GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY

Lecture 23

Stable Mineral Assemblages in Metamorphic Rocks

March 30, 2009

EQUILIBRIUM MINERAL ASSEMBLAGES

Evidence of Chemical Equilibrium

- Lack of disequilibrium textures (replacement textures, corona, compositional zoning, ....)

- Each mineral type shares contacts every other mineral phase in the rock

- Layers are rare or are homogeneous within the layers

- Rocks are in textural equilibrium

- Rocks conform to Gibbs Phase Rule

- Minerals lack chemical zoning

PHASE RULE IN METAMORPHIC ROCKS

Phase rule, as applied to systems at equilibrium:

F = C - + 2 = the number of phases in the system

C = the number of components: the minimum number of chemical constituents required to specify every phase in the system

F = the number of degrees of freedom: the number of independently variable intensive parameters of state (such as temperature, pressure, the composition of each phase, etc.)

In natural systems, there are multiple compositional variables in addition to independent changes in P & T. If F 2 is the most common situation, then the phase rule may be adjusted accordingly:

F = C - + 2 2, or C Goldschmidt’s mineralogical phase

rule, or simply the mineralogical phase rule

PHASE RULE IN METAMORPHIC ROCKS

2) < C

Common with minerals that exhibit solid solution (e.g., Plagioclase - single mineral, but two components)

Suppose we have determined C for a rockConsider the following three scenarios:

1) = C

The standard divariant situation of the Phase Rule

The rock probably represents an equilibrium mineral assemblage from within a metamorphic zone

PHASE RULE IN METAMORPHIC ROCKS

3) > CA more interesting situation and at least one of three

situations must be responsible:A)  F < 2

The sample is collected from a location right on a univariant reaction curve (isograd) or invariant point

B)  Equilibrium has not been attained

C) The number of components were not properly chosen

PHASE RULE IN METAMORPHIC ROCKS

Choosing Components to define Metamorphic Systems

As with igneous rocks, it is not reasonable to choose every chemical constituent of a rock as a component.

Stick to: - Essential Components that generate a new phase with a limited P&T range (garnet – yes; plagioclase – no) - Three Components (or component combinations) that can be graphically portrayed in 2DAvoid: - Components that are major constituents of single phases (e.g., P2O5 – apatite, TiO2 – ilmenite)- Components that substitute for other components (e.g. Ab-An, Fa-Fo, Mn for Fe, Al for Si, Na for K)- “Perfectly mobile” components (H2O, CO2, ...)

Winter (2001)Figure 24-1. P-T diagram for the reaction brucite = periclase + water. From Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

PHASE RULE IN METAMORPHIC ROCKS

“Perfectly Mobile” H2O or Not

Implies that fluid may come and go based on external conditions not controlled by mineral reactions. It is typically not considered a component. It is added as needed and leaves when in excess.

Prograde reaction will go regardless if H2O fluid is present

Retrograde reaction requires H2O to be present to go, but it is not a component of this system (one comp – MgO)

MgO

Mg(OH)2

Prograde

Retrograde

PHASE RULE IN METAMORPHIC ROCKS

How do you know if you have chosen the proper components? The rocks should tell you

The phase rule is an interpretive tool, not a predictive tool, and does not tell the rocks how to behave

If you only see low- assemblages (e.g. Per or Bru in the MgO-H2O system), then some components may be mobile

If assemblages have many phases in an area it is unlikely that so much of the area is right on a univariant curve, and may require the number of components to include otherwise mobile phases, such as H2O or CO2, in order to apply the phase rule correctly

CHEMOGRAPHIC DIAGRAMSChemographics refers to the graphical representation of

the chemistry of mineral assemblages

A simple example: the plagioclase system as a linear C = 2 plot:

3-C mineral compositions are plotted on a triangular chemographic diagram

x, y, z, xz, xyz, and yz2

CHEMOGRAPHIC DIAGRAMS

Divariant EquilibriumDivariant Equilibrium

Mineral AssemblagesMineral Assemblages

(A) x-xy-x(A) x-xy-x22zz

(B) xy-xyz-x(B) xy-xyz-x22zz

(C) xy-xyz-y(C) xy-xyz-y

(D) xyz-z-x(D) xyz-z-x22zz

(E) y-z-xyz(E) y-z-xyz

Compatibility Diagrams determines the equilibrium mineral assemblage that should develop for a particular whole rock composition defined by three components

CHEMOGRAPHIC DIAGRAMS

Valid compatibility diagram must be referenced to a specific range of P-T conditions, such as a zone in some metamorphic terrane, because the stability of the minerals and their groupings vary as P and T vary

Previous diagram refers to a P-T range in which the fictitious minerals x, y, z, xy, xyz, and x2z are all stable and occur in the groups shown

At different grades the diagrams change Other minerals become stable Different arrangements of the same minerals (different

tie-lines connect different coexisting phases)

CHEMOGRAPHIC DIAGRAMSW/ SOLID SOLUTION

Phases with SS between Y and Z

Phases with SS between Y, X. and Z

CHEMOGRAPHIC DIAGRAMSW/ SOLID SOLUTION

Tie lines link coexisting compositions

CHEMOGRAPHIC DIAGRAMSACF DIAGRAM

Figure 24-4. After Ehlers and Blatt (1982). Petrology. Freeman. And Miyashiro (1994) Metamorphic Petrology. Oxford.

Best Suited to Mafic Igneous Rocks and Sedimentary Rocks (Graywackes)

The three pseudo-components are all calculated on an atomic basis:

A = Al2O3 + Fe2O3 - Na2O - K2O

C = CaO - 3.3 P2O5

F = FeO + MgO + MnO

CHEMOGRAPHIC DIAGRAMSACF DIAGRAM

Water is omitted under the assumption that it is perfectly mobile

Note that SiO2 is simply ignored. We shall see that this is equivalent to projecting from quartz

In order for a projected phase diagram to be truly valid, the phase from which it is projected must be present in the mineral assemblages represented

e.g. Alkali Feldspar

By creating these three pseudo-components, Eskola reduced the number of components in mafic rocks from 8 to 3

CHEMOGRAPHIC DIAGRAMSAKF DIAGRAM

Best Suited to Pelitic (clay-rich) Sedimentary Rocks

A = Al2O3 + Fe2O3 - Na2O - K2O - CaO

K = K2O

F = FeO + MgO + MnO

Projected From Quartz & Plagioclase

CHEMOGRAPHIC DIAGRAMSAPICAL PHASE PROJECTIONS

MgO SiO2Fo En Di' QPer

Only valid if phase projecting from is present with all phases

Mathematically, the same as ignoring CaO in the Di formula and normalizing MgO and

SiO2 to 100%

CHEMOGRAPHIC DIAGRAMSAPICAL PHASE PROJECTIONS

x = ABCQy = A2B2CQ

Possible Phase Possible Phase assemblagesassemblages

(q)-b-x-c(q)-b-x-c

(q)-a-x-y(q)-a-x-y

(q)-b-x-y(q)-b-x-y

(q)-a-b-y(q)-a-b-y

(q)-a-x-c(q)-a-x-c

Fig. 24-13

Mathematically the same as ignoring SiO2 in the X & Y formulas and normalizing A, B,

and C to 100%