g eol 5310 a dvanced i gneous and m etamorphic p etrology phase diagrams october 26, 2009
TRANSCRIPT
GEOL 5310 ADVANCED IGNEOUS AND METAMORPHIC PETROLOGY
Phase Diagrams
October 26, 2009
MAKAOPUHI LAVA LAKE, HAWAIIWATCHING A MAGMA CRYSTALLIZE
From Wright and Okamura, (1977) USGS Prof. Paper, 1004.
TIME
TEM
PER
ATU
RE
MAKAOPUHI LAVA LAKE, HAWAII
10090706050403020100
Percent Glass
900
950
1000
1050
1100
1150
1200
1250
Tem
pera
ture
o c
80Winter (2001), Figs. 6-1 & 6-2. From Wright and Okamura (1977) USGS Prof. Paper, 1004.
1250
1200
1150
1100
1050
1000
9500 0 10 20 30 40 0 0 1010 10 20 30 40
Liquidus
MeltCrust
Solidus
Olivine
Clinopyroxene Plagioclase
Opa
que
100
90
80
70
60
50.7.8.9 .9 .8 .7 .6 80 70 60
AnMg / (Mg + Fe)
We
igh
t %
Gla
ss
Olivine Augite Plagioclase
Mg / (Mg + Fe)
Winter (2001), Fig. 6-3. From Wright and Okamura, (1977) USGS Prof. Paper, 1004.
MAKAOPUHI LAVA LAKE, HAWAII
COMPOSITIONAL CHANGES IN SOLID SOLUTION MINERALS
CRYSTALLIZATION BEHAVIOR OF MAGMASFROM NATURAL AND EXPERIMENTAL
OBSERVATIONS AND THERMODYMANIC PREDICTIONS
Cooling melts crystallize from a liquid to a solid over a range of temperatures (and pressures)
Several minerals crystallize over this T range, and the number of minerals increases as T decreases
The minerals that form do so sequentially, generally with considerable overlap
Minerals that involve solid solution change composition as cooling progresses
The melt composition also changes during crystallization The minerals that crystallize (as well as the sequence)
depend on T and X of the melt Pressure can affect the temperature range at which a
melt crystallizes and the types of minerals that form The nature and pressure of volatiles can also affect the
temperature range of xtallization and the mineral sequence
WHY DO MAGMAS CRYSTALLIZE THIS WAY?
PREDICTED BY PHASE DIAGRAMS
Although magmas (melts + crystals) are some of the most complex systems in nature, we can evaluate how they form and crystallize by simplifying them into their basic chemical constituent parts and empirically determine (observe) how these simple systems react to geologically important variables – temperature and pressure.
We portray this behavior through the construction of PHASE DIAGRAMS
PHASE DIAGRAMSTERMINOLOGY
PHASE of a System A physically distinct part of a system that may be mechanically separated from other distinct parts. (e.g., in a glass of ice water (the system), ice and water are two phases mechanically distinct phases)
COMPONENTS of a System The minimum number of chemical constituents that are necessary to define the complete composition of a system (e.g. for the plagioclase system, components are NaAlSi NaAlSi33OO88 – albite and CaAlCaAl22SiSi22OO88 - - anorthite)
VARIABLES that define the STATE of a SystemExtensive – dependent on the quantity of the system – volume, mass, moles, ...Intensive – properties of the phases of a system that are independent of
quantities (temperature, pressure, density, molecular proportions, elemental ratios, ...)
Note that ratios of extensive variables become intensive (V/m = density,V/moles=molar volume)
GIBBS PHASE RULE
F = C - F = C - + + 22F = # degrees of freedom
The number of intensive parameters that must be specified in order to completely determine the system, or the number of variables that can be changed independently and still maintain equilibrium
= # of phasesphases are mechanically separable constituents
C = minimum # of components (chemical constituents that must be specified in order to define all phases)
2 = Two intensive parametersUsually = temperature and pressure
ONLY APPLIES TO SYSTEMS IN CHEMICAL EQUILIBRIUM!!ONLY APPLIES TO SYSTEMS IN CHEMICAL EQUILIBRIUM!!
PHASE RULE IN A ONE-COMPONENT SYSTEM
F = C - F = C - + + 22
Divariant FieldFF = 1 – 1 + 2 = 22
Univariant LineFF = 1 – 2 + 2 = 11
Invariant PointFF = 1 – 3 + 2 = 00
SiO2
PHASE RULE IN A ONE-
COMPONENT SYSTEM
H2O
Fluid
Sublimation
Note that HEAT is different than TEMPERATURE.
A boiling pot of water must be continuously heated to completely turn to steam, all the while sitting at 100oC
This heat is called the latent heat of vaporization
The heat require to turn solid into liquid is the latent heat of fusion
TWO-COMPONENT SYSTEM WITH SOLID SOLUTION
COMPARE X AND T AT A CONSTANT P
System – Plagioclase
Phases – Liquid and Plagioclase mineral
Components – Ab (NaAlSiNaAlSi33OO88)
An (CaAlCaAl22SiSi22OO88) coupled substitution!
An content = An / (Ab + An)F = C - F = C - + + 1 1 (only 1 variable since P is constant)
Divariant FieldFF = 2 – 1 + 1 = 22
Univariant FieldFF = 2 – 2 + 1 = 11
Phase Relationships determined by Experimental Data
TWO-COMPONENT SYSTEM WITH SOLID SOLUTION
EQUILIBRIUM CRYSTALLIZATION
a – Starting bulk composition of melt = An60
b – Beginning of crystallizationT= 1475oC
c – Composition of first plagioclase to crystallize
= An87
TWO-COMPONENT SYSTEM WITH SOLID SOLUTION
EQUILIBRIUM CRYSTALLIZATIONa – Starting bulk composition of melt = An60
b – Beginning of crystallizationT= 1475oC
c – Composition of first plagioclase to crystallize
at 1475oC = An87
d – Melt composition at 1450oC= An48
e – Bulk composition of Magma (Melt + Crystals =
An60)
f – Composition of Plagioclase at 1450oC = An81
TWO-COMPONENT SYSTEM WITH SOLID SOLUTION
EQUILIBRIUM CRYSTALLIZATIONUSING THE LEVER RULE TO DETERMINE CRYSTAL:MELT
RATIO
40%40% 60%60%
%Melt%Melt%Melt%Plag%Plag
TWO-COMPONENT SYSTEM WITH
SOLID SOLUTIONEQUILIBRIUM CRYSTALLIZATION
a – Starting bulk composition of melt = An60
b – Beginning of crystallizationT= 1475oC
c – Composition of first plagioclase to crystallize
at 1475oC = An87
d – Melt composition at 1450oC= An48
e – Bulk composition of Magma (Melt + Crystals =
An60)
f – Composition of Plagioclase at 1450oC = An81
g – Last melt composition at 1340oC = An18
h – Final composition of plagioclase at 1450oC = An60
i – Subsolidus cooling of plagioclase
TWO-COMPONENT SYSTEM WITH SOLID SOLUTION
FRACTIONAL CRYSTALLIZATION
As crystals form, they are removed (fractionated) from the system and thus are not allowed to reequilibrate with the cooling melt.
This has the effect of incrementally resetting the bulk composition of the liquid to a lower An content with each crystallization step.
Consequently, the final melt may have a composition of An0 (pure Ab end member)
TWO-COMPONENT SYSTEM WITH SOLID SOLUTION
FRACTIONAL CRYSTALLIZATION
uts.cc.utexas.edu/~rmr/CLweb/volcanic.htm
Because of coupled substitution of Ca-Na and Al-Si in plagioclase, reequilibration is difficult with T decrease, leading to chemically zoned crystals like this one.
Avg. An=60
TWO-COMPONENT SYSTEM WITH SOLID SOLUTION
OLIVINESonju Lake Intrusion
FayaliteFe2SiO4
FosteriteFe2SiO4
TWO-COMPONENT SYSTEM WITH A EUTECTIC
PYROXENE - PLAGIOCLASE
EutecticPoint
TWO-COMPONENT SYSTEM WITH A EUTECTIC
PYROXENE - PLAGIOCLASE
EutecticPoint
a – bulk starting composition = An70
TWO-COMPONENT SYSTEM WITH A EUTECTIC
PYROXENE - PLAGIOCLASE
EutecticPoint
a – bulk starting composition = An70b – crystallization begins at 1450oCc - pure plagioclase (An) crystallizes
TWO-COMPONENT SYSTEM WITH A EUTECTIC
PYROXENE - PLAGIOCLASE
EutecticPoint
a – bulk starting composition = An70b – crystallization begins at 1450oCc - pure plagioclase (An) crystallizes
b-d – magma composition changes as plagioclase crystallizes
d – reaction stays at 1274oC until liquid is consumed
An 30%An 30% Liq 70%Liq 70%
An 50%An 50% Liq 50%Liq 50%
An 70%An 70% Di 30%Di 30%
Lever Lever RuleRule
TWO-COMPONENT SYSTEM WITH A EUTECTIC
PYROXENE – PLAGIOCLASEEVOLUTION OF LIQUID AND SOLID
DURING CRYSTALLIZATION
EutecticPoint
Equilibrium vs. Fractional
TWO-COMPONENT SYSTEM WITH A EUTECTIC
PYROXENE – PLAGIOCLASE EQUILIBRIUM MELTING
TWO-COMPONENT SYSTEM WITH A EUTECTIC
PYROXENE – PLAGIOCLASE FRACTIONAL (OR BATCH) MELTING
Three phases2MgSiO3 (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
ii
FoFo EnEn
15571557
Bulk X
TWO-COMPONENT SYSTEM WITH A PERITECTIC
OLIVINE-ORTHOPYROXENE-QUARTZ
Liq60%
Ol40%
Opx67%
Ol33%
Proportional amount of Ol that must be converted to Opx
Mg2SiO4 (Ol) + SiO2 (Liq) 2MgSiO3 (Opx)
Opx
Ol
Opx –reaction rim
Ol
15431543cc
dd
iikkmm
FoFo EnEn
15571557
bulk Xbulk X
xx
yy
CrCr
TWO-COMPONENT SYSTEM WITH A PERITECTIC
OLIVINE-ORTHOPYROXENE-QUARTZ
System at:
- pertectic point 10%Ol +90%Liq 50%Opx+50%Liqi.e. all original Ol recrystallizes to Opx (if equilibrium is maintained)
- 80% Opx + 20% Liq
- eutectic point 90%Opx +10%Liq 94%Opx+6%Qtz
ii
mm
cc
Incongruent Melting of EnstatiteIncongruent Melting of Enstatite Melt of En does not Melt of En does not melt of same composition melt of same composition
Rather Rather En En Fo + Liq Fo + Liq ii at the peritectic at the peritectic
Partial Melting of Fo + En Partial Melting of Fo + En
(harzburgite = mantle)(harzburgite = mantle) En + Fo also En + Fo also first liq = first liq = ii Remove Remove ii and cool and cool Result = Result = ??
15431543
ccdd
ii
FoFo EnEn
15571557CrCr
TWO-COMPONENT SYSTEM WITH A PERITECTIC
OLIVINE-ORTHOPYROXENE-QUARTZ
PRESSURE EFFECTSPRESSURE EFFECTSDifferent 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 PERITECTIC
OLIVINE-ORTHOPYROXENE-QUARTZ
LIQUID LIQUID IMMISCIBILIIMMISCIBILITYTY
TWO-COMPONENT SYSTEM WITH A SOLVUS
OLIVINE-ORTHOPYROXENE-QUARTZ
Hyper-liquidusSolvus
SOLID SOLUTION WITH A EUTECTIC
TWO-COMPONENT SYSTEM WITH SOLID SOLUTION, A EUTECTIC AND A SOLVUS
PLAGIOCLASE AND ALKALI FELDSPAR
Subsolidus Solvus Perthitic Exsolution
TWO-COMPONENT SYSTEM WITH A SOLVUS
PRESSURE EFFECTS
THREE COMPONENT SYSTEM WITH EUTECTICS
OLIVINE-PLAGIOCLASE-PYROXENE (FO-AN-DI)
Liquidus surface showing temperature contoursF = 3 – 2 + 1 = 2 (divariant field)
Cotectic (or binary eutectic)F = 3 – 3 + 1 = 1 (univariant line)
Liquidus surface showing temperature contoursF = 3 – 2 + 1 = 2
Ternary eutecticF = 3 – 4 + 1 = 0 (invariant point)Pressure =
0.1 MPaWinter (2001) Figure 7-2. Winter (2001) Figure 7-2. Isobaric diagram illustrating the Isobaric diagram illustrating the liquidus temperatures in the Di-liquidus temperatures in the Di-An-Fo system at atmospheric An-Fo system at atmospheric pressure (0.1 MPa). After Bowen pressure (0.1 MPa). After Bowen (1915), A. J. Sci., and Morse (1915), A. J. Sci., and Morse (1994), Basalts and Phase (1994), Basalts and Phase Diagrams. Krieger Publishers.Diagrams. Krieger Publishers.
THREE COMPONENT SYSTEM WITH EUTECTICS
OLIVINE-PLAGIOCLASE-PYROXENE (FO-AN-DI)
Pressure = 0.1 MPa
XX – starting magma compositionSS – starting bulk solid composition
X’ X’ – magma composition when plagioclase become saturated along with olivine S’ S’ – bulk solid composition when magma at X’ (olivine composes ~ 30% of system)
X’X’
X’’X’’
XX
SS
S’’S’’
S’S’
X”X” – magma comp at 50% crystallized (based on lever rule of tie-line through X)
S”S” – bulk solid comp when magma at X” (composed of 68%Ol & 32%Pl)
X*X*
S*S*
X*X*=M – magma reached ternary eutectic at 65% crystallizedS*S* – bulk solid comp when magma reaches ternary eutectic (composed of 60%Ol & 40%Pl) X*X*
SSff
X*X*=M – magma comp fixed at ternary eutectic until 100% crystallizedSSff – final bulk solid = X-starting liquid comp
Equilibrium Crystallization
THREE COMPONENT SYSTEM WITH EUTECTICS
OLIVINE-PLAGIOCLASE-PYROXENE (FO-AN-DI)
Pressure = 0.1 MPa
X’X’
X’’X’’
XX
SS
S’’S’’
S’S’
X*X*
S*S*
X*X*
SSff
Fractional Crystallization
O
PO
PCO
3 binary systems:Fo-An eutecticAn-SiO2 eutecticFo-SiO2 peritectic
Pressure = 0.1 MPa
THREE COMPONENT SYSTEM WITH A PERITECTIC
PLAGIOCLASE-OLIVINE-(ORTHOPYROXENE)-QUARTZ (AN-FO-(EN-)SIO2)
Winter (2001) Figure 7-4. Isobaric diagram illustrating the cotectic and peritectic curves in the system forsterite-anorthite-silica at 0.1 MPa. After Anderson (1915) A. J. Sci., and Irvine (1975) CIW Yearb. 74.
Liquidus contours not shown to reduce clutter
THREE COMPONENT SYSTEM WITH A PERITECTIC
PLAGIOCLASE-OLIVINE-(ORTHOPYROXENE)-QUARTZ (AN-FO-(EN-)SIO2)
aa – – Starting Liquid CompStarting Liquid Compa-ba-b – – liquid path due to Fo liquid path due to Fo
crystallizationcrystallizationb- b- En crystallization (and En crystallization (and
partial replacement of partial replacement of Fo) beginsFo) begins
b-c – b-c – liquid path due to liquid path due to Fo+En crystallizationFo+En crystallization
c – c – An joins En and Fo as An joins En and Fo as crystallizing phases;crystallizing phases;last liquid comp last liquid comp for equilibrium for equilibrium crystallizationcrystallization
ABC
F
AB AB –– Fo only crystallization Fo only crystallization drives liquid from drives liquid from aabb
AB-C –AB-C – En crystallization En crystallization (and replacement) (and replacement) enriches bulk solid in En enriches bulk solid in En to to CC (where liquid (where liquid reaches reaches cc))
C-FC-F – – when liquid when liquid reaches c, An is added reaches c, An is added to bulk solid and is 100% to bulk solid and is 100% crystallized when reach crystallized when reach starting composition Fstarting composition F
LIQUID PATHLIQUID PATH SOLID PATHSOLID PATH
THREE COMPONENT SYSTEM WITH A PERITECTIC
PLAGIOCLASE-OLIVINE-(ORTHOPYROXENE)-QUARTZ (AN-FO-(EN-)SIO2)
All Ol is consumed
g-d leg only possible with fractional crystallization
final rock is En + An under equilibrium crystallization
THREE COMPONENT SYSTEM WITH SOLID SOLUTION
PLAGIOCLASE-PYROXENE (AN-AB-DI)
Liquid – An content Tie-lines
Winter (2001) Figure 7-5. Isobaric diagram illustrating the liquidus temperatures in the system diopside-anorthite-albite at atmospheric pressure (0.1 MPa). After Morse (1994), Basalts and Phase Diagrams. Krieger Publishers.
THREE COMPONENT SYSTEM WITH SOLID SOLUTION
PLAGIOCLASE-PYROXENE (AN-AB-DI)
Last Liquid (EC)
Final Plag (EC) First Plag
Starting Liquid Composition
THREE COMPONENT SYSTEM WITH SOLID SOLUTION
PLAGIOCLASE-PYROXENE (AN-AB-DI)
First Plag
Starting Liquid Composition
Liquid composition arcs due to decreasing An content of plagioclase
Last Liquid (EC)
Final Plag (EC)
FOUR COMPONENT SYSTEMSFO-DI-AN-AB
y
An
Winter (2001) Figure 7-12. The system diopside-anorthite-albite-forsterite. After Yoder and Tilley (1962). J. Petrol.
Becoming difficult to visualizeTime to revert to multi-dimensional mathematical models
BACK TO THERMODYNAMICSThink about Gibbs Free Energy again
dG = VdP – SdT
at a given P (0.1 MPa) and T (1300ºC)
Low Volume, trumps Entropy Minerals more stable
High Entropy trumps volume Liquid more stable
Liq