Ch 25 Metamorphic Facies

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Ch 25 Metamorphic Facies
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• V.M. Goldschmidt (1911, 1912a) contact
  metamorphosed pelitic, calcareous, and psammitic
  hornfelses Oslo s. Norway
• fewer than six major minerals in the aureoles
  around granitoid intrusives
• first to note that the equilibrium mineral
  assemblage of a metamorphic rock could be related
  to Xbulk
• Aluminous pelites contained Al-rich minerals,
  such as cordierite, plagioclase, garnet, and/or
  an Al2SiO5 polymorph
• Calcareous rocks contained Ca-rich and Al-poor
  minerals such as Di, Wo, and/or amphibole

http//www.weizmann.ac.il/ICS/booklet/20/pdf/bob_w
eintraub.pdf
3
Victor Moritz Goldschmidt- Oslo

• Certain mineral pairs (e.g. anorthite
  enstatite) were consistently present in rocks of
  appropriate composition, whereas the
  compositionally equivalent pair
  (diopside  andalusite) was not
• If two alternative assemblages are X-equivalent,
  we must be able to relate them by a reaction
• In this case the reaction is simple
• MgSiO3 CaAl2Si2O8 CaMgSi2O6 Al2SiO5
• En An Di Als

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Metamorphic Facies

• Pentii Eskola (1914, 1915) Orijärvi, S. Finland
• Rocks with K-feldspar cordierite at Oslo
  contained the compositionally equivalent pair
  biotite muscovite at Orijärvi
• Eskola difference must reflect differing
  physical conditions
• Finnish rocks (more hydrous and lower volume
  assemblage) equilibrated at lower temperatures
  and higher pressures than the Norwegian ones

5
Metamorphic Facies

• Oslo Kfs Mg-Crd
• Orijärvi Phlo Ms Qtz
• Reaction
• 2 KMg3AlSi3O10(OH)2 6 KAl2AlSi3O10(OH)2 15
  SiO2
• Phlo Ms Qtz
• 3 Mg2Al4Si5O18 8 KAlSi3O8 8 H2O
• Crd Kfs water (missing at Oslo)

6
Metamorphic Facies

• Pentii Eskola (1915) developed the concept of
  metamorphic facies
• In any rock or metamorphic formation which has
  arrived at a chemical equilibrium through
  metamorphism at constant temperature and pressure
  conditions, the mineral composition is controlled
  only by the chemical composition. We are led to a
  general conception which the writer proposes to
  call metamorphic facies.
• Eskola s dual basis facies Xbulk mineralogy
• A metamorphic facies is a set of repeatedly
  associated metamorphic mineral assemblages

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Metamorphic Facies

• Eskola aware of the P-T implications and
  correctly deduced the relative temperatures and
  pressures of facies he proposed
• Modern lab results can now assign relatively
  accurate temperature and pressure limits to
  individual facies

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Metamorphic Facies

• Eskola (1920) proposed 5 original facies
• Greenschist
• Amphibolite
• Hornfels
• Sanidinite
• Eclogite
• Easily defined on the basis of mineral
  assemblages that develop in mafic rocks

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Metamorphic Facies

• In his final account, Eskola (1939) added
• Granulite
• Epidote-amphibolite
• Glaucophane-schist (now called Blueschist)
• ... and changed the name of the hornfels facies
  to the pyroxene hornfels facies

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Metamorphic Facies

• Fig. 25-1 The metamorphic facies proposed by
  Eskola and their relative temperature-pressure
  relationships. After Eskola (1939) Die Entstehung
  der Gesteine. Julius Springer. Berlin.

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Metamorphic Facies

• Several additional facies types have been
  proposed. Most notable are
• Zeolite
• Prehnite-pumpellyite
• ...resulting from the work of Coombs in the
  burial metamorphic terranes of New Zealand
• Fyfe et al. (1958) also proposed
• Albite-epidote hornfels
• Hornblende hornfels

12
Metamorphic Facies
Fig. 25-2. Temperature-pressure diagram showing
the generally accepted limits of the various
facies used in this text.

• The limits are approximate and gradational,
  because the reactions vary with rock composition
  and the nature and composition of the fluid phase
  
• The 30oC/km geothermal gradient is an example of
  an elevated orogenic geothermal gradient.

13
Metamorphic Facies

• Table 25-1. The definitive mineral assemblages
  that characterize each facies (for mafic rocks).

14
Metamafic and pelitic grade correspondence
Fig. 25-9. Typical mineral changes that take
place in metabasic rocks during progressive
metamorphism in the medium P/T facies series. The
approximate location of the pelitic zones of
Barrovian metamorphism are included for
comparison. Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
15
Miyashiro extended the facies concept to
encompass broader progressive sequences facies
series A traverse up grade through a metamorphic
terrane should follow one of several possible
metamorphic field gradients (Fig. 21-1, next
slide), and, if extensive enough, cross through a
sequence of facies
Facies Series
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Metamorphism of Mafic Rocks

• Mineral changes and associations along T-P
  gradients characteristic of the three facies
  series
• Hydration of original mafic minerals generally
  required
• If water unavailable, mafic igneous rocks will
  remain largely unaffected, even as associated
  sediments are completely re-equilibrated
• Coarse-grained intrusives are the least permeable
  and likely to resist metamorphic changes
• Tuffs and graywackes are the most permeable and
  so subject to metamorphic changes.

17
Metamorphism of Mafic Rocks

• The principal mineral changes are due to the
  breakdown of the two most common basaltic
  minerals plagioclase and clinopyroxene
• Plagioclase
• More Ca-rich plagioclases become progressively
  unstable as T lowered
• General correlation between temperature and
  maximum An-content of the stable plagioclase
• At low metamorphic grades only albite (An0-3) is
  stable
• The An-content of plagioclase jumps as grade
  increases
• More Anorthite content ( more calcic)
  plagioclases are stable in the upper amphibolite
  and granulite facies

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Metamorphism of Mafic Rocks

• Clinopyroxene breaks down to a number of mafic
  minerals (less hot) chlorite, actinolite,
  hornblende (hotter), etc. depending on T P

19
Greenschist, Amphibolite, and Granulite Facies

• The greenschist, amphibolite and granulite facies
  constitute the most common facies series of
  regional metamorphism on Mafic rocks

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Greenschist Facies

• ACF diagram
• The most characteristic mineral assemblage of the
  greenschist facies is chlorite albite
  epidote actinolite ? quartz

Fig. 25-6. ACF diagram illustrating
representative mineral assemblages for
metabasites in the greenschist facies. The
composition range of common mafic rocks is
shaded. Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall.
21
Greenschist Facies

• Greenschist amphibolite facies transition
  involves two major mineralogical changes
• 1. Albite oligoclase (increased Ca-content with
  temperature across the peristerite gap)
• 2. Actinolite hornblende (amphibole accepts
  increasing aluminum and alkalis at higher Ts)
• Both transitions occur at approximately the same
  grade, but have different P/T slopes

Exist exsolution lamellae on cooling in the
peristerite miscibility gap, An5-An18
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Amphibolite Facies

• ACF diagram
• Typically two-phase Hbl-Plag
• Amphibolites are typically black rocks with up to
  about 30 white plagioclase
• Like diorites, but differ texturally
• Garnet in more Al-Fe-rich and Ca-poor mafic rocks
• Clinopyroxene in Al-poor-Ca-rich rocks

Fig. 25-7. ACF diagram illustrating
representative mineral assemblages for
metabasites in the amphibolite facies. The
composition range of common mafic rocks is
shaded. Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall.
23
Amphibolite to Granulite Facies

• Mafic rocks generally melt at higher temperatures
• If water is removed by the earlier melts the
  remaining mafic rocks may become depleted in
  water
• Hornblende decomposes and orthopyroxene
  clinopyroxene appear

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Hornblende gt orthopyroxene clinopyroxene
Low water
Fig. 26-19. Simplified petrogenetic grid for
metamorphosed mafic rocks showing the location of
several determined univariant reactions in the
CaO-MgO-Al2O3-SiO2-H2O-(Na2O) system
(C(N)MASH). Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
25
Granulite Facies

• Granulite facies characterized by a largely
  anhydrous mineral assemblage

• Critical meta-basite mineral assemblage is
  orthopyroxene clinopyroxene plagioclase
  quartz
• Garnet, minor hornblende and/or biotite may be
  present
• mineralogy plagioclase and pyroxene similar to
  basalt
• The texture gneissic, and grain shapes decussate
  or polygonal, no resemblance to the original
  basalt

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Granulite Facies

• Origin of granulite facies general agreement on
  two points
• 1) Granulites represent unusually hot conditions
• Temperatures gt 700oC (geothermometry has yielded
  some very high temperatures, even in excess of
  1000oC)
• Average geotherm temperatures for granulite
  facies depths should be in the vicinity of 500oC,
  suggesting that granulites are the products of
  crustal thickening and excess heating

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Granulite Facies

• 2) Granulites are dry
• Rocks dont melt due to lack of available water
• Granulite facies terranes represent deeply buried
  and dehydrated roots of the continental crust
• Fluid inclusions in granulite facies rocks of S.
  Norway are CO2-rich, whereas those in the
  amphibolite facies rocks are H2O-rich

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Fig. 25-2. Temperature-pressure diagram showing
the generally accepted limits of the various
facies used in this text. Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
29
Mafic Assemblages of the High P/T Series
Blueschist and Eclogite Facies

• Mafic rocks (not pelites) develop definitive
  mineral assemblages under high P/T conditions
• High P/T geothermal gradients characterize
  subduction zones
• Mafic blueschists are easily recognizable by
  their color, and are useful indicators of ancient
  subduction zones. Formation requires abundant
  water.
• The great density of eclogites subducted
  basaltic oceanic crust becomes more dense than
  the surrounding mantle
• Begins to sink , extended necks thin and break
  off pieces

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Blueschist Facies

• The blueschist facies is characterized in
  metabasites by the presence of a sodic blue
  amphibole stable only at high pressures (notably
  glaucophane)
• The association of glaucophane lawsonite
  (CaAl2Si2O7(OH)2H2O) is diagnostic of the high
  pressure.
• Albite breaks down at high pressure by reaction
  to jadeite (a pyroxene) quartz
• NaAlSi3O8 NaAlSi2O6 SiO2 (reaction 25-3)
• Ab Jd Qtz
• Jadeite without quartz would be stable into the
  albite field at lower pressure gtlow pressure
  blueschist

Specimen Jadeite Glaucophane w/o quartz
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Blueschist and Eclogite Facies
Fig. 25-10. ACF diagram illustrating
representative mineral assemblages for
metabasites in the blueschist facies. The
composition range of common mafic rocks is
shaded. Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall.
32
Eclogite Facies

• Eclogite facies mafic assemblage omphacitic
  pyroxene pyrope-grossular garnet

Glaucophane Paragonite Pyrope Jadeite
Qtz H2O
Glaucophane Na2Mg3Al2Si8O22(OH)2 Paragonite is
NaAl2(AlSi3O10)(OH)2 Pyrope is Mg3Al2(SiO4)3
Jadeite is NaAlSi2O6
Fig. 25-11. ACF diagram illustrating
representative mineral assemblages for
metabasites in the eclogite facies. The
composition range of common mafic rocks is
shaded. Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall.
33
BTW Omphacite compositions are intermediate
between calcium-rich augite and sodium-rich
jadeite

• Non-quad pyroxenes

Jadeite
Aegirine
NaAlSi2O6
NaFe3Si2O6
0.8
Omphacite
aegirine- augite
Na Na / (Na Ca)
Ca-Tschermacks molecule
0.2
CaAl2SiO6
Augite (Ca, Na) (Fe, Al) Si2O6
Ca(Mg,Fe)Si2O6
Diopside-Hedenbergite
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Pressure-Temperature-Time (P-T-t) Paths

• The facies series concept suggests that a
  traverse up grade through a metamorphic terrane
  should follow a metamorphic field gradient, and
  may cross through a sequence of facies (spatial
  sequences)
• Progressive metamorphism rocks pass through a
  series of mineral assemblages as they
  continuously equilibrate to increasing
  metamorphic grade (temporal sequences)
• Are the temporal and spatial mineralogical
  changes the same?

35
Pressure-Temperature-Time (P-T-t) Paths
Staurolite (St) as relict within poikiloblastic
andalusite (And).

• Metamorphic P-T-t paths may be addressed by
• 1) Observing partial overprints of one mineral
  assemblage upon another
• The relict minerals may indicate a portion of
  either the prograde or retrograde path (or both)
  depending upon when they were created

Muscovite chlorite quartz staurolite
andalusite / sillimanite biotite H20
Staurolite (St) as relict within poikiloblastic
andalusite (And).
36
Pressure-Temperature-Time (P-T-t) Paths

• Metamorphic P-T-t paths may be addressed by
• 2) Apply geothermometers and geobarometers to the
  core vs. rim compositions of chemically zoned
  minerals to document the changing P-T conditions
  experienced by a rock during their growth

37
Pressure-Temperature-Time (P-T-t) Paths

• Classic view regional metamorphism is a result
  of deep burial or intrusion of hot magmas
• Plate tectonics regional metamorphism is a
  result of crustal thickening and heat input
  during orogeny at convergent plate boundaries
  (not simple burial)

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Chapter 26 Metamorphic Reactions

• Isograds are reaction
• lines

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1. Phase Transformations

• Isochemical phase transformations (the polymorphs
  of SiO2 or Al2SiO5 or graphite-diamond or
  calcite-aragonite
• The transformations depend on temperature and
  pressure only

Aragonite is the stable CaCO3 polymorph commonly
found in blueschist facies terranes
40
1. Phase Transformations
Independent of other minerals present, fluids,
etc. Andalusite -gt Sill as T and P increase
regardless of other phases Stau, Mus, Qtz
41
1. Phase Transformations

• Small DS for most polymorphic transformations
• ? small DG between two alternative polymorphs,
  even several tens of degrees from the equilibrium
  boundary
• ? little driving force for the reaction to
  proceed common metastable relics in the
  stability field of other
• Coexisting polymorphs may therefore represent
  non-equilibrium states (overstepped equilibrium
  curves)

Staurolite poikiloblast
42
2. Exsolution
Some solid solutions are unstable at lower T, and
exsolve as T falls. Classic example K-spar and
Ab form a solid solution at 1000C but separate
into Microcline Albite -gt Perthitic Texture
Figure 6-16. T-X phase diagram of the system
albite-orthoclase at 0.2 GPa H2O pressure. After
Bowen and Tuttle (1950). J. Geology, 58, 489-511.
Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
43
3. Solid-Solid Net-Transfer Reactions

• Differ from polymorphic transformations involve
  solids of differing composition
• Material must diffuse from one site to another
  for the reaction to proceed

• NaAlSi2O6 SiO2 NaAlSi3O8
• Jd Qtz Ab
• MgSiO3 CaAl2Si2O8 CaMgSi2O6 Al2SiO5
• En An Di And
• 4 (Mg,Fe)SiO3 CaAl2Si2O8
• Opx An Plag
• (Mg,Fe)3Al2Si3O12 Ca(Mg,Fe)Si2O6 SiO2
• Gnt Cpx Qtz

Reaction curves typically pretty straight DS and
DV change little
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3. Solid-Solid Net-Transfer Reactions

• If minerals contain volatiles, but the volatiles
  conserved in the reaction so that no fluid phase
  is generated or consumed
• For example, the reaction
• Mg3Si4O10(OH)2 4 MgSiO3 Mg7Si8O22(OH)2
• Talc Enstatite
  Anthophyllite
• involves hydrous phases, but conserves H2O
• It may therefore be treated as a solid-solid
  net-transfer reaction

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4. Devolatilization Reactions

• For example the location of the reaction line on
  a
• P-T phase diagram of the dehydration reaction
• KAl2Si3AlO10(OH)2 SiO2 KAlSi3O8
  Al2SiO5 H2O
• Musc Qtz Kfld Sill Water
• depends upon the partial pressure of H2O (pH2O)

46
4. Devolatilization Reactions

• Here the equilibrium curve represents equilibrium
  between the reactants and products under
  water-saturated conditions (pH2O PLithostatic)

P-T phase diagram for the reaction Ms Qtz Kfs
Al2SiO5 H2O showing the shift in equilibrium
conditions as pH2O varies (assuming ideal H2O-CO2
mixing). Calculated using the program TWQ by
Berman (1988, 1990, 1991). After Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
47
KAl2Si3AlO10(OH)2 SiO2 KAlSi3O8 Al2SiO5
H2O Ms Qtz Kfs Sill
water

• Suppose H2O is withdrawn from the system at some
  point on the water-saturated equilibrium curve
  pH2O lt Plithostatic
• According to Le Châteliers Principle, removing
  water at equilibrium will be compensated by the
  reaction running to the right, thereby producing
  more water
• This has the effect of stabilizing the right side
  of the reaction at the expense of the left side
• So as water is withdrawn the Kfs Sill H2O
  field expands slightly at the expense of the Mu
  Qtz field, and the reaction curve shifts toward
  lower temperature

Pfluid lt PLith by drying out the rock and
reducing the fluid content Pfluid PLith, but
the water in the fluid can become diluted by
adding another fluid component, such as CO2 or
some other volatile phase
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4. Decarbonization Reactions

• CaCO3 SiO2 CaSiO3 CO2 (26-6)
• Cal Qtz Wollastonite

Figure 26-5. T-XCO2 phase diagram for the
reaction Cal Qtz Wo CO2 at 0.5 GPa assuming
ideal H2O-CO2 mixing, calculated using the
program TWQ by Berman (1988, 1990, 1991). Winter
(2001) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
Figure 26-1. A portion of the equilibrium
boundary for the calcite-aragonite phase
transformation in the CaCO3 system. After
Johannes and Puhan (1971), Contrib. Mineral.
Petrol., 31, 28-38. Winter (2001) An Introduction
to Igneous and Metamorphic Petrology. Prentice
Hall.
49

• 5. Continuous Reactions
• Occur when F ? 1, and the reactants and products
  coexist over a temperature (or grade) interval

Fig. 26-9. Schematic isobaric T-XMg diagram
representing the simplified metamorphic reaction
Chl Qtz ? Grt H2O. From Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall. Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
50
6. Ion Exchange Reactions

• Reciprocal exchange of components between 2 or
  more minerals
• MgSiO3 CaFeSi2O6 FeSiO3 CaMgSi2O6
• Enstatite Hedenbergite Ferrosilite Diopside
• Expressed as pure end-members, but really
  involves Mg-Fe (or other) exchange between
  intermediate solutions
• Basis for many geothermobarometers
• http//www.springerlink.com/content/ghr3426g336865
  22/

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Ch 27b Geothermobarometry

• For any reaction with one or more variable
  components, at any given P,T ,we can solve for
  the equilibrium curve using
• DG0 DG0 RT ln K (27-17)
• where DG0 is the Gibbs Free Energy at the
  pressure and temperature of interest
• So ln K - DG0/RT
• At constant P, a reaction obeys
• DG DH TDS
• and dDG DVdP DSdT
• are the corrections as T,P change

52

• lnK - DG0/RT
• DG DH TDS DVdP (last correction if T const.)
• Gives lnK - DH/RT DS/R - (DV/RT)dP (27-26)
• Consider the Fe-Mg exchange in the reaction
  between biotite and Ca-free garnet
• Fe3Al2Si3O12 KMg3Si3AlO10(OH)2
• Mg3Al2Si3O12 KFe3Si3AlO10(OH)2
• Almandine Phlogopite Pyrope Annite

53
Geothermobarometry

• The Garnet - Biotite geothermometer

lnK - DH/RT DS/R - (DV/RT) dP
This is a line! From (27-26) we can extract DH
from the slope and DS from the intercept!
Figure 27-5. Graph of lnK vs. 1/T (in Kelvins)
for the Ferry and Spear (1978) garnet-biotite
exchange equilibrium at 0.2 GPa from Table 27-2.
Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
54
Geothermobarometry

• The GASP geobarometer
• Garnet-aluminosilicate-silica-plagioclase

Figure 27-8. P-T phase diagram showing the
experimental results of Koziol and Newton (1988),
and the equilibrium curve for reaction (27-37).
Open triangles indicate runs in which An grew,
closed triangles indicate runs in which Grs Ky
Qtz grew, and half-filled triangles indicate no
significant reaction. The univariant equilibrium
curve is a best-fit regression of the data
brackets. The line at 650oC is Koziol and
Newtons estimate of the reaction location based
on reactions involving zoisite. The shaded area
is the uncertainty envelope. After Koziol and
Newton (1988) Amer. Mineral., 73, 216-233