Transcript GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY Lecture 7 More About Phase Diagrams for Two-Component Systems February 6, 2009

GEOL 2312
IGNEOUS AND METAMORPHIC
PETROLOGY
Lecture 7
More About Phase Diagrams for
Two-Component Systems
February 6, 2009
TWO-COMPONENT SYSTEM WITH A EUTECTIC
PYROXENE - PLAGIOCLASE
Eutectic
Point
TWO-COMPONENT SYSTEM WITH A EUTECTIC
PYROXENE - PLAGIOCLASE
a – bulk starting composition = An70
Eutectic
Point
TWO-COMPONENT SYSTEM WITH A EUTECTIC
PYROXENE - PLAGIOCLASE
a – bulk starting composition = An70
b – crystallization begins at 1450oC
c - pure plagioclase (An) crystallizes
Eutectic
Point
TWO-COMPONENT SYSTEM WITH A EUTECTIC
PYROXENE - PLAGIOCLASE
b-d – magma composition changes as
plagioclase crystallizes
d – reaction stays at 1274oC until liquid
is consumed
a – bulk starting composition = An70
b – crystallization begins at 1450oC
c - pure plagioclase (An) crystallizes
Lever
Eutectic
Point Rule
An 30%
An 50%
An 70%
Liq 70%
Liq 50%
Di 30%
TWO-COMPONENT SYSTEM WITH A EUTECTIC
PYROXENE – PLAGIOCLASE
EVOLUTION OF LIQUID AND SOLID
DURING CRYSTALLIZATION
Equilibrium vs. Fractional
Eutectic
Point
TWO-COMPONENT SYSTEM WITH A EUTECTIC
PYROXENE – PLAGIOCLASE
EQUILIBRIUM MELTING
TWO-COMPONENT SYSTEM WITH A EUTECTIC
PYROXENE – PLAGIOCLASE
FRACTIONAL MELTING
TWO-COMPONENT SYSTEM WITH A PERITECTIC
OLIVINE-ORTHOPYROXENE-QUARTZ
Three phases
2MgSiO3 (Opx) =
Mg2SiO4 (Ol) + SiO2 (Qtz)
Si-rich magma (a)
(eutectic relationship)
Winter (2001) Figure 6-12.
Isobaric T-X phase diagram of the
system Fo-Silica at 0.1 MPa. After
Bowen and Anderson (1914) and
Grieg (1927). Amer. J. Sci.
TWO-COMPONENT SYSTEM WITH A PERITECTIC
OLIVINE-ORTHOPYROXENE-QUARTZ
Mg-rich magma (f)
i - Peritectic Point
Winter (2001) Figure 6-12.
Isobaric T-X phase diagram of the
system Fo-Silica at 0.1 MPa. After
Bowen and Anderson (1914) and
Grieg (1927). Amer. J. Sci.
TWO-COMPONENT SYSTEM WITH A PERITECTIC
OLIVINE-ORTHOPYROXENE-QUARTZ
Bulk
X
Opx
Ol
i
Ol
Opx –reaction rim
1557
Liq
60%
Opx
67%
Fo
Ol
40%
Ol
33%
En
Proportional amount of Ol that
must be converted to Opx
Mg2SiO4 (Ol) + SiO2 (Liq)
2MgSiO3 (Opx)
TWO-COMPONENT SYSTEM WITH A PERITECTIC
OLIVINE-ORTHOPYROXENE-QUARTZ
System at:
x
i
m
k
y
i
- pertectic point
10%Ol +90%Liq
 50%Opx+50%Liq
i.e. all original Ol recrystallizes to
Opx (if equilibrium is maintained)
m
- 80% Opx + 20% Liq
1557
d
Cr
c
bulk X
Fo
En
c
1543
- eutectic point
90%Opx +10%Liq
 94%Opx+6%Qtz
TWO-COMPONENT SYSTEM WITH A PERITECTIC
OLIVINE-ORTHOPYROXENE-QUARTZ
Incongruent Melting of Enstatite

Melt of En does not  melt of same composition

Rather En  Fo + Liq i at the peritectic
Partial Melting of Fo + En
(harzburgite = mantle)



i
En + Fo also  first liq = i
Remove i and cool
Result = ?
1557
Fo
d
En
1543
c
Cr
TWO-COMPONENT SYSTEM WITH A PERITECTIC
OLIVINE-ORTHOPYROXENE-QUARTZ
PRESSURE EFFECTS
Different phases have different compressibilities
Thus P will change Gibbs Free Energy differentially


Raises melting point (lower volume (solid) phase is favored at higher P)
Shifts from a peritectic relationship at low P to a dual eutectic relationship at high P
with a thermal divide separating them.
Figure 6-15. The
system Fo-SiO2
at atmospheric
pressure and 1.2
GPa. After
Bowen and
Schairer (1935),
Am. J. Sci.,
Chen and
Presnall (1975)
Am. Min.
TWO-COMPONENT SYSTEM WITH A SOLVUS
OLIVINE-ORTHOPYROXENE-QUARTZ
Hyper-liquidus
Solvus
LIQUID
IMMISCIBILITY
TWO-COMPONENT SYSTEM WITH SOLID SOLUTION, A
EUTECTIC AND A SOLVUS
PLAGIOCLASE AND ALKALI FELDSPAR
SOLID SOLUTION
WITH A EUTECTIC
Subsolidus Solvus 
Perthitic Exsolution
TWO-COMPONENT SYSTEM WITH A SOLVUS
PRESSURE EFFECTS