Metamorphic Petrology

Francis, 2014

4.3 Ga “Faux Amphibolite”

Reactions

The processes of progressive metamorphism are dominated by de-watering and de-carbonation reactions, with the final production of anhydrous mineral assemblages at the highest metamorphic grades.

Mixed Volatiles and Buffering

800

Temp o C Periclase

700 600 Per + H O 2 Bru

Brucite

B

A

ru + C O 2 M ag + H 2 O 500

MBP

Per + CO 2 Mag

Magnesite

400 0.0

0.1

XCO in fluid 2

0.2

Thermobarometry

garnet – biotite thermometer garnet – plagioclase barometer

Eastern Acadian Terrane

- Low pressure – high temperature ”

Buchan

” style metamorphism with the highest grade zones being cored by granitic intrusions.

Counter clockwise P-T path is interpreted to reflect crustal extension in the pre Acadian continental margin.

Western

“

Barrovian”

Acadian Terrane

Regional style metamorphism with clockwise P-T metamorphic paths interpreted to be the late over thrusting of the Eastern Acadian terrane during the main Acadian deformation.

Acadian Orogeny

Solid - Solid Reactions

Single Component Systems:

SiO

2 When a solid consist of 2 coexisting minerals (phases):

F = C – P + 2 = 1 - 2 + 2 = 1

Such a system is invariant at any given pressure, and thus a single component solid phase will melt at 1 unique temperature at any specified pressure.

The boundary between the 2 phases in P - T space will be a univariant line with a slope approximated by:

d

 G =  S

d

T +  V

d

P = 0

d

P/

d

T =  S/  V This is also true for solid - liquid phase boundaries because, to a first approximation,  H o and  S o are constant for small changes in temperature (true for all reactions not involving compressible vapour phase).

a relatively

1) Solid - Solid Reactions

andalusite

Al 2 Si0 5

sillimanite

Al 2 Si0 5 From the phase rule, we know that this is reaction is univariant and thus can be represented by a line in P-T space:

F = C - P + 2 = 1

At equilibrium:  G (P,T) =  H o (1bar,T) - T  S o (1bar,T) + (P-1)  V = 0 d  G (P,T) = 0 =  S o dT +  VdP  S,  V, and  H vary little with T and P for solid - solid reactions because the changes in the reactants with T and P tend to parallel those in the products. Thus the above equation approximates that of a straight line in P - T space with a slope of:

dP/dT =



S/



V = - 3.3 bar / K

 V = - 1.44 Joules / bar mol  S = 4.72 Joules / mol K

1) Solid - Solid Reactions

Similarly:

albite

NaAlSi 3 O 8

jadeite + quartz

NaAlSi 2 O 6 + SiO 2

dP/dT =



S/



V = 25.02 bar / oK

 V = - 1.734 Joules / bar mol  S = - 43.39 Joules / mol K Most solid - solid reactions have positive slopes in P - T space because the higher temperature side of the reaction typically has both higher entropy and volume.

F = C – P + 2 F = 2 – 3 + 2 F = 1 univariant

Jd Ab Qtz Jd Qtz Jd Ab Qtz

2) Simple Dehydration and Other Devolatilization Reactions

Water analcite + qtz

NaAlSi 2 O 6 H 2 O + SiO 2

pyrophyllite

Al 2 Si 4 O 10 (OH) 4

paragonite + qtz

NaAl 2 (Al,Si 3 )O 10 (OH) 2

muscovite + qtz

KAl 2 (Al,Si 3 )O 10 (OH) 2

albite

NaAlSi 3 O 8

+ H

2

O

+ H 2 O

kyanite + 3 qtz

Al 2 Si0 5 + 3SiO 2

+ H

2

O

+ 2H 2 O

Al-silicate + albite

Al 2 Si0 5 + NaAlSi 3 O 8

+ H 2 O

+ H

2

O

sillimanite + K-felds + H 2 O

Al 2 Si0 5 + KAlSi 3 O 8 + H 2 O

Dehydration Reactions - the general case :

H hydrous A anhydrous

 G (P,T) =  G o (P,T) + RTln (

a

H 2 O)(

a

A) = 0 (

a

H)

+ water

If the solids have constant compositions and water behaves as an ideal gas, then:  G (P,T) =  G o (P,T) + 0  G o (P,T) =  H o (1bar,T) - T  S o (1bar,T) + (P-1)  V = 0 At relatively low pressures, both  V and  S are positive, and the reaction has a positive slope:

(dP/dT =



S/



V)

With increasing pressure, however, H 2 O compresses, the  V of the reaction decreases and the slope of the reaction increases and may even become negative because typically V A < V H .

Dehydration Reactions - the general case :

H hydrous A anhydrous + water



G (P,T) =



G

o

(P,T) + RTln (

a

H

2

O)(

a

A) = 0 (

a

H)

If H and A are pure phases, but the fluid phase is diluted by another component such as CO 2 , then the maximum thermal stability of H is reduced by an amount given by:  G (P,T) =  G o (P,T) + RTln (

a

H 2 O) = 0 

G

o

(P,T) = - RTln (

a

H

2

O)

Assuming water behaves as an ideal gas: 

G o (P,T) = - RTln (XH 2 O)

3 ) CO 2 - Decarbonation reactions

dolomite + qtz

CaMg(CO 3 ) 2 + SiO 2

calcite + qtz

CaCO 3 + SiO 2 Similar to the general case for dehydration reactions: C carbonate D decarbonated + carbon dioxide 

G (P,T) =



G o (P,T) + RTln(aCO 2 )(aD) = 0 (aC)

Again, if C and D are pure phases, and CO 2 is an ideal gas: 

G o (P,T) = - RTln(XCO 2 ) diopside + CO

2 CaMgSi 2 O 6 + CO 2

wollastonite + CO

2 CaSiO 3 + CO 2

Mixed Volatile Reactions Reduced water pressure (PH 2 O < Ptotal), either because a system is open to water or because other components are present in the fluid phase reducing the activity of water, will shift simple dehydration reactions to lower pressures.

If, however, there are reactions involving the additional components that are diluting water in the fluid phase, more dramatic effects occur. This is particularly true for CO 2 , whose presence in the fluid phase may stabilize carbonates at the expense of Ca and Mg bearing silicate.

5CaMg(CO 3 ) 2 + 8SiO 2

dolomite qtz

+ H 2 O Ca 2 Mg 5 Si 8 O 22 (OH) 2 + 3CaCO 3

tremolite calcite

+ 7CO 2 Greenwood’s Classification of Mixed Volatile Reactions:

bB + …… dD + ….. + mH 2 0 + nCO 2

1) m = n = 0 2) m > 0, n = 0 3) m = 0, n > 0 4) m = n > 0; 4a) m = 1, n = 3 5) m > 0, n < 0 6) m < 0, n > 0

4) Net Transfer Reactions: Reactions that cause a change in the number of moles of minerals are termed net-transfer reactions. Net transfer reactions with large Δ V make the best geobarometers. Net-transfer reactions are also the most useful for fieldwork, as they typically mark the appearance or disappearance of a phase that can be mapped in the field as a metamorphic

“isograd”.

Tie-line switching (2D) or piercing plane (nD)

reactions

Terminal

reactions at which phases appear or disappear B + C = A appearance A = D + E disappearance

Three Component Systems:

Tie-line switching (2D) or piercing plane (nD)

reactions

F = 3 - P + 2

F = 2, if P =3 F = 1, if P = 4, univariant Temperature

Terminal

reactions at which phases appear or disappear B + C = A appearance D = A + B + C disappearance

F = 3 - P + 2

F = 2, if P =3 F = 1, if P = 4, univariant Temperature

5)

Exchange Reactions: Mg 2 SiO 4 + 2FeSiO 3

forsterite ferrosillite

Fe 2 SiO 4 + 2MgSiO 3

fayalite enstatite

 G o (P,T) = -RTlnK = -RTln(

a

Fa)(

a

En) 2 (

a

Fo)(

a

Fs) 2 Mg 3 Al 2 Si 3 O 12 + KFe 3 AlSi 3 O 10 (OH) 2

pyrope annite

Fe 3 Al 2 Si 3 O 12 + KMg 3 AlSi 3 O

almandine phlogopite

10 (OH) 2 2Fe 7 Al 4 Si 4 O 15 (OH) 12 + 7Mg 2 Al 9 Si 4 O 23 (OH) 2Mg 7 Al 4 Si 4 O 15 (OH) 12 + 7Fe 2 Al 9 Si 4 O 23 (OH)

Fe-chlorite Mg staurolite Mg chlorite Fe staurolite

Exchange reactions typically have small volume changes and tend to be relatively insensitive to pressure.

They commonly make the best geothermometers for calculating the last temperature assemblage.

of equilibrium of the mineral

6) Continuous Reactions: (Mg,Fe) 7 Al 4 Si 4 O 15 (OH) 12 + KAl 2 AlSi 3 O 10 (OH) 2

chlorite muscovite

K(Mg,Fe) 3 AlSi 3 O 10 (OH) 2 + Fe 2 Al 9 O 6 (SiO 4 ) 4 (O,OH) 2

biotite staurolite + water

Reactions that have a variance of 2, or more, are termed continuous reactions and describe a region in P - T space.

Many reactions are continuous with increasing metamorphic grade because of Fe-Mg exchange between reactants and products.

Other continuous reactions involve the exchange of Na and Ca, or other substituting cation pairs.

Pure exchange reactions are always continuous, and many net transfer reactions are also continuous because they are sensitive to concomitant exchange reactions.

7) Discontinuous Reactions: (Mg,Fe) 7 Al 4 Si 4 O 15 (OH) 12 + KAl 2 AlSi 3 O 10 (OH) 2 + Fe 2 Al 9 O 6 (SiO 4 ) 4 (O,OH) 2

chlorite muscovite staurolite

K(Mg,Fe) 3 AlSi 3 O 10 (OH) 2 + Al 2 SiO 5

biotite kyanite

+

water

Reactions that have a variance of 1 are discontinuous and describe a line in P - T space. Most discontinuous reactions are terminal or piercing plane net-transfer reactions that are not sensitive to exchange reactions.

Multi-Component Systems The curves for the dehydration reactions presented for metapelites and metabasites are drawn for average bulk compositions of shale and basalt respectively, and assume that the activity of water is 1 and P total = PH 2 O. The majority of these reactions, however, are not univariant and their positions are sensitive to variations in bulk composition and the controls on the composition of the fluid phase. Commonly the effect of additional components on a reaction can be predicted in a qualitative way by considering whether they dissolve preferentially in the product or the reactant phases, the stability of the phase(s) that preferentially accepts the additional component is enhanced at the expense of that which does not. These preferences can be predicted on the basis of the crystal-chemical preferences of minerals.

The equilibrium constant : K = activities of products / activities reactants

A + 3B 2C + D

Le Châtelier’s Principle or the Law of Mass action enables us to use such equilibria to predict the effect of other chemical components on reactions of interest:

K = RTln ([

a

C

]

2

[

a

D

]) ([

a

A

][

a

B

]

3

)

Bulk Compositional Effects:

Fe and Mg

In general, increasing Fe shifts most reactions to lower pressures and increasing Mg shifts them to higher pressures. There are exceptions, however, particularly in the case of Fe preferentially partitioning into garnet - higher Fe contents tend to increase the stability field of garnet, to both lower and higher pressures.

For the same reason, Mn has a similar effect on increasing the stability field of garnet at the expense of other minerals.

Ca, Ti, Fe 3+

The presence of Ca in metapelites may have a similar effect on garnet as Fe 2+ , whereas Ti and Fe 3+ , in contrast, partition preferentially into the phyllosilicates biotite and chlorite, thus enhancing their stability.

Additional components permit the existence of more mineral phases compared to that present at equivalent degrees of freedom in the simpler system. In general, for an assemblage to exist across a 3-D space (and thus a range of P & T), the number of phases is less than or equal to the number of components:

Goldschmidt’s mineralogical phase rule: C



P.

This follows from the phase rule that requires 2 or more degrees of Freedom:

F

Freedom

= C

Components

P

Phases

+ 2

Metamorphic Facies : A metamorphic facies is the set of mineral assemblages that are stable over a given range of P and T. The actual mineral assemblage within this set of possible mineral assemblages, the one that a given rock exhibits is a function of its chemical composition.

The delineation of the metamorphic facies commonly used today is a matter of historical development that predates actual experimental determination of pressures and temperatures. The division of the P-T metamorphic regime into the following metamorphic facies developed from field observations on the persistence of certain mineral assemblages for specific bulk compositions in geographic and thus P-T space: Zeolite -

zeolites or clay minerals, calcite and/or quartz-filled amygdules

Greenschist -

green minerals: chlorite, actinolite, epidote

Blueschist -

blue amphibole, aragonite

Amphibolite -

dark amphibole (hornblende), staurolite, garnet

Granulite -

absence of hydrous minerals and thus schistocity

Eclogite -

pyropic garnet & jadeiitic clinopyroxene – high pressure