GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY

Lecture 6

Phase Diagrams for

One- and Two-Component Systems

February 4, 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