Chapter 24. Stable Mineral Assemblages in Metamorphic Rocks.

Chapter 24. Stable Chapter 24. Stable Mineral Assemblages in Mineral Assemblages in

Metamorphic RocksMetamorphic Rocks

Equilibrium Mineral AssemblagesEquilibrium Mineral Assemblages At equilibrium, the mineralogy (and the At equilibrium, the mineralogy (and the

composition of each mineral) is determined composition of each mineral) is determined by T, P, and X . by T, P, and X . “Mineral paragenesis”“Mineral paragenesis”

The Phase Rule in Metamorphic The Phase Rule in Metamorphic SystemsSystems

at equilibrium:F = C - + 2 the phase rule

(6-1) = #phases at a point

C = # components: the minimumminimum number of chemical constituents required to specify every phase in the system. Many poss. choices

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.)

Goldschmidt’s Goldschmidt’s RuleRule

If F If F 2 (phase field) 2 (phase field)thenthenF = C - F = C - + 2 + 2 2 2SoSo C C (24-1)(24-1)

http://serc.carleton.edu/research_education/equilibria/phaserule.html

Useful in evaluating whether or not a rock is at equilibrium

Picking the ComponentsPicking the Components C is the C is the minimumminimum number of chemical components required number of chemical components required

to constitute all the phases. Consider the reaction: to constitute all the phases. Consider the reaction:

CaMg(COCaMg(CO33))22 + 2 SiO + 2 SiO22 = CaMgSi = CaMgSi22OO66 + 2 CO + 2 CO22 Dolomite + 2 Quartz = Diopside + 2 Carbon Dioxide Dolomite + 2 Quartz = Diopside + 2 Carbon Dioxide

Normally we would pick the 4 components: CaO, MgO, SiONormally we would pick the 4 components: CaO, MgO, SiO22, , and COand CO22 based on the simple oxides. However, because Ca based on the simple oxides. However, because Ca and Mg are in a 1:1 ratio in both dolomite and diopside (and and Mg are in a 1:1 ratio in both dolomite and diopside (and not present in quartz or carbon dioxide), we could choose a not present in quartz or carbon dioxide), we could choose a ternary system with components: CaMgOternary system with components: CaMgO22, SiO, SiO22, and CO, and CO22

Easier to plotEasier to plot


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

Case 1 Case 1 = C = C

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


b)b) < C < C– Common with mineral systems that Common with mineral systems that

exhibit solid solutionexhibit solid solution

PlagioclasePlagioclase

Liquid

LiquidLiquid

plus

Plagioclase

•For example, in the plagioclase or the olivine system we can see that under metamorphic conditions (below the solidus) these 2-C systems consist of a single mineral phase• = 1 and C = 2 so < C

Consider the following three scenarios:Consider the following three scenarios:Subcase a: C = Subcase a: C =

11 = 1 is = 1 is

commoncommon = 2 Isograd = 2 Isograd

less commonless common = 3 only at the = 3 only at the

specific P-T specific P-T conditions of the conditions of the invariant pointinvariant point

(~ 0.37 GPa (~ 0.37 GPa and 500and 500ooC)C)

Figure 21-9. The P-T phase diagram for the system Al2SiO5

calculated using the program TWQ (Berman, 1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Case 3: > C

Case 3 Case 3 > C > CSubcase b: Subcase b: Equilibrium has not been attained Equilibrium has not been attained

The phase rule applies only to systems at The phase rule applies only to systems at equilibrium, and there could be any equilibrium, and there could be any number of minerals coexisting if number of minerals coexisting if equilibrium is not attainedequilibrium is not attained

http://serc.carleton.edu/research_education/equilibria/PTtPaths.html

Relict inclusions in garnet

Case 3 Case 3 > C > CSubcase 3: We didn’t choose the # of components correctlySubcase 3: We didn’t choose the # of components correctly

Some guidelines for an appropriate choice Some guidelines for an appropriate choice of Cof C– Begin with a 1-component system such as Begin with a 1-component system such as

CaAlCaAl22SiSi22OO88 (anorthite), there are 3 common (anorthite), there are 3 common types of major/minor components that we can types of major/minor components that we can add:add:

Case a) Components that generate a Case a) Components that generate a new phasenew phase

Adding a component such as CaMgSiAdding a component such as CaMgSi22OO66 (diopside), results in an additional phase: in(diopside), results in an additional phase: in

the binary Di-An system diopside coexists the binary Di-An system diopside coexists with anorthite below the soliduswith anorthite below the solidus

Case 3 Case 3 > C > C

– Begin with a 1-component system, such as CaAlBegin with a 1-component system, such as CaAl22SiSi22OO88 (anorthite), (anorthite), there are 3 common types of major/minor components that we can there are 3 common types of major/minor components that we can add:add:

Case b) Components that substitute for other componentsCase b) Components that substitute for other components

Adding a component such as NaAlSiAdding a component such as NaAlSi33OO88 (albite) (albite) to the CaAlto the CaAl22SiSi22OO88 anorthite system. Albite would anorthite system. Albite would dissolve in the anorthite structure, resulting in a dissolve in the anorthite structure, resulting in a single solid-solution mineral (plagioclase) below single solid-solution mineral (plagioclase) below the solidusthe solidus

Fe and Mn commonly substitute for MgFe and Mn commonly substitute for Mg Al may substitute for SiAl may substitute for Si Na may substitute for KNa may substitute for K

Case 3 Case 3 > C > CBegin with a 1-component system, such as CaAlBegin with a 1-component system, such as CaAl22SiSi22OO88 (anorthite), there are 3 common (anorthite), there are 3 common

types of major/minor components that we can add:types of major/minor components that we can add:

c) “Perfectly mobile” components, e.g. abundant Hc) “Perfectly mobile” components, e.g. abundant H22O with excellent O with excellent permeabilitypermeability

These are components are either a freely These are components are either a freely mobile fluid or a component that dissolves mobile fluid or a component that dissolves readily in a fluid phase and can be transported readily in a fluid phase and can be transported easily (e.g. LILs)easily (e.g. LILs)

Perfectly Mobile ComponentsPerfectly Mobile ComponentsConsider the very simple metamorphic system, Consider the very simple metamorphic system,

MgO-HMgO-H22OO– Possible natural phases in this system are Possible natural phases in this system are

periclase (MgO), aqueous fluid (Hpericlase (MgO), aqueous fluid (H22O), and O), and brucite (Mg(OH)brucite (Mg(OH)22))

> C> C– How we deal with HHow we deal with H22O depends upon O depends upon

whether water is perfectly mobile or notwhether water is perfectly mobile or not– A reaction can occur between the potential A reaction can occur between the potential

phases in this system: phases in this system:

MgO + HMgO + H22O O Mg(OH) Mg(OH)22 Per + Fluid = Per + Fluid = BruBru

BTW this is a retrograde reaction as written BTW this is a retrograde reaction as written (occurs as the rock cools and hydrates)(occurs as the rock cools and hydrates)

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

•The reaction will proceed until either periclase or water is consumed

•Periclase can react only with the quantity of water that can diffuse into the system

•If water is not perfectly mobile, and is limited in quantity, it will be consumed first

Once the water is gone, the excess periclase remains stable as conditions change into the brucite stability field

•The reaction line is not the absolute stability boundary of periclase, or of water, because either can be stable across the boundary if the other reactant is absent

If water is not abundant or perfectly mobile it may become a limiting factor in the reaction

•A reaction involving more than one reactant will proceed until any one of the reactants is consumedMgO + HMgO + H22O O Mg(OH) Mg(OH)22 Per + water = BruPer + water = Bru

Chemographic DiagramsChemographic DiagramsChemographics refers to the graphical representation Chemographics refers to the graphical representation

of the chemistry of mineral assemblagesof the chemistry of mineral assemblages

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

Metamorphic components typically expressed as molar quantities, and not as weight %

Any intermediate composition is simply plotted an appropriate distance along the line

An50 would plot in the center, An25 would plot ¼ of the way from Ab to An, etc.

Chemographic DiagramsChemographic DiagramsSuppose you had a small area of a metamorphic terrane Suppose you had a small area of a metamorphic terrane

in which the rocks correspond to a hypothetical 3-in which the rocks correspond to a hypothetical 3-component system with variable proportions of the component system with variable proportions of the components x-y-zcomponents x-y-z

as shown in Fig. 24-2as shown in Fig. 24-2The rocks in the area are found to contain 6 minerals with the fixed compositions x, y, z, xz, xyz, and yz2Example: components x,y and

z are MgO, FeO, and SiO2 respectively

Actual choices more complex

Suppose that the rocks in Suppose that the rocks in our area are found to our area are found to have the following 5 have the following 5 assemblages:assemblages:

– x-xy-xx-xy-x22zz

– xy-xyz-xxy-xyz-x22zz

– xy-xyz-yxy-xyz-y

– xyz-z-xxyz-z-x22zz

– y-z-xyzy-z-xyzFigure 24-2 Figure 24-2 Phases that coexist at equilibrium in a rock are connected by tie-lines

Compatibility Diagram

MgO, FeO, and SiO2

x2z = Mg2SiO4 Forsterite

Note that this subdivides the chemographic Note that this subdivides the chemographic diagram into 5 sub-triangles, labeled (A)-(E)diagram into 5 sub-triangles, labeled (A)-(E)

– A:x-xy-xA:x-xy-x22z z

– B:xy-xyz-xB:xy-xyz-x22zz

– C:xy-xyz-yC:xy-xyz-y

– D:xyz-z-xD:xyz-z-x22zz

– E:y-z-xyzE:y-z-xyzAny point within the diagram represents a specific bulk rock composition

The diagram determines the corresponding mineral assemblage that develops at equilibrium

For example, a point within the sub-triangle (E), the corresponding mineral assemblage corresponds to the corners = y - z - xyz

Any rock with a bulk composition plotting within triangle (E) will develop that same mineral assemblage

What happens if you find a place that falls What happens if you find a place that falls directly on a tie-line, such as point (f)?directly on a tie-line, such as point (f)?

In this case the mineral assemblage consists of xyz and z only (the ends of the tie-line), since by adding these two phases together in the proper proportion you can produce the bulk composition (f)

In such a situation = C = 2, as required at equilibrium

In the unlikely event that the In the unlikely event that the bulk composition equals that bulk composition equals that of a single mineral, such as of a single mineral, such as xyzxyz, then , then = 1 = 1, but , but C = 1C = 1 as as wellwell

““compositionally compositionally degenerate” i.e degenerate” i.e “requiring fewer “requiring fewer components components than normalthan normal

Chemographic Diagrams for Chemographic Diagrams for Metamorphic RocksMetamorphic Rocks

Most common natural rocks contain the Most common natural rocks contain the major elements: SiOmajor elements: SiO22, Al, Al22OO33, K, K22O, CaO, O, CaO, NaNa22O, FeO, MgO, MnO and HO, FeO, MgO, MnO and H22O such that O such that C = 9C = 9

Three components is the maximum Three components is the maximum number that we can easily deal with in number that we can easily deal with in two dimensionstwo dimensions

What is the “right” choice of What is the “right” choice of components? components?

Some simplifying choices:Some simplifying choices:

The ACF DiagramThe ACF Diagram

• Eskola (1915) proposed the ACF diagram as a way to illustrate metamorphic mineral

assemblages in mafic rocks on a simplified 3-C triangular diagram• He wanted to concentrate only on the minerals that appeared or disappeared during metamorphism, thus acting as indicators of metamorphic grade

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

Fig. 24-4 illustrates the positions of several common metamorphic minerals on the ACF diagram. Note: this diagram is presented only to show you where a number of important phases plot.It is not specific to a P-T range and therefore is not a true compatibility diagram, and has no petrological significance

The ACF DiagramThe ACF Diagram The three pseudo-components are The three pseudo-components are

all calculated on an atomic basis: all calculated on an atomic basis: A = AlA = Al22OO33 + Fe + Fe22OO33 - Na - Na22O - KO - K22OO

C = CaO - 3.3 PC = CaO - 3.3 P22OO55

F = FeO + MgO + MnOF = FeO + MgO + MnO

For A we are here interested only in For A we are here interested only in the Al that occurs in excess of that the Al that occurs in excess of that combined with K and Na to make combined with K and Na to make any feldsparany feldspar


Water is omitted under the assumption that it is Water is omitted under the assumption that it is perfectly mobileperfectly mobile

Note that SiONote that SiO22 is simply ignored is simply ignored– We shall see that this is equivalent to projecting from quartzWe shall see that this is equivalent to projecting from quartz

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

WHERE DOES A MINERAL PLOT?WHERE DOES A MINERAL PLOT? HOW DO SOLID SOLUTIONS OR MIXTURES PLOT?HOW DO SOLID SOLUTIONS OR MIXTURES PLOT?

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


Anorthite CaAlAnorthite CaAl22SiSi22OO88

AA = 1 + 0 - 0 - 0 = = 1 + 0 - 0 - 0 = 11, , CC = 1 - 0 = = 1 - 0 = 11, and , and FF = = 00 Provisional values sum to 2, so we can Provisional values sum to 2, so we can

normalize to 1.0 by multiplying each value by normalize to 1.0 by multiplying each value by ½, resulting in ½, resulting in

A = 0.5A = 0.5

C = 0.5C = 0.5

F = 0F = 0

An example:An example:

Anorthite thus plots half way between A and C on the side of the ACF triangle, as shown in Fig. 24-4

A = AlA = Al22OO33 + Fe + Fe22OO33 - Na - Na22O - KO - K22OO

C = CaO - 3.3 PC = CaO - 3.3 P22OO55


X

Figure 24-4.

Example 1: Where does K-feldspar plot?

•For KAlSi3O8 A = 0.5 + 0 - 0.5 = 0, C = 0, and F = 0•Kspar doesn’t plot on the ACF diagram

•Example 2: If you try this for albite NaAlSi3O8 you will find that it doesn’t plot either A = AlA = Al22OO33 + Fe + Fe22OO33 - Na - Na22O - O -

KK22OO

C = CaO - 3.3 PC = CaO - 3.3 P22OO55


For albite NaAlSi3O8

A = 0.5 + 0 - 0.5 -0 = 0

C = 0 + 0

F = 0 + 0 + 0

A typical ACF compatibility diagram, referring to a specific A typical ACF compatibility diagram, referring to a specific range of P and T (the kyanite zone in the Scottish Highlands)range of P and T (the kyanite zone in the Scottish Highlands)

Figure 24-5. After Turner (1981). Metamorphic Petrology. McGraw Hill.

•Plot all phases and connect coexisting ones with tie-lines

• Field R: Mafic rocks Mafic rocks in the hb-plag –gt (almandine) triangle, and thus most metabasaltic rocks occur as amphibolites or garnet amphibolites in this zone

•Field S: More aluminous rocks develop kyanite and/or muscovite and not hornblende

• Field T: More calcic rocks lose Ca-free garnet, and contain diopside, grossularite, or even calcite (if CO2 is present)

•The diagram allows us to interpret the relationship between the chemical composition of a rock and the equilibrium mineral assemblage

T

S

R

NEW for Pelitics:The AKF DiagramNEW for Pelitics:The AKF Diagram

In the AKF diagram, the pseudo-In the AKF diagram, the pseudo-components are:components are:

A = AlA = Al22OO33 + Fe + Fe22OO33 - Na - Na22O - KO - K22O - O - CaOCaOK = KK = K22OO

F = FeO + MgO + MnOF = FeO + MgO + MnOWe are now interested only in the Al that occurs in excess of

that combined with K, Na, and Ca to make any feldspar

Because pelitic sediments are high in AlBecause pelitic sediments are high in Al22OO33 and K and K22O, O,

and low in CaO, Eskola proposed a different diagram and low in CaO, Eskola proposed a different diagram that included Kthat included K22O to depict the mineral assemblages O to depict the mineral assemblages

that develop in themthat develop in them

Figure 24-6. After Ehlers and Blatt (1982). Petrology. Freeman.

Another figurative AKF to show where metapelitic minerals plot

AKF compatibility diagram (Eskola, 1915) AKF compatibility diagram (Eskola, 1915) illustrating paragenesis of pelitic hornfelses, illustrating paragenesis of pelitic hornfelses, Orijärvi region FinlandOrijärvi region FinlandFigure 24-7. After Eskola (1915) and Turner (1981) Metamorphic

Petrology. McGraw Hill.

• Region R:•Al-poor rocks contain biotite K(Mg,Fe)2(AlSi3O10)(F,OH)2, and may contain an amphibole (e.g. Anthophyllite (Mg, Fe)7Si8O22(OH)2 )

if sufficiently rich in Mg and Fe, or microcline KAlSi3O8 if not.

• Region S: Rocks richer in Al contain andalusite Al2SiO5 ,cordierite (Mg,Fe)2Al4Si5O18

and Muscovite KAl2(AlSi3O10)(OH)2

S

R

Three of the most common minerals in metapelites: Three of the most common minerals in metapelites: andalusite, muscovite, and microcline, all plot as andalusite, muscovite, and microcline, all plot as distinct points in the AKF diagramdistinct points in the AKF diagram

Figure 24-7. After Ehlers and Blatt (1982). Petrology. Freeman. And & Ms plot as And & Ms plot as

the same point in the same point in the ACF diagram, the ACF diagram, and Micr doesn’t and Micr doesn’t plot at all, so the plot at all, so the ACF diagram is ACF diagram is much less useful much less useful for pelitic rocks for pelitic rocks (rich in K and Al)(rich in K and Al)

Projection from Apical PhasesProjection from Apical Phases ACF and AKF diagrams eliminate SiOACF and AKF diagrams eliminate SiO22 by by

projecting from quartz, so quartz MUST be projecting from quartz, so quartz MUST be presentpresent

The shortcoming is that these projections The shortcoming is that these projections compress the true relationships as a compress the true relationships as a dimension is lostdimension is lost

Let’s examine projections on another diagram, Let’s examine projections on another diagram, the A(K)FM of Thompson.the A(K)FM of Thompson.

J.B. Thompson’s A(K)FM DiagramJ.B. Thompson’s A(K)FM Diagram

An alternative to the AKF diagram for An alternative to the AKF diagram for metamorphosed pelitic rocksmetamorphosed pelitic rocks Although the AKF is useful in this capacity, Although the AKF is useful in this capacity,

J.B. Thompson (1957) noted that J.B. Thompson (1957) noted that Fe and Mg Fe and Mg do not partition themselves equally do not partition themselves equally between the various mafic minerals between the various mafic minerals in in most rocksmost rocks

Biotite is useful in assessing temperature Biotite is useful in assessing temperature histories of metamorphic rocks, because histories of metamorphic rocks, because the partitioning of iron and magnesium the partitioning of iron and magnesium between biotite and garnet is sensitive to between biotite and garnet is sensitive to temperaturetemperature

J.B. Thompson’s A(K)FM DiagramJ.B. Thompson’s A(K)FM DiagramA = Al2O3

K = K2OF = FeOM = MgOIn order to avoid dealing with a three-dimensional tetrahedron, Thompson projected the phases in the system to the AFM face, thereby eliminating K2O

He recognized that projecting from K2O would not work, since no phase corresponds to this apical point, and projections would cross important tie-lines

J.B. Thompson’s J.B. Thompson’s A(K)FM A(K)FM

DiagramDiagram

Since muscovite is the most widespread K-rich phase in metapelites, he decided to project from muscovite (Mu) to the AFM base as shown

Biotite is outside this volume, and projecting it from Mu causes it to plot as a band (of variable Fe/Mg) outside the AFM triangle

J.B. Thompson’s A(K)FM DiagramJ.B. Thompson’s A(K)FM Diagram At high grades muscovite At high grades muscovite

dehydrates to K-feldspar dehydrates to K-feldspar as the common high-K as the common high-K phasephase

Then the AFM diagram Then the AFM diagram should be projected from should be projected from K-feldspar, because K-feldspar, because Muscovite IS NOT Muscovite IS NOT PRESENTPRESENT

When projected from Kfs, When projected from Kfs, biotite projects biotite projects withinwithin the the F-M base of the AFM F-M base of the AFM triangletriangle

Figure 24-18. AKFM Projection from Kfs. After Thompson (1957). Am. Min. 22, 842-858.

Chapter 24. Stable Mineral Assemblages in Metamorphic Rocks.

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Transcript of Chapter 24. Stable Mineral Assemblages in Metamorphic Rocks.