18-3. The Ab-Or-Qtz system with the
ternary cotectic curves and eutectic minima
from 0.1 to 3 GPa. Included is the locus of most
granite compositions from Figure 11-2 (shaded)
and the plotted positions of the norms from the
analyses in Table 18-2. Note the effects of
increasing pressure and the An, B, and F
contents on the position of the thermal minima.
From Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice
Hall.
5um powder
12.7mm
Incongruent melting reactions
(Limpopo SMZ, Ga-Mathule village, E. of Bandelierkop)
Chapter 18:
Granitoid Rocks
Figure 18-5. a. Simplified P-T phase diagram and b. quantity of
melt generated during the melting of muscovite-biotite-bearing
crustal source rocks, after Clarke (1992) Granitoid Rocks.
Chapman Hall, London; and Vielzeuf and Holloway (1988)
Contrib. Mineral. Petrol., 98, 257-276. Shaded areas in (a) indicate
melt generation. Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
• 2 generations of
melt in a single
outcrop ?
– M1: Bt still stable
(Q+KSp+Ph+H2O
=M)
– M2: incongruent
melting yielding
crd
(Velay dome, french
hercynian belt)
Melting of an heterogeneous crust
• Orthogneiss: Qz-Pg-Bt
• Paragneiss: Or-Ab-Qz-Bt-AlS
• Shear zone: add water to the above
• What will melt, at what temperature, with
which melting reaction?
NB- this is a simplified model!
• Slides not used
Figure 18-8. Schematic models for the
uplift and extensional collapse of
orogenically thickened continental
crust. Subduction leads to thickened
crust by either continental collision
(a1) or compression of the continental
arc (a2), each with its characteristic
orogenic magmatism. Both
mechanisms lead to a thickened crust,
and probably thickened mechanical
and thermal boundary layers (“MBL”
and “TBL”) as in (b) Following the
stable situation in (b), either
compression ceases (c1) or the thick
dense thermal boundary layer is
removed by delamination or
convective erosion (c2). The result is
extension and collapse of the crust,
thinning of the lithosphere, and rise of
hot asthenosphere (d). The increased
heat flux in (d), plus the
decompression melting of the rising
asthenosphere, results in bimodal postorogenic magmatism with both mafic
mantle and silicic crustal melts.
Winter (2001) An Introduction to
Igneous and Metamorphic Petrology.
Prentice Hall.
Partial melting
3. Migmatites and melt extraction
Partially molten rocks
MELANOSOME = Residue
LEUCOSOME = Liquid = granitic magma
MESOSOME = Not melted
Qz
KF
Biot
Plg
Qz
Biot
KF
Plg
Metatexites
Diatexites
« Dirty » granites
Rheology of partially molten systems
Melt extraction
Brown, 1994
Melt
depletion
An experimental study of melt
extraction
J. Barraud, PhD 2000
Films
Exp39.avi
(Films)
Exp32.avi
Exp43.avi
Evolution de la déformation
1) 5% shortening
2) 22% shortening
3) 30% shortening
Zone de
cisaillement
Asymetrical fold; shear zone on one flank
4) 36% shortening
Bande de
cisaillement
Strain localization in liquid patches.
Melt extraction: role of deformation
Melting and migmatitic domes – migmatites
in orogenic belts
The Velay dome
Burg et al., 1994
• Slides not used