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Ch 25 Metamorphic Facies. 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

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slide2

V.M. Goldschmidt (1911, 1912a) contact metamorphosed pelitic, calcareous, and psammitichornfelses 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_weintraub.pdf

victor moritz goldschmidt oslo

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

metamorphic facies

Metamorphic Facies

PentiiEskola (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

metamorphic facies5

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

CrdKfswater (missing at Oslo)

metamorphic facies6

Metamorphic Facies

PentiiEskola (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

metamorphic facies7

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

metamorphic facies8

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

metamorphic facies9

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

metamorphic facies10

Metamorphic Facies

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

metamorphic facies11

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

metamorphic facies12

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.

Metamorphic Facies

metamorphic facies13

Metamorphic Facies

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

slide14

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.

slide15

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

metamorphism of mafic rocks

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.

metamorphism of mafic rocks17

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 granulitefacies

metamorphism of mafic rocks18

Metamorphism of Mafic Rocks

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

greenschist amphibolite and granulite facies

Greenschist, Amphibolite, and GranuliteFacies

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

greenschist facies

GreenschistFacies

ACF diagram

The most characteristic mineral assemblage of the greenschistfacies 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.

greenschist facies21

GreenschistFacies

Greenschist®amphibolitefacies 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

amphibolite facies

AmphiboliteFacies

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.

amphibolite to granulite facies

Amphibolite to GranuliteFacies

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

slide24

Hornblende => 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.

granulite facies

GranuliteFacies

  • 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

Granulitefacies characterized by a largely anhydrousmineral assemblage

granulite facies26

GranuliteFacies

Origin of granulitefacies: general agreement on two points

1) Granulites represent unusually hot conditions

Temperatures > 700oC (geothermometry has yielded some very high temperatures, even in excess of 1000oC)

Average geotherm temperatures for granulitefacies depths should be in the vicinity of 500oC, suggesting that granulites are the products of crustal thickening and excess heating

granulite facies27

GranuliteFacies

2) Granulites are dry

Rocks don’t melt due to lack of available water

Granulitefaciesterranes represent deeply buried and dehydrated roots of the continental crust

Fluid inclusions in granulitefacies rocks of S. Norway are CO2-rich, whereas those in the amphibolitefacies rocks are H2O-rich

slide28

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.

mafic assemblages of the high p t series blueschist and eclogite facies

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

blueschist facies

BlueschistFacies

The blueschistfacies 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)2·H2O) 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)

AbJdQtz

Jadeite without quartz would be stable into the albite field at lower pressure =>low pressure blueschist

Specimen: Jadeite + Glaucophane w/o quartz

blueschist and eclogite facies

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.

eclogite facies

Eclogite Facies

Glaucophane + Paragonite = Pyrope + Jadeite + Qtz +H2O

Glaucophane Na2Mg3Al2Si8O22(OH)2

Paragonite is NaAl2(AlSi3O10)(OH)2

Pyrope is Mg3Al2(SiO4)3

Jadeite is NaAlSi2O6

Eclogite facies: mafic assemblage omphacitic pyroxene + pyrope-grossular garnet

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.

btw omphacite compositions are intermediate between calcium rich augite and sodium rich jadeite
BTW Omphacite compositions are intermediate between calcium-rich augite and sodium-rich jadeite

“Non-quad” pyroxenes

Jadeite

Aegirine

NaAlSi2O6

NaFe3+Si2O6

0.8

Omphacite

aegirine- augite

Na# =

Na / (Na + Ca)

Ca-Tschermack’s molecule

0.2

CaAl2SiO6

Augite (Ca, Na) (Fe, Al) Si2O6

Diopside-Hedenbergite

Ca(Mg,Fe)Si2O6

pressure temperature time p t t paths

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?

pressure temperature time p t t paths35

Staurolite (St) as relict within poikiloblastic andalusite (And).

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

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).

pressure temperature time p t t paths36

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

pressure temperature time p t t paths37

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)

chapter 26 metamorphic reactions
Chapter 26: Metamorphic Reactions
  • Isograds are reaction
  • lines
1 phase transformations

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

slide40

1. Phase Transformations

Independent of other minerals present, fluids, etc.

Andalusite -> Sill as T and P increase

regardless of other phases Stau, Mus, Qtz

1 phase transformations41

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

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 -> Perthitic Texture

2. Exsolution

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.

3 solid solid net transfer reactions

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

  • JdQtzAb

MgSiO3 + CaAl2Si2O8 = CaMgSi2O6 + Al2SiO5

  • En An Di And

4 (Mg,Fe)SiO3 + CaAl2Si2O8 =

  • Opx An Plag
  • (Mg,Fe)3Al2Si3O12 + Ca(Mg,Fe)Si2O6 + SiO2
  • GntCpxQtz

Reaction curves typically pretty straight

DS and DV change little

3 solid solid net transfer reactions44

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

4 devolatilization reactions

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

MuscQtzKfld Sill Water

depends upon the partial pressure of H2O (pH2O)

4 devolatilization reactions46

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.

kal 2 si 3 alo 10 oh 2 sio 2 kalsi 3 o 8 al 2 sio 5 h 2 o ms qtz kfs sill water

KAl2Si3AlO10(OH)2 + SiO2 = KAlSi3O8 + Al2SiO5 + H2OMs QtzKfs Sill water

Suppose H2O is withdrawn from the system at some point on the water-saturated equilibrium curve: pH2O < Plithostatic

According to Le Châtelier’s 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 < 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
4 decarbonization reactions

CaCO3 + SiO2 = CaSiO3 + CO2 (26-6)

  • Cal Qtz Wollastonite

4. Decarbonization Reactions

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.

slide49
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.

6 ion exchange reactions
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/ghr3426g33686522/

ch 27b geothermobarometry
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
  • DG=0= 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

slide52

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
geothermobarometry
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.

geothermobarometry54
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 Newton’s 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