g eol 5310 a dvanced i gneous and m etamorphic p etrology phase diagrams october 26, 2009

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


EQUILIBRIUM CRYSTALLIZATION

a – Starting bulk composition of melt = An60

b – Beginning of crystallizationT= 1475oC

c – Composition of first plagioclase to crystallize

= An87


EQUILIBRIUM CRYSTALLIZATIONa – Starting bulk composition of melt = An60



at 1475oC = An87

d – Melt composition at 1450oC= An48

e – Bulk composition of Magma (Melt + Crystals =

An60)

f – Composition of Plagioclase at 1450oC = An81


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



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


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)


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


OLIVINESonju Lake Intrusion

FayaliteFe2SiO4

FosteriteFe2SiO4

TWO-COMPONENT SYSTEM WITH A EUTECTIC

PYROXENE - PLAGIOCLASE

EutecticPoint



EutecticPoint

a – bulk starting composition = An70



EutecticPoint

a – bulk starting composition = An70b – crystallization begins at 1450oCc - pure plagioclase (An) crystallizes



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


PYROXENE – PLAGIOCLASEEVOLUTION OF LIQUID AND SOLID

DURING CRYSTALLIZATION

EutecticPoint

Equilibrium vs. Fractional


PYROXENE – PLAGIOCLASE EQUILIBRIUM MELTING


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.



ii

FoFo EnEn

15571557

Bulk X



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



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



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.



LIQUID LIQUID IMMISCIBILIIMMISCIBILITYTY

TWO-COMPONENT SYSTEM WITH A SOLVUS


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.



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



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



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



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.



Last Liquid (EC)

Final Plag (EC) First Plag

Starting Liquid Composition



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

g eol 5310 a dvanced i gneous and m etamorphic p etrology phase diagrams october 26, 2009

Documents

plagioclase system

system volume

number of minerals

t range

number of variables

distinct phases components

pressuresseveral minerals

t decreasesthe minerals